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Electrical System

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Automotive Electrics Basics - Part 1 - Terminology and Part 2 - Typical faults, symptoms, and diagnostic techniques

Ammeters_and_Voltmeters    Alternator/Dynamo    Anti_Run-on_Valve    Batteries_and_Chargers    Battery_Cut-off_Switch    Brake_Balance_and_Handbrake_Warning    Bulbs    Cable_and_Pipe_Routing    Clocks    Connectors_and_Terminals    Cooling_Fans    Cruise_Control    Fan_Belt    Fuel_Pumps    Fuses_and_Fusebox    Hazard_Flashers    Heated_Rear_Window    Heater_Fan    Horns    Ignition_Switch    Ignition_System    Ignition_Warning_Light    Indicators/Turn_Signals    Instruments    Lighter_Socket    Lighting    North_American_'Key_in'_Warning    Overdrive    Polarity    Radio    Relays    Schematics    Screen_Washers    Seat_Belt_Warning    Sealed_Wiring_Junctions    Starter    Steering_Lock    Switches_in_General    Tachometer    Wipers    Wire_Colours,_Terminal_Numbering    Wiring_Harness_Replacement    Won't_Start    Won't_Switch_Off!    Torque_Values    Links   

(Image posted by Geoff Hutton on the MGOC forum)

Probably more problems crop up with electrics than anything else, possibly everything else put together. Not surprising, considering the number of electrical components and connectors in the car. For those new to classic car electrics basic electrics terminology is covered here, and further generalised information including typical faults, symptoms, causes and diagnosis can be found here.

Today's computerised cars are even more prone to electrical problems since they are as complex as laptops and other computer equipment.

Bad connections are a frequent cause of problems in classic cars, and high-resistance connections can be the most confusing to deal with, small increases in resistance having a disproportionate effect on the circuits affected. The Lucas Fault Diagnosis Service Manual states:

"The acceptable volt-drop figure for most circuits is 10% of system voltage (1-2v on a 12v system) but there are exceptions to this rule as in the case of the starter circuit where the maximum voltage drop is 0.5v." The manual also states: "The majority of procedures involve circuit testing and the principle used will be that of checking for 'voltage drop' where a voltmeter is connected in parallel with the particular circuit to be tested.

"As voltage drop exists only when current is flowing and varies according to the amount of current it is essential that the circuit is tested 'under load', i.e. whilst passing its normal current. In certain cases this current will be measured using a test ammeter."

That is exactly the principle that is followed in these pages. And as the Lucas manual says the majority will be using a voltmeter in parallel with a circuit to look for a volt-drop, which will indicate a bad connection or an open-circuit fault. About the only case where an ammeter will be used is for testing the LH overdrive circuit, as that is the only way of determining if current is passing through the solenoid or not, as well as whether it is to the correct level or not. Practically every other circuit has a visual (lighting, gauges) or audible (heater fan) indication that something is happening, even if it is not to the correct level, and a voltmeter on the device terminals will show whether the full voltage is reaching it or not. Similarly use of an ohmmeter is very rare - like for the D-type overdrive (this initially takes 17 amps which is beyond the capability of a typical automotive multi-meters), or testing the internal resistance of the two-speed heater assembly, although they can also be used for doing a simple go/no-go check of a condenser, and even checking the dwell/gap of points-based ignition systems.

Understanding how the circuit is wired and what it shares its supply and earth with will help immensely, and for that you will need the Workshop Manual, glovebox handbook, or Haynes wiring diagrams. The colour codes for your model and year are essential, but the factory diagrams can be difficult to follow as they generally place the components on the page as they are in the car, which means a lot of wiring snaking about. Where a component simply isn't working you can use either the individual circuit elements as here, or these Advance Autowire diagrams. But if you are getting strange interactions between various circuits you will need the factory diagrams to see how the supply and earths are shared, as bad connections in these are a frequent cause of problems.

The information that follows is mainly geared towards situations where the car has been working but now has a fault. Obviously, if there are faults when you buy the car, or after someone has been making changes, then absolutely anything could have happened, i.e. multiple faults and incorrect wiring, but the basic diagnosis techniques should allow you to resolve the problems.

I have created individual schematics of virtually every circuit in all variations of the MGB - hopefully you will find them a little clearer than the official diagrams. If you hover your cursor over a wire it should change shape to indicate a link, and then display a 'tool-tip' to confirm the wire colour. Where such a schematic exists you will see an icon somewhere in the main text that talks about that circuit, click on this to see the schematic in a separate window. Clicking on the graphic here displays a list of available schematics.

Ammeters and Voltmeters

As John Twist has said: "Except in the RAREST of circumstances, the ignition warning light indicates any problem with the charging. AMMETERS properly connected into alternator circuits provide at least two more connections which can corrode and cause the alternator to fail. Buy some driving gloves instead." That presupposes that your warning light is working, of course ...

A car ammeter displays the current flowing into (charging) or out of (discharging, except cranking current) the battery hence has a centre zero and moves left of that to indicate a battery discharge and to the right of zero to indicate battery charging. Almost all need you to remove the brown wires from the starter solenoid and run two very heavy gauge wires capable of taking at least 45 amps from there to the ammeter. As well as these two new connections which can corrode, those on the back of the gauge can come loose, and either wire can short to earth and being unfused could cause a fire. There are reputed to be 'remote shunt' ammeters around (although I've never seen one) where you make the same interruption down by the solenoid but connect an insulated bar between them to carry the main current, then run two much thinner wires up to the gauge. This does away with potential failures up at the gauge but still leaves those down by the solenoid and the risk of shorting-out. An analogue ammeter needs a scale running from at least -45 amps to +45 amps, and is practice can be even higher, which means the 'normal' range of needle movement is compressed into a tiny section either side of zero. It will almost certainly be a moving-iron meter and be unstabilised i.e. have a trembling needle. Both these factors make it difficult to see whether it is showing a slight charge as it should, or a slight discharge which is bad. Under fault conditions the battery voltage could be reducing but an ammeter stills shows a slight charge, or conversely an overcharging fault could gradually be raising voltage higher and higher but still not be showing an excessive current. If you really want to connect an ammeter then see here.

A voltmeter avoids all these issues and is a much simpler proposition requiring just two light-gauge wires to an ignition switched source and earth. Under normal circumstances it is the charging circuit that is supplying all the electrical loads, even at idle in the case of an alternator, as well as trickle-charging the battery once the cranking losses have been replaced. Ordinarily a voltmeter will show something above 14v with a charged battery and a minimal electrical load, reducing as the current load goes up and gets towards the maximum capacity of the (say) alternator. When the current load exceeds the output of the alternator the voltage will drop below 12.5v and the battery will then be supplying part of the load, and hence discharging. This voltmeter has a coloured scale which is useful for showing at a glance if the voltage is in or out of limits. Up to 15v is shown as 'green' and dynamo-equipped cars can show this although an alternator should not get that high. At the lower end 12v is also shows as green, and between 11v and 12v as 'marginal', but for me if it shows that for any length of time I'd be concerned. Others show up to 16v as green which would be high even for a dynamo, yet other show 10v to 14v as 'good' which is too low for both upper and lower limits.

Voltage can drop for a number of reasons including an owner having added some high-current loads but not uprated the alternator, or the alternator is failing, or it could just be some iffy connections somewhere. In all cases a voltmeter will indicate these problems - also the problem of overcharging - much sooner and clearer than an ammeter will. The only added fault liability of a voltmeter is one of shorting of the 12v connection, but as long as this is fused even this is eliminated. The voltmeter must be connected to an ignition switched source, particularly the analogue type with the expanded scale from about 8 to 18 volts (the coloured scale which not all instruments have is particularly helpful as you can see at a glance whether it is correct or not instead of having to read the numbers and know what they mean) as these are usually thermal (slow-acting) devices and will reduce the charge in the battery over time if permanently registering. Digital instruments take less current but I still wouldn't want to leave them powered all the time.

However, unless you have an ignition relay and connect the voltmeter via an in-line fuse to that, the green circuit (fused ignition, which is probably the most obvious place to connect one) on most MGBs will show a voltage which can be significantly lower (up to 2v lower is classed as 'acceptable' by Lucas) than the alternator and battery voltages, even worse with a digital gauge where owners get paranoid about tenths of a volt. I've also seen one digital gauge that displayed half a volt less than two other test meters connected to the same point! Beware of voltmeter vendor claims that the instrument will tell you the 'strength' of your battery. It doesn't, all it does is tell you is system voltage, which isn't true battery voltage.

Both ammeter and voltmeter will tell you if the battery is being charged or not in their different ways, and a correctly operating warning light will do so as well. But none of them will tell you if the car is going to start next morning! You could say, if you are really desperate to win the argument, that the warning light might fail when you were driving along, and something else might happen to stop charging. But like I say, you would have to be desperate.

January 2015 Well, I said desperate, but Adam Liptrot did experience a situation where the warning light did NOT indicate a problem, where especially a voltmeter and possibly an ammeter would have, as recounted on the MGOC bulletin board. On a wet winter's night on country lanes over the Pennines he drove through a large puddle, and after that became aware that his indicators were slower, his lights were dimming, and about half an hour later he ground to a halt and had to be recovered home. A flat battery was diagnosed, but subsequent testing showed that whilst the system voltage was about 14v with the engine running with minimal electrical load, it dropped to 11.5v with the lights on. Turning them off again it climbed back to 14v. That is a symptom of a very weak alternator i.e. only able to put out a fraction of the current it is supposed to be capable of. A replacement alternator delivered 13.7v at idle with lights, fan and indicators all on, so somehow the water splash had damaged the alternator. Because it was still putting out some current, as indicated by the 14v with minimal electrical load, that was enough to keep the ignition warning light extinguished, even when the system voltage dropped to 11.5v. The reason the warning light didn't come on is because it is comparing the voltage at the alternator with the voltage at the rest of the cars electrical system. When these are both the same the light will not glow, and when both alternator and system voltages are low as in this case it still will not glow. In this case a voltmeter would have immediately and clearly shown the problem, but it has to be said that dimming lights and slowing indicators should also have alerted him. Unlikely to have enabled him to do anything about it at the time, but he would perhaps have been able to stop at a warm pub to ring his recovery organisation, rather than being stranded in the middle of nowhere. I've often wondered, if that happened to me on some of our jaunts around the country and Europe before I had a phone with sat nav, just how I would describe to the AA (in my case) exactly where I was!


Repolarising a Dynamo
Charging System Basics
Ignition Warning Light and Charging Theory
Dynamo Control Box
Alternator Operation
Testing Output
Trickle-charge current
Charging Faults
Alternator Brush Replacement
Alternator Harness Plug Clip
Fan/drive belts
Converting Dynamo to Alternator
Converting 16AC Alternator with separate regulator to later alternator with integral regulator
'One wire' Alternators


Dynamo converted to Alternator
Alternator with external regulator (16AC, 1968)
Alternator with external regulator (16AC) converted to Alternator with internal regulator (16/17/18ACR)
Alternator with internal regulator (16/17/18ACR) - without Ignition Relay (69-76)
Alternator with internal regulator (16/17/18ACR) - UK with Ignition Relay (77-on)
Alternator with internal regulator (16/17/18ACR) - North American with Ignition Relay (77-on)
Alternator internals


Alternatives/Replacements April 2021

January 2021: A dynamo does not output AC as Ant Anstead said in the Wheeler Dealers episode on the TR4. Both dynamo and alternator output DC at their terminals, they just use different methods internally to do so. A dynamo generates DC internally and outputs that directly, an alternator generates AC internally but rectifies it to DC before outputting it. What Ant Anstead held up is the 'control box' which regulates the voltage and current the dynamo outputs to charge the battery and run the cars electrics, using relays and resistors. The first alternators used on the MGB - the 16AC used on early Mk2s - also had external regulation, albeit electronic instead of relays and resistors, but after that that 16ACR, 17ACR and 18ACR had internal regulation and all modern alternators (except possibly very specialised applications) will be the same.

A dynamo and separate control box were used on Mk1 MGBs. The first MGB alternators (MkII models in 1967) were the 16AC (remote regulator) and 16ACR (integral regulator) which are rated at 34 amps. It was changed to the 17ACR in February 1973 and finally the 18ACR in about June 1976. There is conflicting information about the output rating of these latter two, some sources say the 17ACR is 36 amps and the 18ACR 45 amps, others say the 17ACR is 43 amps and the 18ACR 45 amps. 43 or 45 amps ought to be sufficient for factory loads, the V8 has a Delco 46 amp alternator and that is sufficient to keep the battery voltage above 12.5v even with headlights, twin cooling fans and the heated rear window running, and at idling speeds. The problem is that people fit voltmeters wired to the green circuit, which can be a couple of volts lower than the solenoid i.e. battery voltage, then get paranoid. It's battery voltage which is important, and any volt-drop between there and the green circuit is down to ageing connections, and the best alternator in the world isn't going to cure them, although it may cover them up.

I had briefly wondered why an AC/Delco was chosen for the V8 when it has almost the same output capacity as the later Lucas items. It was only during a discussion with someone about alternators for a V8 conversion that it became clear - the Lucas alts are too long to fit in the factory position as the rocker cover is in the way! It wasn't a problem for Costello as he swung the alternator out from the engine, but the factory fitted the MGB V8 filter high up on the inner wing which precluded that. Hence a shorter alt that would fit directly in front of the rocker cover. Only just though, as the harness plug has to be wiggled off at an angle.


There are several different connection arrangements for Lucas alternators over the years ranging from 4-pin of the 16AC with remote regulator (best avoided for a conversion), then a 5-pin using two connectors on the early internally regulated 16ACR and finally a 3-pin single connector for other 16/17/18ACR variants. 5-pin/two plug systems have two Indicator spades in one of the connectors which are linked together by a loop of brown/yellow wire in the plug, possibly to protect the alternator if the engine is run with the IND/B+ plug removed. 3-pin have two variants - one with two large spades side-by-side and a single standard-sized spade to one side, and another with a single large spade and one standard-sized spade either side of it, one possibly with chamfered corners. Where there are two large spades both are outputs (+), with the standard-sized spade being for the indicator wire (IND). With the other type the single large spade is the output (+), the standard spade is for the indicator wire (IND), and the one with the chamfered corners is for the voltage sensing wire (B+ or BATT+).

The B+ (or BATT+) voltage sensing terminal is wired back to the solenoid with a standard gauge brown wire. This is used to sense the voltage at the solenoid rather than the alternator for voltage regulation purposes, and would ensure that under high current conditions any volt-drop occurring in the main output wires (thick brown and black) between alternator and solenoid/body is ignored and the voltage at the solenoid (and hence the battery) is maintained at the correct level, this system is called 'battery sensing'. This was the case in the 5-pin 2-plug 16ACR from 69 to 71.

Initially the 3-pin single-plug alternators used machine sensing (i.e. the correct voltage is maintained at the alternator terminals, but could be lower at the solenoid and hence battery under high current conditions) with just a single thick brown and a standard gauge brown/yellow in the alternator plug with the third position in the harness plug empty. This is a '2-wire' alternator, seen on a 1972 model.

Possibly because of problems with low battery voltage for 1973 the Workshop Manual diagram shows the alternators have reverted to battery sensing again having 3-wire alternator wiring. Clausager states that a new version of the 16ACR with modified regulator and surge protection was provided in March 72, and is how Bee a 73-model built September 72 is wired with an additional standard-gauge brown in the alternator plug wired back to the solenoid as before, and seems to have remained the case up to and including the 77 model year at least.

Clausager states the 17ACR was fitted from February 73 so higher output, possibly associated with the GT HRW having being made standard equipment. This is a 3-wire alternator, but can be used with a 2-wire harness by connecting the third spade to the output spade in the alternator plug. A '2-wire' alternator i.e. one with two large output spades can be connected to a 3-wire harness as-is i.e. each brown wire to either of the large output spades, although the harness plug may need to be changed to fit the alternator.

The final variation was the 18ACR. Clausager says the exact change point is unavailable, but thinks it was from June 76, borne out by the Parts Catalogue which shows the 18ACR being used before September 76. The schematics get confusing here, with UK 1979 from the WSM and 'later' models for both UK and North America indicating it had reverted to machine-sensing with two thick brown wires from the two large output spades to the solenoid. This would give increased current carrying capacity and lower volt-drop now cars had electric cooling fans, offsetting the loss in voltage caused by the regulator sense terminal moving from the solenoid back to the alternator again, and despite the three wires is effectively a '2-wire' system. However JCR Supplies at one time showed this diode pack which is the 3-wire battery-sensing type, and I have seen the harness plug on a 78 which does match that. However they said it was for an 18ACR, which other sources indicate should be machine sensing. They also showed one with the two large spades i.e. the Euro plug, correctly described as machine sensing, but saying it is for 15/16/17ACR which is only partially correct.

  October 2014:

Click the thumbnail for details on converting between 4-wire, 3-wire and 2-wire alternators.

And to beat it to death, put a bullet in its brain, and hang, draw and quarter it, there are additional changes to the above in the Parts Catalogue:

  • The original 16ACR as detailed above - part No. 37H 4194
  • A different 16ACR in Jan 71 - 37H 6983
  • The modified 16ACR as detailed above in March 72 - 37H 7503
  • The 17ACR as detailed above in Feb 73 - 37H 7959
  • A different 17ACR (no date) - 37H 8208
  • An 18ACR (no date, but prior to September 76 and used to the end of production) - AAU1013

'Two browns' (what a terrible thought) wiring will cope equally well with both battery sensing and machine sensing alternators, but battery sensing alternators must have the 2nd brown wire, or at least a link in the harness plug between the + and B+, to operate correctly. In addition to the brown/yellow Indicator wire a friends 72 only has one brown (large), my 73 has one large and one smaller brown, and another friends 74 is the same, so those at least conform to the above. For completeness my 75 V8 (AC-Delco) uses the same plug, both large spades are output terminals, however only one is wired (as per the factory schematic) with a heavy gauge brown, the other is unused (and has allowed me to use it as a direct output to the cooling fan relay).

So some care needs to be taken to determine just which type of wiring, plug and alternator you have when making changes, even swapping alternators which take the same plug. If by looking at the two large spades on the alternator you can see they are clearly connected together, then you have a machine-sensing alternator and can use either or both large spades for the output. But if the two are clearly insulated from one another, then you have a battery sensing alternator. On these you must have a large gauge brown wire on the output spade at the very least, and a smaller gauge at least on the sense terminal. If in doubt as to which you have, it may be possible to determine by voltage measurement. Turn all the electrical loads on you possibly can, alternator plugged in, engine running at a fast idle, then connect a voltmeter between the two large spades. If you can measure any voltage between the two (may only be in the order of tenths of a volt) then you probably have a battery sensing alternator. If there is zero volts between the two large spades, then you probably have a machine sensing alternator. Or simply provide large gauge brown wires to both large spades to cover both eventualities, and get the benefit of a lower volt-drop under high-current conditions if you have a machine sensing alternator.

Tip: If you carry an alternator as a spare at any time, then it's a good idea to make sure it already has a pulley fitted. The large nut is very tight and makes it very difficult if not impossible to remove the pulley from a failed unit as there is no easy way of holding the rotor still (except perhaps by wrapping a fan-belt right round the pulley and gripping it firmly). If your spare alt has a pulley, then compare the size with what's on the car. If it's the same size then all well and good. If it's a different size then check now by trial-fitting that it is compatible with your fan-belt! And remember, if the pulley is smaller the alternator will rotate faster than normal, so you may want to limit engine revs a little to avoid over-revving the alternator. If it is larger then it will rotate slower, so you may find the engine needs to be revved a bit higher before it starts charging, will stop charging sooner as the revs fall, and it may not charge at idle. The charge voltage and current during normal driving will also be lower than usual, but if you keep the revs up and/or the electrical load down it should still charge well enough to get you where you are going.


Front: With the top front mounting bolted to an ear on the water pump, there is an adjuster link on the lower front mounting. Prior to the 77 model year both dynamo and alternator used a flat link 12H67 or 2A497 and a pillar 2A128 on the engine front cover. However for 1977 on a different link was used - 12G2627, which the Parts Catalogue (and a couple of supplier drawings) seems to show is cranked, without the pillar. However Googling 12G2627 shows a flat link, albeit curved with a curved slot, so the pillar is still needed.

Rear: Both dynamo and alternator use their own versions of an angle bracket to attach a single mounting point on the rear end-plate to two bosses on the block. Early blocks only had two for the dynamo, whereas later blocks have one pair for a dynamo and another pair further forwards for an alternator. After-market adapters are available to mount an alternator to an early block which only has the rear-most pair of bosses.

Alternatives/Replacements April 2021

A friend of a friend had an alternator seize on his 72 MGB which burnt out the belt. We got a 'universal' 16/17/18ACR replacement rated at 55amps complete with fan and pulley which was about half the price of 'standard' types available from the usual sources. I compared the pulley against the original and it was very slightly smaller, which means it would spin faster for a given engine speed and so boost output, but not so fast that it would damage the alt. However when we came to fit it the belt would just not go on, even using the this tip to get tight belts on. The new belt was the same as the old i.e. a 10900 for CB and pre-77 RB. The alt body was sitting on the engine mount i.e. down as far as it would go, but there was still a good 1/2" of adjustment on the adjuster link that we could not use. In the end we had to remove the front bolt altogether and slacken the rear right off, and that allowed the alt to tip down and across just enough to get the belt in the alt pulley, and than into the fan pulley groove. Getting the front bolt back in was very marginal, and hardly any movement at the adjuster link was needed to get correct belt tension. I wondered about using the later 10950 belt, but felt that might be too long. Anyway it was on, so we tested it - firstly to make sure that the short period of running without a belt and hence no water pump which drive the temp gauge right up had done no damage, and then checked the output. 14.5v at a fast idle, and turning everything on that I could think of i.e. headlights, heater fan, wipers, brake lights, and holding the clutch down in reverse didn't drop the voltage at all. I was a bit bothered by how fast the temp gauge rose towards N, but once it reached there it stopped bang over the N, and stayed there on a short test drive and idling after our return. It's a long time I've seen that - both mine only get about 2/3rds the way from C to N, although the V8 is over N when the fans cut in. However the oil gauge only got as far as 20psi, and stayed there no matter what the engine was doing, and took a couple of seconds to start dropping when the engine was switched off, so I suspect that is a sticking needle in the mechanism. Subsequently asking on the MGOC forum two people said they had found the same thing with this type of alt and had to use the same technique to get the belt on. One of them had been offered a 10925 or a 10950 belt as an alternative to the 10900 but not surprisingly stayed with the original. So I suspect that because he was offered alternatives, the vendor knew there was an issue, and maybe the 10925 belt would be a better bet.

We scrapped a Metro years ago but I kept the alt which is an A115 45 amp. A direct replacement on the roadster, even though the pulley is slightly larger so rotates slower which reduces output, but the belt still fits, and it's good enough to carry as a 'get you home' spare. As it happens the roadster came to me with what looks like an A127 (metal end-plate instead of plastic on the 16/17/18 ACR and A115 types) of unknown output, which are available in 45, 55 and 65 amps with standard Euro plug spade terminals (as mine is), and 70 amps with stud output terminal. These are available from various Mini parts suppliers (including Moss), and given that the Metro item seems to be a direct replacement, these Mini items may be as well if you have additional loads and need a higher output. However given the availability of a universal 16/17/18ACR at 55 amps as recounted above maybe that is a better bet even at the cost of the faff with the belt, and it would be worth trying a 10925.

Whilst fitting the 55amp round at my pal's house his neighbour came round asking if we had any use for an 18ACR from his TVR days. Pal didn't want it so I had it, thinking I might be able to juggle things around and end up with a spare for the V8 as well. It's too long for the V8 which has the rocker cover immediately behind it - 5.5" as opposed to 4.6" of the AC/Delco. But it is a direct replacement for the A127 the roadster came with so was fitted and tested and gives good output, which makes the A127 available as a spare for the V8.

Whilst both alts are on the same - driver's - side of the engine 4-cylinder cars have the mounting ears on top and the adjuster link underneath, but the V8 is the other way round. This makes the V8 a 'left-hand' as opposed to the 'right-hand' of the roadster. But by removing three through-bolts you can rotate one half of the casing with respect to the other and perform a 'gender reassignment'. That needs the pulley and fan to be removed as the fan covers the heads of the bolts, which was where we came 'unstuck' with the 72 MGB above. Thinking we would need to transfer them over as some sources showed a 'bare' alt, but the pulley was stuck fast onto the shaft although the nut itself came off easily with a chatter-gun and one hand holding the old belt wrapped round the pulley. It did eventually come off, but not before a piece of the cast pulley broke off, meaning we had to get one with fan and pulley already fitted. The Woodruff key also took some shifting. But this nut came off with my air-gun and belt as before, and the pulley and fan just fell off and no provision for Woodruff key. After that altering the case was easy, and a trial-fit before refitting the fan and pulley showed that it was a goer, so refitted the original AC Delco. The belt has always been too long on that - right at the end of the adjuster link, and while it was off I noticed the part number was VS1138, indicating that it was 1138mm long instead of 1125mm as it should be. I have a note that I bought a spare on getting the car, fitted when the one that came with the car frayed (and pulled a coil wire off ...). At that time I bought a new 11125 which has been in the back ever since, so that went on now and is the correct size leaving a bit of adjustment left. So another 11125 bought as a carried spare, and the 1138 kept 'just in case' at home. Trial fit both to check the belt fits, it works, and the pulley is aligned correctly.

Ignition Warning Light (aka 'Idiot Light') and Charging Theory Schematics

Change to LED?

Why 'Idiot' light? I don't know, but it seems to be an Americanism (that is, it's Americans that seem to use the term, not that Americans are idiots as one seemed to think I meant ...). The only thing I can think of is a point of view that says "Only an idiot would need a warning light telling them the ignition was on." Which shows a complete misunderstanding of the purpose of the light, so who's the idiot now? However someone else said that he has heard the oil warning light (provided in lieu of an oil pressure gauge) referred to as the 'idiot' light, because only idiots ignore it when it comes on then seize their engine. But another view has it that even idiots should be able to understand when a warning light comes on, whereas you need intelligence to understand a gauge. So maybe, in terms of the ignition warning light, only an idiot ignores it until the battery goes flat, and as Jochen Beyer has pointed out the ignition warning light also lets you know your fan-belt has broken before you boil your coolant out.

The ignition warning light always had two wires connected to it which must be insulated from earth as at various times one side receives 12v from the ignition switch and the other side 12v from the alternator or control box. Most of the other bulb holders on CB 4-cylinder cars only need one wire as they can pick up an earth from being plugged into to metal panel or instrument case. V8s and RB 4-cylinder cars have the other warning lights with two wires as well as they are fitted in plastic panels so need a wired earth.

The warning light is like a pair of balance scales between the ignition circuit and the charging circuit, and that is how it is connected - from the white of the ignition circuit, through the bulb, and to the dynamo/alternator via the brown/yellow. (Note that the lamp-holder is unique in that it has two wires - one to each side of the bulb - and the body of the holder should not be connected to earth like the panel and main-beam lamps are.) If both circuits have the same voltage then there is no potential difference across the bulb and it will not light. This is irrespective of whether there is 0v on both circuits (ignition off, engine stationary) or 12v (actually around 14v when charging) on both circuits (ignition switched on and engine running and charging). If the two circuits show a potential difference i.e. ignition switched on but engine stationary, or ignition switched on and engine running but not charging, then the lamp will light. This latter condition is a fault (and incidentally the main purpose of the light) which should be investigated before you get stranded. You may also note that when you switch off the ignition but while the engine is still spinning the ignition warning light glows again until the engine stops. This is because the charging system is still outputting while the engine is spinning down, so outputting 14v to the brown/yellow, but with the ignition off there is no longer any voltage on the white. By itself that wouldn't cause the warning light to glow, but as there are several other circuits also powered from the white, each with their own path to earth, current from the brown/yellow passes through the warning light bulb and these other circuits to their earths, and that is enough to make the light glow, until the charging system stops outputting. It is this feature that gives rise to the "won't switch off!" problem that can affect 1977 and later North American spec.

At the simplest level, a glowing warning light tells you that the ignition is switched on but the dynamo/alternator is not charging. It may be obvious that the dynamo/alternator isn't charging if you haven't even started the engine yet, but the beauty is that you can see the warning light itself is working. So if the engine is running and the charge does fail at some point, then you have a very good chance that the warning light will come on and tell you about it. On alternator-equipped cars the warning light acts as a priming system to start it charging from about 1000rpm. Without the warning it may need to be revved to several thousand rpm to start, and new alts out of the box may not do that. Dynamos are self-priming as below.

Change to LED? The short answer is no. As stated above the warning light on alternator-equipped cars is used as a primer to start it charging, using the current through the incandescent bulb. If changed to an LED the current is reduced to a fraction of its previous value and won't allow it to start charging at the normal point. The warning is also part of the on-board diagnostics, and current can flow in either direction under various conditions. An LED only glows with current flowing in one direction - unless you go to the bother of installing it with a full-wave bridge rectifier. Although this effect is not an issue with dynamos there is another aspect relating to the warning light acting like a pair of balance scales. It is inevitable that there will be some differences in voltage between the ignition supply and the dynamo/alternator output and this difference in voltage increases as the demand for current goes up i.e. lights, wipers, heater fan and so on are turned on. Because an incandescent bulb is relatively insensitive to small voltages it will not glow, unless there is a fault resulting in a bigger difference in voltage then there should be. But LEDs are much more sensitive to small voltages and will glow when incandescents wouldn't, including when there is no actual fault as such. Even some suppliers will tell you not to fit one in this position.

With the exception of RHD cars from 1977 on with the ignition relay, the white for the warning light always comes off the ignition switch. On RHD 1977 and later cars it comes off the output of the ignition relay which starts off white/brown, then changes to white at the multi-way plug behind the dash. See ignition schematics for more info.

On a dynamo system the warning light brown/yellow is connected to the dynamo output at the control box and hence has a low-resistance path to earth to light it when the ignition is turned on. The initial excitation for the dynamo field always comes from its own residual magnetism, which is why you have to 'flash' the field terminal to battery when you install a new dynamo or when you are converting from one polarity to another. NEVER, I repeat, NEVER flash an alternator's terminals to battery. This residual magnetism results in a dynamo output of a couple of volts, which is passed through low-resistance windings on the cut-out and current regulator relays in the control box to the field winding. This voltage now causes the dynamo to output its full voltage, which operates the cut-out relay to connect the dynamo output to the battery so charging it. The cut-out relay has a normally open contact which disconnects the dynamo when the engine is stopped, or the output voltage drops below a certain level. In fact it usually releases at idle, lighting or flickering the warning lamp. If this did not happen the battery would rapidly discharge through the dynamo, which would be acting like a motor trying to turn the engine. The cut-out relay has two windings, one of which ensures the relay releases as the voltage falls. IMPORTANT NOTE: If you manually operate the cut-out relay with the engine stopped it will latch in, connecting battery voltage to the dynamo, which will try to turn the engine. This passes a high current through the control box and dynamo which will burn them out in quite a short time.

Because it has connections to both the ignition supply and the alternator or control box the bulb holder is a special in that it always has two terminals which are insulated from the body of the bulb holder so cannot short to an earthed metal panel. Various other bulb holders also had two terminals at various times - Mk1 indicator tell-tales for the same reason as the ignition warning light, V8 and RB indicator tell-tales and main-beam warning lights as they are fitted to a plastic panel and so need a wired earth.

Dynamo Control Box April 2013

Click the thumbnail on the left for step-by-step information on how the control box does what it does. Considering its technology is so old its method of voltage and current regulation is really clever - a form of time-division multiplexing if you want to be technical. Basically when the battery voltage rises above a certain point the voltage regulator relay will operate. Its contacts (normally closed) open, introducing a resistance into the field circuit. This reduces the voltage at the field winding, which reduces the output voltage and current, and hence the charging current, allowing the battery voltage to fall back slightly. But it doesn't operate just once, oh dear me no, it is usually operating and releasing rapidly all the time unless the battery is significantly discharged. When the full dynamo voltage is connected to the battery, the battery voltage can't rise immediately, but takes a period of time. As it's voltage rises so does the voltage across the voltage regulator relay, which eventually operates. This reduces the dynamo current and voltage, but again the battery voltage can't drop instantly, but takes a period of time. It's only when battery voltage has dropped below a certain point that the voltage regulator relay releases again, so connecting full dynamo voltage and current to the battery again. The really clever bit is that when the battery needs charging it takes a relatively long time for its voltage to rise enough to operate the voltage regulator relay, and a relatively short time for the voltage regulator relay to release again. The result is a relatively high average charging current, to rapidly recharge the battery. As the battery becomes charged the time taken for the voltage regulator relay to release each time increases gradually, and the time taken for it to operate again reduces gradually, giving a lower average charging current over time. The average current as seen on a graph has a relatively steep rise initially when recharging starts, gradually flattening out as it approaches a horizontal 'fully charged' line (ranging from 14.9 to 15.v at 10C to 14.3 to 14.9v at 40C), until it just touches it, at which point just a trickle charge is being put into the battery. In practice, unless the battery is significantly discharged, the voltage regulator relay operates and releases very rapidly, this can be felt as a rapid vibration of the relay armature as it is lightly touched with a finger-tip, and a continuous electrical arc can be seen at the contacts.

The current regulator relay operates on a similar principle, but it only comes into play when the maximum design current of the dynamo is reached. The relay operates, introducing the same resistance into the field circuit, which again reduces the field voltage and hence the output current, to protect the dynamo against overheating and damage. This reduction in current causes the relay to release again, so giving full current, which causes the relay to operate again and so on, giving an average current over time as before. The system is designed such that this average current (19 to 22 amps) is the safe current for the dynamo. With large non-original electrical loads connected to the system it will be the current drawn by these that will cause the current regulator relay to operate to protect the dynamo. The loads are still connected of course, and so is the battery, and it will be the battery that will be supplying them then, at least partially, so gradually discharging it, even though the engine is running and the dynamo is operating correctly.

I've heard a claim from someone who studied the workings of the control box at college (many years ago!) that there is a weakness in the system in that if the battery is less than half charged the characteristics of the control box are such that the battery will never recharge, and you would have to recharge it using a charger before you could use it normally again. Personally I can't see it, the cut-out operates independently of the battery voltage, and even if that then tries to take so much current that the current regulator relay operates, the dynamo is still going to be delivering some voltage. As long as that is more than battery voltage then the battery will charge. Subsequently perusing the Lucas Fault Diagnosis Manual I found the statement that if a battery has been fully discharged, the on-board charging system will never put back more than half the original capacity, so I think that is where the 'half charged' thing comes from. Boost charging will be required to put back the full charge, and this applies to cars with alternators more so than dynamos as the regulated alternator voltage is less than the dynamo regulated voltage under most operating conditions. It's particularly relevant to cars equipped with electronic alarm systems, including modern cars, where these are used infrequently i.e. there is a constant trickle discharge from the battery.

  Alternator operation

By contrast an alternator system takes much less explanation - unless you get into the theory of semiconductors! In an alternator the warning light brown/yellow connection is connected to the field windings, which because they are relatively low resistance and connected to earth, offers an earth-path to the bulb to light it when the ignition is turned on. So it is the warning light current through the bulb and the field windings which generates the initial excitation for the output windings. This generates an initial output voltage, which is fed back to the fields as well as the output terminal by a set of diodes, to give the full excitation voltage and hence the full output voltage. It is at this point that the bulb has full system voltage on both sides and therefore extinguishes, which is usually at about 900rpm. With the alternator charging, as the engine is slowed the alternator output voltage drops, and hence the field excitation current, until at about 600rpm charging suddenly stops and the warning light will glow.

From this it can be seen that the ignition warning light is necessary to give the alternator its initial excitation, and some schematics do show a resistor wired across the warning light to ensure that this initial excitation current is available even if the bulb has blown or is removed. However, I have never known of this resistor being provided in practice, and also in practice a used alternator has a little residual magnetism that is usually enough to 'kick-start' it into charging, although the engine may have to be revved to 2000 or 3000 rpm before this starts happening. Once it has started charging, it will charge normally i.e. down to about 600rpm as before, but then need to be revved to 2k or 3k again to start charging again. A new alternator just out of the box may not have this residual magnetism and so may not be able to kick-start itself, in which case the ignition warning light circuit is essential. ON NO ACCOUNT should you try to generate this magnetism by 'flashing' the alternator connections across the battery like you would polarise a dynamo, you may well blow the diodes or other electronics.

The voltage regulator is a sealed electronic module which constantly varies the voltage fed back to the field windings from the output, according to the voltage of the output - i.e. a closed-circuit feedback system. There is no current regulator circuit as such, the books say that the inherent design of the alternator is such that current is automatically self-regulating. This is possibly from the thickness of the output windings and hence their resistance (higher output units having thicker wires), that being all that is required as unlike a dynamo an alternator has its output windings attached to the case, hence no brushes or commutator to limit current. It does mean that an alternator naturally generates an alternating current in its output windings (3-phase in fact), hence the requirement for a network of diodes to convert this to pulsed direct current at the output terminals and field windings.

Warning light resistor:

My Leyland Workshop Manual has a schematic of the 16AC alternator charging circuit which shows a resistor connected in parallel with the warning light, but none of the full schematics show this, and as far as I know one has never existed in practice. Apparently the RV8 schematic also shows one, but Nic Houslip has confirmed with one of the engineers that worked on the RV8 that it doesn't exist on that either. If you Google 'ignition warning light resistor' you get a lot of chatter about whether this resistor exists or not. One can see that with alternators if the warning light blows, then theoretically there is nothing to start the alternator charging (although as indicated above used alternators at least will start charging when revved to about 2k to 3k, then charge normally after that down to about 600 rpm). On cars with a dynamo and control box the warning light is purely an indicator, it has no priming function, as the residual magnetism in the dynamo makes it self-priming. A number of comments are incorrect, some say it should be in series and not in parallel, and some cause even more confusion by talking about the use of LEDs in place of incandescents (what on earth for?). But it does appear that there are schematics for marques other than MG that also include them, and for a lot more recent models, for example BMW seem to have started providing this resistor from 1987. An interesting one is Bud Krueger's MG TD site where he shows pictures (reproduced here) of what he believes are original warning light holders for his car, with a length of resistance wire wrapped around the holder of both the ignition and low fuel warning lights. However Bud tells me they are in series with the lamps to reduce the brightness. The TD has a dynamo which as I say above doesn't need the warning light to start charging, and a low-fuel warning circuit wouldn't require a bypass circuit in any event.

Testing Output:

Alternator Bench Test

Under normal circumstances, with minimal electrical load, you should have 14v to 15v on, say, the brown wires at your fusebox. As you switch on more and more electrical circuits you will take more and more current from the dynamo or alternator. As that approaches its maximum output current, the system voltage will start to drop, and when the system voltage drops to 12.5v you have reached the maximum capacity of your charging system. Any further increase in electrical load will reduce the system voltage below 12.3v, and some current will be taken from the battery. You can use this 'feature' - voltage dropping as current increases - to check the output of your charging system, either dynamo or alternator. You could put an ammeter in series with the dynamo or output wire, but most home-use multi-meters only go up to 10 amps which is less than half the output of a dynamo let alone that of an alternator, and connecting an ammeter to an alternator is not straightforward. It is easier to monitor the system voltage while gradually switching on more and more electrical circuits, and varying the engine speed, and seeing at what point the system voltage dips below 12.5 volts. Lighting circuits have stated wattages for all the bulbs which can be used to calculate current - add all the wattages together, then divide that by 12 volts to give amps. For example by turning on the parking lights of an MGB you can have four bulbs at the corners and four number plate bulbs all at 5 watts each and four 2.2 watt bulbs in the instruments, giving 48.8 watts in total, or 4 and a bit amps (North American cars with side markers have another four 5 watt bulbs or 1.7 amps). A pair of 45 watt dipped beams adds 7.5 amps, if you have a headlamp flasher and 60 watt main beams that will add another 10 amps. A pair of 21W brake lights will add 3.5 amps, as will a pair of reversing light bulbs, and all that totals 28 amps (30 amps for North America with side markers). A pair of 21W indicator bulbs adds 3.5 amps, and hazard lights with four 21w bulbs will add 7 amps, but of course these and the indicators will be flashing so their effect on the voltage will not be steady so making it difficult to read. However these can be added by bypassing the respective flashers to get a steady glow, and hence a steady effect on the voltage.

For dynamo-equipped cars i.e. with a 22 amp output capacity parking lights, dipped beams, brake lights, reversing lights and indicators take about 22 amps. For alternator-equipped cars these lighting circuits plus headlamp flasher and heater fan give 35 amps. On GTs the HRW adds about another 7 amps, although mine takes 9 amps as it is on a relay, and my V8 twin fans add another 10 amps. With all these there is more than enough load (54 amps) to overwhelm the 46 amp alternator. But don't expect to see the voltage drop below 12.5 volts at exactly the calculated load on your dynamo or alternator. Even if your circuits seem to be working well there are bound to be some unwanted resistances in switches and connections which will reduce the current taken by the circuits, which means you may be able to take more than the theoretical output of your dynamo or alternator and still be at more than 12.5 volts. However if the voltage drops below 12.5v at less than the theoretical figure, then either your dynamo/alternator is giving less output than it should or there are bad connections between it and the solenoid. So take another voltage measurement right at the dynamo control box 'B' terminal or alternator output terminal, and if that is more than about 0.5v higher then you have some resistance between the two measurement points which is limiting the maximum effective output.

  The Workshop Manual quotes charge voltage for the dynamo at 3000rpm as follows:

10C/50F - 14.9 to 15.5v
20C/68F - 14.7 to 15.3v
30C/86F - 14.5 to 15.1v
40C/104F - 14.3 to 14.9v
The variation with temperature is because the regulator is electro-mechanical and more susceptible to ambient temperature than electronic systems.

For the alternator it quotes 14.3 to 14.7v, the closer tolerance due to temperature-compensated electronics. However some suppliers of replacement voltage regulators quote 14 to 14.5v for their products, and another quotes 13.6 to 14.4v. I remember reading some time ago that Mercedes had started using higher voltages to protect against premature battery failure, as the lower the voltage the less capacity the charging system will be able to restore. Lucas states that a battery having lost just 25% of its charge will never be fully recharged by the vehicles charging system - even at the higher of the above voltages, and a battery that has gone completely flat will only regain 50% of its capacity from the vehicles charging system, both situations requiring an external charger at a higher voltage and current for maybe several hours. All this came about from a pal finding that his car took several seconds of cranking to start from cold normally, and he never got more than about 14.1v at the alternator terminals, even after a 30 mile run, even revving the engine, but after being on a conventional trickle charger it fires up straight away. He wondered whether his voltage regulator was faulty, which led to finding the above figures for replacements. However Bosch regulators are available in 14.2, 14.6 and 14.7v flavours, and maybe as high as 15v, and it would be interesting to see if there is a higher voltage Bosch unit that would fit a Lucas alt. There are about 30 Lucas voltage regulator part numbers just for the 16/17/18ACR, and almost as many Lucas equivalent numbers for each Bosch regulator. One would have to try and match each Lucas number with each Bosch Lucas equivalent - patience required! It had occurred to me that perhaps one could modify how the existing voltage regulator was connected and so 'encourage' it to output a higher voltage. A diode would be the obvious way, these have a forward volt-drop of about 0.5v regardless of current, and if connected correctly should result in an increase of 0.5v in the output. Whilst looking for higher voltage Bosch units I came across someone with a similar problem as my pal, and had done just that.

Alternator Bench Test: For an alternator this is quite easy, but you have to be careful with a dynamo as it is very easy to exceed its maximum voltage and cause an internal fault.

For the alternator all you need is a 12v supply, a small bulb to simulate the ignition warning light, a voltmeter, and something to spin it such as a drill.

12v goes to an output spade (usually one of two large spades on the 3-pin connector on all but the earliest alts) and to one side of the bulb. MAKE ABSOLUTELY SURE THE POLARITY IS CORRECT OR YOU WILL SERIOUSLY DAMAGE THE ALT. The other side of the bulb goes to the smallest of the three spades where the output plug goes. Earth goes to the body of the alternator. The meter goes between the 12v supply and earth.

My alternators all have a 22mm pulley nut, and a long bolt through that with the head and washer inside the socket, and a nut and washer on the back of the socket, means the bolt shank can be clamped in the drill chuck.

The bulb should be glowing, set the drill to spin the alternator clockwise as you look at the pulley, and increase the speed. At some point the bulb will suddenly dim right down (but not go completely out) and the voltage on the meter will step up from 12v to 14v+, which indicates the alternator is now charging.

Note 1: The engine typically has to be spinning at 900-1000 rpm to start the alternator charging normally, and the alternator is spinning quite a bit faster than that. With my mains drill, multi-speed Bosch, on the highest setting, two of my alternators need the drill to be at nearly maximum speed before they start, and another one does not start to charge. With that alternator on a car the warning light doesn't go out until the tach shows 1100rpm, so my drill probably isn't quite fast enough.

Note 2: Unless you have 12v connected to the output terminal of the alternator when you spin it, when it does start charging it can generate a high voltage of 30v or more. This is why alternator-equipped cars usually have a label warning that the engine must not be run without a battery connected. I have read of all the (powered) bulbs on a vehicle blowing when the battery became inadvertently disconnected.

  Trickle-charge current:

I've also measured the charging current on what should be fully charged nearly new batteries. This was after a 30 mile run i.e. fully charged, still hot so after connecting the ammeter it restarted pretty-well instantly i.e. took very little out of the batteries. Switched off I removed the alternator plug and used a jumper wire with a female spade connector at one end and a male spade connector at the other to link the brown/yellow from the plug to the alternator. Then I connected the ammeter switched to its 10 amp scale between the brown in the plug and the output spade in the alternator. If you have an alternator with a metal back like my A127 instead of the plastic back like the 16/17/18ACR make absolutely sure the connection at the alternator end can't short to the case, also that the one in the plug can't short to anything, and they are secure i.e. do not fall off as the engine rocks on cranking. Remember that with the engine running if either ammeter wire comes off its connection it is live and can cause damage if it shorts to earth. Also that even when switched off, with one side of the ammeter connected to the alternator plug, the other end of the ammeter is live. On a hot start the alternator output went to 5 amps, then over a period of two or three minutes gradually reduced to 3.8 amps. It may well have gone slightly lower if I had left it longer. With everything else turned off the alternator is powering the ignition system as well as charging the battery, and with points ignition at least both ballasted and unballasted systems will take about 4 amps while the points are closed. However with the engine running they are taking that 4 amps for only about half the time, and zero current for the other half of the time, so the ignition is taking an average of about 2 amps as viewed on an analogue meter. So 3.8 amps minus 2 amps gives us a nominal battery trickle charge current of 1.8 amps. Remember, if performing these tests take great care that neither ammeter connection can short to earth, regardless of whether the engine is running or switched off.

Charging Faults:

It doesn't glow when it should:
Your ignition warning light should always glow when you first turn on the ignition, before you have started the engine. If it doesn't, and at some point in the future your dynamo or alternator stops charging, you are unlikely to know about it until the car conks out and you are stranded. At which point you will probably blame Lucas instead of yourself.

  • Check that with the ignition on (the engine needn't be running) you have 12v on the white at the warning light lamp-holder.
  • Check that this 12v flows through the bulb and appears on the brown/yellow at the lamp-holder.
  • Check that the body of the lamp-holder is not in contact with an earth when it is plugged into the tach.
  • Check that the 12v reaches the control box/alternator on the brown/yellow (wire removed from control-box terminal or harness unplugged from alternator).
  • Ignition warning light doesn't glow, and I can't turn the engine off! November 2013: Ordinarily at this point I'd advise removing the wiring plug from the alternator, connect an earth to the brown/yellow (NOT a brown wire!!), turn the ignition on, and the warning lamp should light. However another scenario has recently presented itself which would make earthing the brown/yellow dangerous. Peter Burgess reported that they had a car in the workshop with a non-working ignition warning light, but also that they couldn't switch off the engine with the ignition key. My first thought for not being able to switch off was a sticking ignition relay, but they were only provided from 1977 and this was a 76. Second thought was that the ignition switch wasn't disconnecting power from the white, and a third possibility was a fault on the ignition ballast bypass circuit, having 12v on it from somewhere when the starter wasn't operating. None of those would have caused the non-functioning ignition warning light, but that could have been from a completely separate fault altogether. Subsequently Peter mentioned the warning light socket showed some heat damage, and when they wiggled the bulb it came on, and after that they could switch off the engine. The light now working could have been due to a loose bulb, but that wouldn't have prevented the engine from being switched off. And it would have to be a significantly higher powered bulb and working some of the time at least to cause heat damage. So I think the likely cause is the wires in the warning light bulb holder shorted together. That would prevent the light from working, and the 12v from the alternator on a running engine would be fed back onto the ignition white in full i.e. without the resistance of the bulb in circuit to limit the current, as if the ignition switch hadn't been turned off. Note this is a different scenario to North American spec cars with an ignition relay i.e. 1977 and later not switching off with the key, where the warning light works normally, which is covered here. Normally the resistance of the bulb is more than enough to reduce the current and voltage on the ignition system to well below what is required to power the ignition. This fault also puts an unrestricted 12v into the alternator via the brown/yellow, so could have damaged that as well. The higher current from the bulb being short-circuited, plus the short probably having some resistance, could well have developed enough heat to damage the holder.

    So only earth the brown/yellow if the car switches off normally. If it doesn't, use another test 2.2w bulb to connect an earth to the brown/yellow. If the warning light is operating correctly both bulbs will glow at half brightness. If neither glow there is an open-circuit (which you should have found in the previous test). If the test bulb glows at full brightness, and the warning light not at all or only very dimly, the warning light wires are shorted together.

    If you have got this far you should have found any faults in the warning light circuit itself. If the warning lamp glows when you earth the brown/yellow but doesn't glow when connected back up to the control box/alternator:

    Dynamo: The warning light terminal is connected to the 'D' terminal on the control box, which has another brown/yellow to the output terminal on the dynamo. With both brown/yellow wires connected back up at the control box, earthing the brown/yellow at the dynamo should light the warning lamp. If this works, but the lamp doesn't light with the wire connected back up to the dynamo, then you have an internal disconnection in the dynamo - follow the circuit through the brushes and the commutator to the body of the dynamo.

    Alternator: There may be a simple internal disconnection. If you know what you are doing it might be worth looking for it and trying to fix it, otherwise replace the alternator. But if you fancy a fiddle, some further diagnostics may help narrow the problem down.

    The ignition warning light is lit when the ignition is switched on but the engine is stopped by 12v coming from the ignition switch, and an earth from the IND terminal on the alternator. This earth comes through the field winding and its slip-rings and brushes, from the field terminal of the voltage regulator. During normal running the voltage regulator varies the resistance of this earth, to vary the current through the field windings, which controls the output voltage of the alternator.

    If there is a problem with the field winding or its slip rings and brushes, or with the voltage regulator itself, then that is almost certainly going to affect the output voltage of the alternator, especially as more electrical loads are switched on. Check the brushes for wear, and try cleaning the slip-rings.

    But if the alternator seems able to maintain the correct system voltage with a range of electrical loads, but the warning light does not glow, then there is probably an internal break in the IND wire where it connects to the field circuit. Note that this fault may need the engine to be revved to 2k or 3k before it starts charging, but after that will charge normally down to about 600rpm. Another symptom of this fault is that with the ignition warning bulb unscrewed or unplugged from its holder at the dashboard, there will be little or no voltage on the IND terminal when there is the normal 14v or so on the brown at the fusebox with the engine running. With a fully working and charging system there should be the same voltage on both sides of the ignition warning light.

    It glows when it shouldn't:
    Typically this is "It glows all the time" or "It glows dimly at night".

    It glows all the time:
    This usually means the dynamo/alternator is not charging, although it could be a fault in the warning light circuit. Check the system voltage with the engine running at a fast idle.

  • If you see at least 14v then the system is charging. Use "It doesn't glow when it should" above to check the warning light circuit, making sure it isn't earthing at any point and is connected to where it should be.
  • If you only see 12v then the system isn't charging:
  •   Dynamo: Test the dynamo by removing both connections and bridging the two spades on the dynamo. Connect a voltmeter between here and earth and start the engine DO NOT REV IT. Slowly increase the engine speed whilst watching the voltmeter. Do not allow the voltmeter to reach 20v, this should happen before the engine reaches 1000 rpm.
  • If the voltmeter reads 0.5v to 1v then the field winding may be faulty.
  • If the voltmeter reads 4 to 5 volts the armature winding may be faulty.
  • If the correct voltage is obtained the control box may be faulty.
  • Control Box: The control box monitors the output voltage from the dynamo and when this has reached 12.7v to 13.3v the cut-out relay operates connecting the dynamo output to the battery. The other two relays are the current regulator to stop an excessive load damaging the dynamo, and a voltage regulator to stop overcharging the battery. Both work by opening a contact when they operate, introducing a series resistance into the field circuit, so reducing the excitation and hence the output current/voltage.


  • Use "It doesn't glow when it should" above to check the warning light circuit, making sure it isn't earthing anywhere, and making sure it does get back to the alternator.
  • Check the voltage on the thick brown(s) at the alternator plug (remove the cap but leave the plug plugged in). If this is also 12v then replace the alternator. If you see higher than this then a break in the thick brown between the alternator and where it picks up the battery cable is indicated. The thick brown connects to the battery cable either at the solenoid, or on the V8 at a copper stud under the toe board on the RHS.
  • It glows dimly at night:
    Usually only relevant to alternators. If the warning light glows dimly at night, and increasingly brightly as the load is increased, then faulty alternator diodes are indicated. Open circuit diodes will cause a reduction in output, either voltage or maximum current, so the battery charging may not be immediately affected. Short-circuit diodes are more serious, usually resulting in a reduced charging voltage, and can cause noticeably increased levels of heat and/or noise in the alternator. It may be possible to replace the diode pack inside the alternator, alternatively replace the alternator.

    For heavens sake don't do what someone said and fit a diode to 'correct' i.e. hide this problem. If you do you may well have stopped the warning light from glowing dimly at night, but you have also stopped it telling you of complete charge failure. If you want to do that you might just as well unscrew and throw away the warning light bulb and save the hassle of fitting the diode!

    However another cause can be bad connections in the white - ignition switch - brown circuit chain which causes a low voltage on the white side of the lamp.

      Low voltage: Update March 2010:
    Mike Polan has reported how low voltage from his alternator was caused by corrosion in the assembly and mounting bolts of the alternator. When charging he discovered that whilst the front of the alternator showed zero volts relative to the engine and body, the rear showed -2v! Cleaning up the assembly and mounting bolts, and the spacer and mounting ears, solved the problem. Incidentally using an ohmmeter with the engine stopped showed no resistances, a reminder that you should only ever use volt-drops in a circuit carrying its design current when looking for bad connections.

    Alternator Brush Replacement March 2013

    I'd noticed Vee's ignition warning light flickering occasionally and wondered whether that was a sign of worn brushes, having done 100k in my ownership and quite possibly over 200k. Rather than open up the alternator to discover that was indeed the case, then have to either put it all back together so I could use the car then take it apart again, or leave it out so the car would be unavailable which I don't like doing, I ordered a new set up front.

    Quite easily done by removing the alternator from the engine and the end cover from the alternator, then the brush carrier from the body of the alt. As you remove this you will hear the brushes ping off the commutator, and wonder how you are going to get them on again. On the V8 AC Delco at least replacement brushes are supplied in a plastic housing, which has a retaining rod holding the brushes inside the housing, pressing back against the springs. You attach the housing to the carrier, and the carrier to the alternator body, and withdraw the rod through a hole in the carrier, and the brushes drop down onto the slip-rings. However. There is no way to grab hold of the end of the rod once the carrier is fitted, and the hole in it is too small for the rod anyway! So you have to replace the rod with a thinner, longer rod or wire at some point, pushing the end through the hole in the carrier, so once the carrier is fitted you can pull the wire out and release the brushes. Refitting old brushes is just the same, depress each brush in turn against its spring while you insert the retaining wire.

    In the event the old brushes were hardly worn, but the slip-rings were a bit manky, so I cleaned them and the brush faces and refitted the old ones. Short-sighted having new ones to hand? Well, if the flickering continued it would indicate some other alternator problem, which might need replacement of the whole thing. In the event cleaning seems to have solved the problem, but it has made me wonder about fitting a voltmeter ...

    Alternator Harness Plug Clip October 2014

    Simon Matthews asked about alternator connections on the MGs mailing list, and I responded. He wrote back directly to thank me and say he had sorted it out, and happened to mention that he had replaced the missing retaining clip as well. I suddenly realised that neither of mine have this clip, nor has a spare I took off a Metro years ago, but I do remember them from earlier cars. Did a bit of digging, and find it is 13H 8163 for the Lucas 16/17/18 ACR on the roadster, but nothing found for the AC Delco on the V8. Nothing shown in any of the usual suppliers, or anything useful Googling that part number, but Googling various descriptions I did find one as part of a repair kit from JCR Supplies. Contacted them and they sell them separately, so ordered two. But looking at my alternators the clips are of two different types. Perhaps not surprising given that one is Lucas and the other AC Delco, but ironically the clips I've ordered look like they will fit the Delco, but not the Lucas! That isn't an ACR but a later 1980s model, and the wire clips for that need the ends pointing outwards, whereas the ACRs and the Delco have them pointing inwards.

    Whilst one of the clips fitted the Delco the plug top was further away than it obviously is in the ACR series, and the clip wouldn't fit over. But it also moved from side to side in the holes, i.e. the clip was wider than a Delco clip, so it was a relatively simple matter to move the bends to make the legs longer and closer together which allows the clip to fit over the plug top.

    But the roadster alt is completely different, the 'feet' point outwards instead of inwards, then the upper part of the plug body is wider than the aperture in the rear case so the legs have to be bent outwards and then upwards again to fit round that, before the top part of the clip can go over the top of the plug. Those extra bends, plus the plug top being further away from the casing like on the Delco, mean the wire isn't long enough to modify. So I sketch the likely shape of the clip, and take a photo of the back of the alt, and send them off the JCR Supplies to see if they can tell me what it is, or ideally have a clip. They get back by return so say the alt is an A127, have located the correct clips and added them to their eBay shop - excellent service! Incidentally web sources indicate the A127 has anything from 35 to 120 amps output, potentially (ho ho) significantly more than the 34 amps of the original 16ACR, although 70 amps and over have an output stud instead of output spades.

    Pulleys August 2019

    The Leyland Parts Catalogue list pulley 12H1278 (in the engine section) for dynamo-equipped MGBs, as does Brown & Gammons, Moss lists 12G2102, and MGOC 12G1178 (not available). Sources indicate the 12G2102 is 3".

    For the alternator the Parts Catalogue lists 12H2516 prior to the 77 model year and 13H9514 for 77 and later. Alternative part numbers for the former are given as AUE1238, BAU1461 and AEU1238 which various sources show as 2.5". No alternatives for the latter number but suppliers like Moss and Brown & Gammons do not differentiate between them. Some sites show 12G1054 at 2.75".

    For the V8 the Parts Catalogue lists 37H8023, no Google hits for that, Brown & Gammons lists BHM7044 but NLA, no size found.

    I didn't know how the above sources measure the size i.e. whether it is the bottom of the groove, the OD, or something in between, but subsequently to originally writing this section spotted in one of my Lucas catalogues a drawing showing it was simply the OD. The roadster has what seems to be a non-standard A127 so the pulley is an unknown quantity, and that measures 2.406" OD and 2.41" on a wrapped belt. Someone else has said theirs (original Lucas as far as they know) measures 2.875" OD. The V8 (original AC/Delco as far as I know) measures 2.96" overall diameter and 2.595" on a wrapped belt so a big difference between measure diameter and running diameter. An old alt from a Metro which I have as a spare for the roadster has a pulley of 2.7" OD and a wrapped belt is the same. OD is not good measure as the flanges can be anywhere from flush with the belt (as on the Metro alt) to several mm above it (as on the V8) and not have any effect on rotational speed.

    The pulley size determines how fast the rotor spins for a given engine speed, and as there is a maximum rotational speed for both dynamos and alternators the pulley size is chosen to suit the maximum engine rpm. I can't see a speed given for the dynamo but the WSM specifies 12500rpm for both the 16AC and the 16ACR and 15,000rpm for the 18ACR. The crank pulley size also has a direct effect on dynamo/alternator rpm at a given engine rpm. An alternator is said to spin safely up to 18,000 rpm and a 2:1 ratio of alternator speed to engine speed is often quoted, which would be safe up to 9000 engine rpm (6250 or 7500 in the case of MGB alts). The maximum for a dynamo is typically 1/2 to 1/3rd of the alternator due to its construction. A dynamo rotor has heavy output windings as well as the commutator carrying output current, with the segments constantly connecting and disconnecting that current. The alternator has a much stronger rotor carrying lighter, lower current field windings, and slip-rings carrying that current as a constant flow. For that reason the pulleys for a given engine will be larger for a dynamo (to give a lower dynamo rpm) than the alternator, i.e. 3" as opposed to 2.5" or 2.75" for an alternator. Dynamo and early alternator engines used the same pulley. The oddity is that the V8, with a slightly lower revving engine, has a larger (as measured) pulley, despite having a significantly greater electrical load from its twin cooling fans and HRW, when 1977 and later 4-cylinder cars that had a single cooling fan got the smaller (higher output) 2.5" pulley in place of the 2.75". But then the V8 has a 6" pulley compared to the 5" of the roadster. I.e. 20% bigger, which would imply a 20% bigger alt pulley for the same maximum alt rpm, which would be 3". 2.96" as measured would be logical for the slightly lower maximum rpm of the engine, but the fly in that ointment is that the V8 belt sits well down in the Vee of the pulley and gives an effective diameter of 2.56", so the V8 alt at maximum revs is spinning some 20% faster than even the smaller 2.5" roadster pulley. The old Metro alternator with its 2.7" OD pulley gives noticeably less output than the 2.44" (measured, 2.2" effective) A127 on the roadster.

    Converting Dynamo to Alternator


    Probably the main reason for converting is to get a higher charging current as the dynamo is limited to 22 amps. Although this is usually adequate for most normal, and particularly 'classic' use, when stuck in traffic with headlights, heater fan etc. on the charge will almost certainly not be adequate which means you will be discharging the battery and on a daily driver this can rapidly reduce the battery to a point where it will no longer start the car. Bad connections will limit current flow, and will contribute to a drop in system and charging voltage, and I think it is this which people are seeing as much as insufficient output from the alternator. Even 'normal' volt-drops up from the solenoid and particularly with a voltmeter on the green circuit can be enough to reduce the indicated voltage below 12.5v, even though the voltage at the solenoid and hence the battery is above this critical point. A good example of a little knowledge being dangerous. Of course if you are going to significantly increase the electrical load then you may well need to consider an even higher rated alternator from another source.

    One common misconception seems to be that fitting a higher rated alternator is automatically going to push more current through the wiring, and people get paranoid about uprating it. The maximum current that will flow depends (to the largest extent) on the electrical load, not the maximum capacity of the alternator fitted. If you only have 40 amps of load then only 40 amps of current will flow, even with a 60 amp, 80 amp or 100 amp alternator fitted. Having said that high-rated alternators like the 80 and 100 amp will be better at maintaining sufficient charge at idle if you have added electrical loads, as well as when running.

    About the first thing to say about the process of converting from dynamo to alternator is that unless it has already been done you almost certainly will have to convert from positive earth to negative earth. Positive earth alternators will probably be very difficult if not impossible to find, a negative earth will be tricky to convert to positive, and the availability of used, rebuilt and new negative earth alternators of various types is almost infinite. Also it might be safer to take things one step at a time and do the polarity conversion first, check everything works OK, and only then do the alternator conversion. Getting the polarity wrong with an alternator connected will probably destroy it, and there is only one very simple step which will be 'wasted'. These notes only cover use of a Lucas alternator, there are too many variations in Bosch and GM Delco alternator connections, although the alternators themselves are quite suitable for use.

    March 2013: As well as a suitable alternator you will also need an extended bracket to connect the rear mounting point of the shorter alternator to the original mounting points on the block. Unless this is done rigidly vibration can fracture the front mounting ear on the water pump, so extending the existing bracket in some way is not a good idea. Fortunately a number of places sell a suitable bracket - 12G1053, such as Leacy, Brown & Gammons, and Sussex Classic Car.

    The usual advice is before starting any of the following work disconnect the battery earth strap first, and only replace it as the final step. However on cars with the remote solenoid it is very easy to remove the brown wire from the spade on the battery cable terminal on the solenoid, and as long as this can't drift back towards its spade while you are doing the work then this is fine.

    Rewritten November 2012 (following the opportunity to work on Chris Mottram's car):

    The dynamo circuit originally had a standard gauge brown/green (field) wire, and a thicker gauge brown/yellow (output) wire. Because of this it's best to use the thicker brown/yellow wire for the output of the alternator, even though this goes against the convention on cars equipped with an alternator from the factory. Therefore the brown/green is used for the INDicator wire, this is method 1.

    There is an alternative method if you are adding a lot of significant loads to the cars electrics, and consequently fitting a high-power alternator, and that is method 2.

    Method 1:

    You can leave the control box in-situ and hence use it's spades for interconnecting most of the wires. But you must cut off the small spade from the brown/green and fit a large spade so it can be moved from the F terminal to the D terminal in place of the thick brown/yellow. The displaced brown/yellow then has to have its large spade cut off and be connected in some way to the brown circuit. The method used with a conversion kit from a well-known supplier is a ScotchLok connector, but these have a habit of working loose over time and are not a good idea on things like the alternator output cable which will carry quite a high current, and the slightest resistance will cause significant heat to develop as well as reduce the output of your shiny new alternator. You could instead use a ring terminal of the appropriate size and add it to the solenoid terminal that carries the battery cable. That would give a good connection, but you MUST disconnect the battery earth cable before working on this terminal. However in both cases you would need to extend the brown/yellow, which has its own problems as to what method to use. I suggest it's better to bite the bullet and remove the control box altogether, make proper soldered joints, and give better access to other components into the bargain.

    Remove all the wires from the control box, and the control box from the panel on the inner wing (easier said than done with those fiddly nuts behind the panel!). The standard gauge brown/yellow from the WL terminal, and the standard gauge brown/green from the F terminal have their spades cut off, a portion of insulation carefully stripped so no strands are cut, twisted together, and soldered. Put a couple of lengths of heat-shrink tubing just over the soldered section, each about 1/4" longer than the soldered joint, heat-shrinking them one at a time. Then fold the 'spare' bit of tubing over, and fit a couple of lengths of larger diameter tubing over that and an inch or so of the insulation on the wires. This circuit supplies the 'priming' voltage to the alternator to start it charging, and indicate charge failure.

    The three original browns from the B terminals on the control box, and the thick brown/yellow that came from the D terminal, are dealt with in much the same way. Cut all the spades off, making the four wires the same length, and strip about half an inch of insulation from each, being careful not to cut through any strands. Check the conductors are clean, using fine emery if tarnished. Twisting four, heavy gauge wires together is not really advisable, so I used the method adopted by the harness manufacture for the sealed, multi-way junctions that exist on some of the MGB harnesses. First I put a short piece of heat-shrink over the insulation of all four wires to keep them together. Then I got a strand of tinned copper wire and wrapped it round the four conductors, for the length of the stripped section, to make a good mechanical joint, only then did I solder the joint. As before a couple of lengths of heat-shrink about a 1/4" longer than the soldered joint are fitted over the soldered part, then the 'spare' bit at the end is folded over, and a couple of pieces of larger diameter heat-shrink fitted over that and an inch or so of insulation, as with the previous two wires.

    Finally that leaves the earth wire from the control-box. It's best not to leave this floating around, but rather than cut the spade off I folded it over and again used heat-shrink to insulate it. That leaves three heat-shrunk spurs sticking out of the main harness at various angles, but a cable-tie can be used to attach those out of the way against the panel the control-box came from.

    Method 2 is very similar but to carry higher currents, if you are adding significantly to the cars electrical load and hence using a high-output alternator, it would be best to use a new heavy gauge (of the appropriate size for your loads) brown wire from the output of the alternator to the battery cable stud on the solenoid at least. You may also need to use similar gauge wire from there to your non-standard, high power loads, the brown from the spade on the solenoid going to the control-box should be enough for all the factory loads. Again, you MUST remove the battery earth connection before starting work on the battery cable stud on the solenoid. After that you can leave the browns on the control-box B terminals, and the brown/yellows on the WL and D terminals, using the heavy gauge brown/yellow at the alternator for its INDicator terminal. The standard gauge brown/green should be removed from the control-box F terminal and its spades taped back and insulated at both ends to prevent them coming into contact with anything else. Alternatively you can discard the control box and join the three browns from the B terminals, and join the two brown/yellows, and insulate the earth wire, using the techniques described above.

    If converting the polarity at the same time leave the alternator unplugged when connecting the batteries the new way round for the first time, if you get it wrong and the alternator is connected you will blow its diodes and burn wiring. Confirm the polarity is correct before continuing by connecting a voltmeter between a brown in the alternator plug (meter +ve) and an engine earth (meter -ve).

    After confirming that the polarity is correct connect an analogue voltmeter (digital meters may give unpredictable results) on its 12v scale in place of the battery earth strap. There should no voltage registered. If there is, it will probably be a full 12v, and means some circuit on the car is switched on (courtesy lights? Boot light?) which should be found and switched off before proceeding. When no voltage is shown plug in the alternator. You may now see a few volts registered, which will be the normal microscopic leakage current of the diodes and can be ignored. If a full 12v is shown the alternator diodes are faulty. If the reading is correct, replace the battery earth strap. If you don't have an analogue meter use a low-wattage 12v bulb instead, such as one from one of the gauges. If there is any glow at all from the bulb, current is flowing, proceed as above. Only a significant current flow will cause the bulb to glow, connecting the alternator should not be enough.

    You now have the alternative of going for broke and reconnecting the battery earth strap, or taking a smaller step. For this connect a high wattage bulb e.g. a headlamp bulb (e.g. an old one with one filament gone - "If you haven't found a use for something yet ...") in place of the earth strap. This will allow a safe amount of current to flow while you turn each thing on in turn. If any circuit is faulty and is full short the bulb will limit the current and prevent damage to wiring and components. Some things (low current items) will work almost normally, higher current items probably not. Low current items might cause a dim glow from the bulb, higher current items a brighter glow. Only turning the key to crank (nothing will happen) should cause the bulb to glow at full brightness, nothing else. When you are happy, reconnect the battery earth strap.

    With the ignition off there should be no glow from the ignition warning light. A glow now indicates faulty alternator diodes or voltage regulator, or incorrect connections to it.

    With the ignition on the warning light should glow. If no glow remove the plug from the alternator and connect an earth to the brown/yellow terminal (NOT the brown!). If the warning light glows now the alternator is faulty if not then the circuit is broken back towards the warning light, possibly where the brown/yellows are joined where the control box was, or a blown bulb. There should be 12v on the white at the bulb holder and an earth on the brown/yellow to light the bulb.

    With the warning light glowing start the car, and with the engine revved above 1000 rpm the light should go out. If the light remains on the alternator is faulty. Only if the revs drop below about 600rpm should the light come back on, stay off till about 1000 rpm, then go out again as before. Early cars had an idle speed of 500 rpm and if the light comes on at idle, particularly with lots of load switched on, then you would be advised to increase the idle speed to, say, 700 rpm to keep the light out at all times. While the light is on the alternator isn't charging and the battery is discharging, which largely negates the effort of converting!

    With the engine at about 1000 rpm, and all loads switched off (and the warning light off), measure the voltage between the brown at the fusebox and earth. You should see about 14.5v, much less or more than this indicates a faulty alternator. Now turn on headlights, brake lights, heater fan etc. The voltage will probably drop, possibly to less than 13v with one of the smaller Lucas alternators. Increase the revs to about 3500 and the voltage should rise above 13v again, indicating the battery is still being charged even with everything switched on. If the voltage doesn't rise above 12.5v check the voltage at the alternator output terminal(s), and if similarly low here it indicates the alternator has a low output current fault, however note that the smaller Lucas alternators will probably not be able to supply anything above the standard factory loads at best. If the voltage is closer to 14v at the alternator then there is a bad connection somewhere between the alternator and the brown at the fusebox, check the voltage on each brown wire and the battery cable at the solenoid.

    Converting 16AC Alternator with Separate Regulator to Later Alternator with Integral Regulator Added October 2007


    First remove the battery earth strap, and don't replace it until you have made all the wiring changes.

    The alternator should have the following wires:

    Heavy gauge brown (output) going to the solenoid
    Black (earth/ground)
    Brown/yellow (Indicator) coming from the voltage regulator
    Brown/green (field) coming from the voltage regulator

    The voltage regulator should have the following wires:

    Black (earth/ground)
    Brown (12v supply to the voltage regulator) coming from the solenoid
    Brown/green (field) going to the alternator
    Brown/yellow (Indicator) going the alternator
    Brown/black (Indicator) coming from the ignition warning light

    These last two (brown/yellow and brown/black) are probably the most important. They are electrically connected together at the voltage regulator, and they must remain connected together and isolated from everything else after the conversion, so that effectively the Ind terminal on the new alternator is connected to the warning light. As well as lighting the warning light, the current flowing through the warning light to the alternator acts as a 'pump primer' and is needed to get the alternator to start charging. If both wires are in the same spade connector then all you have to do is securely insulate that connector so it cannot come into contact with anything else. If they have two separate spade connectors then these should be cut off, the wires twisted and soldered together, and the joint insulated with at least two layers of heat-shrink tubing (slip these on before twisting and soldering!).

    Of the other wires the heavy gauge brown goes to the output terminal of the new alternator (See here for Lucas 16/17/18ACR terminals) and remains on the solenoid.

    The two black wires and two brown/green wires at the alternator and old voltage regulator are no longer required and should be taped back out of the way of anything else.

    Note that the earthing point to the body for the black wires from the regulator and alternator is also the earthing point for four other wires (heater fan, instruments, wipers and headlights), even if the regulator and alternator earth wires are removed the other four wires must still be earthed, you will get some very strange results without it.

    That should leave the brown going from the voltage regulator to the solenoid, and this is also no longer required. If the two standard gauge wires at the solenoid (there should only be two apart from the large battery cable and the heavy gauge brown from the alternator) have separate spade connectors it should be easy to determine with an ohmmeter which goes to the voltage regulator and which to the fusebox. The one going to the fusebox must remain connected, but the one going to the old voltage regulator can be taped back both ends. But if these two brown wires terminate in the same spade connector then you have a 50/50 chance of cutting the right wire off. You could cut one wire off and then test, and if you have cut the wrong wire reterminate it on a new spade connector and tape the other one back, or cut both off and test and then reterminate the wire going to the fusebox, taping the other one back. But the best thing to do would be to cut both wires from the existing spade connector, identify the one going to the old voltage regulator and tape that back both ends, then for the other wire that goes to the fusebox reterminate that on a ring connector that will go on the stud with the battery cable. This makes a more secure connection than the spades, as all the current for the cars electrics goes through it. While you are doing that you can do the same with the output cable from the alternator, for the same reason. Later starters did have all the browns terminated with ring connectors on the battery cable stud as standard.

    With all the wiring changes done, the new alternator mounted but the brown and brown/yellow not yet connected (make sure they can't short out on anything), and everything in the car switched off including doors etc. closed so the courtesy lights aren't on, check to make sure you haven't shorted any of the brown wires to earth/ground. The safest way to do this is to connect a voltmeter on its 12v scale in place of the battery earth strap. If the voltmeter shows any reading at all, there is something drawing current. Anything less that 12v shown is a tiny current, could be a clock or the 'keep alive' circuit of a radio. If it shows a full 12v it could be a small current e.g. something simple like a courtesy light left on, or it could be a full short. Connect a test-lamp or other 12v bulb (an old headlamp bulb is best) in place of the earth strap, and if it glows brightly it is a full short which must be investigated and fixed before you proceed.

    A cruder check is to tap the battery earth strap very briefly on the -ve post of the battery. You should not get any kind of a spark. If you get a small spark maybe one of the courtesy lights or similar is still on. If you get a big flash then it looks like one of the browns is shorting to earth somewhere, which again must be investigated and fixed before you proceed.

    With the brown at the alternator still not connected and protected from shorting to anything, and with the battery earth strap reconnected, connect the brown/yellow to one of the standard sized spades and turn on the ignition. If the warning light glows you can proceed. If it doesn't then turn off the ignition, move the brown/yellow to the other standard sized spade and try again. If the warning light glows now again you can proceed. If not, disconnect the brown/yellow from the alternator and connect an earth to instead end and try again. If the light glows now then possibly the alternator is faulty, or possibly the wire should go to yet another spade if you have non-Lucas alternator. If the light doesn't glow with the earth connected however, then either there is a problem where the brown/yellow joins the brown/black, or the bulb has failed, or there is some other open-circuit between the end of the wire at the alternator and where the white from the bulb joins the others at the ignition switch. This must be found and fixed before you proceed, or the new alternator probably won't charge.

    With the bulb glowing with the ignition on, carefully connect the heavy gauge brown to the output spade, remembering it is live and unfused. You may prefer to disconnect the battery earth strap again while you attach the brown output wire, then go through the same tests for a short as before. With other types of alternator there can be different connection arrangements, some have an output stud as well as an output spade, use the stud as it will have a better current carrying capacity.

    With both output and indicator wires connected to the alternator, start the engine revving it as little as possible, and watch the warning light. The warning light may still be glowing, so slowly raise the revs, and at about 900 rpm the light should go out. Now use a volt-meter on the brown at the fusebox and you should see around 14v. If so the new alternator is charging. With the engine idling turn on the lights, press the brake pedal, switch on any other electrical loads you can, and the voltage will drop to some extent, possibly towards 12v. Rev the engine to about 3k and the voltage should rise again above 12.5v. With everything turned off, and a fully charged battery, and the engine revved to about 3k, you should see a maximum of about 14.5v. With the engine idling again select 4th gear, handbrake and footbrake on, and slowly lift the clutch pedal up so the revs start to drop. The warning light should come on again at about 600 rpm. Dip the clutch again and take it out of gear, slowly raise the revs again and the light should go out again at about 900 rpm. If your normal idle causes the warning light to come on again anyway, it might be an idea to raise the revs a bit so it stays out, that way the alternator will still be charging at idle, rather than the electrical loads of the car draining the battery.

    If the light doesn't go out when revved, and you have two standard sized spades on the alternator, switch off, and move the brown/yellow to the other standard spade. Turn on the ignition and if the light glows start up and try the tests above again. If the warning light doesn't go out when the engine is revved, with the brown/yellow on any of the standard sized spades that it glows on with the ignition on, or the voltages don't show as above, then possibly the alternator is faulty.

    'One-wire' Alternators Added January 2010

    Some confusion over these. Some people use this term to describe 'one output wire' alternators i.e. where there is also an excitation wire, as compared to the 3, 4 and 5 wire alternators used on MGBs at various times. Others think that any alternator with internal voltage regulation is a 1-wire alternator - very few are. On the other hand there really are one-wire alternators that do not have an excitation or ignition wire, just a single heavy gauge wire from the output terminal to the battery or starter solenoid. These still need excitation, and it is achieved by having some device that either senses rotation, or senses a drop in battery voltage i.e. cranking. In both cases it then internally connects the output wire (which has 12v from the battery) to the field circuit to commence charging. In the former case it can need revving up to 1200rpm before charging commences, and in the latter it triggers before the engine is turning fast enough to charge, or even when simply turning a light on. In this latter case it will be discharging the battery until it senses that the alternator isn't going to charge any time soon so disconnects again. Not only do these alternators cost more, they also have extra things to go wrong. Hot-rodders with an engine bay stripped of every possible thing like them, but for the rest of us they aren't really relevant, and if you don't mind revving your engine soon after starting you can actually use a conventional alt without the warning light wire connected, previously used examples of which will start charging when revved to 2k or so. If you really are going to use a one-wire, i.e. without a warning light, then you are going to need a voltmeter ... which rather negates the loss of a couple of inches of visible wire from the harness wrapping to the plug in the engine bay!

    Batteries and Chargers

    Safety First!
    Securing the batteries
    What Polarity is my Car!?
    Which Terminal is Which?
    Twin-6v Link Cable
    Cable Size and Current-carrying Capacity
    Battery Cover
    Replacement Batteries
    Battery Drain January 2014
    Cut-off Switch
    Battery Types - Lead Acid, Gel, Advanced Glass Mat etc.
    Do I need plastic battery bins?
    Fused Battery Terminal January 2014
    Battery Chargers
    Lucas Battery Catalogue

      Safety First!
    It is absolutely vital that batteries are securely clamped into their cradles. Many who switch to a single 12v, or fit battery boxes, seem to leave the batteries unsecured. Although it is an MOT requirement to have a secured battery the tester is not allowed to 'dismantle' any part of the car so unless he looks up into the cradle and prods the battery to see if it moves they are unlikely to spot an unsecured one. Roger Parker formerly of the West Midland Police motorway patrol unit has seen the effects of an unsecured battery and recounts the following:

    The (car) was totally destroyed following an accident on my section of the motorway. Unfortunately the driver died in horrifying circumstances which I believe to have been avoidable. I can now relate the circumstances as the inquest has recently been closed as I feel there are important lessons to be noted.

    What happened was the car was travelling along the M6 at about 4am when, for reasons unknown, the vehicle left the road on the nearside and took out a traffic sign. The impact caused very severe damage to the underside of the car as the concrete base to the sign was at cross-member height. This impact also took out the fuel pipes. Now as we all know the electric fuel pump keeps on pumping until the electricity supply is cut. With the electrical circuits still open after the crash this is exactly what happened consequently soaking the underside of the car, which after the crash had come to rest on its offside. The driver had suffered serious but not fatal injuries - MGBs are strong cars - but however he was trapped by one leg.

    When a passing motorist stopped shortly after the accident, he saw the driver was trapped and able to talk to him as he was conscious. At this time poor maintenance in the battery compartment then contributed to subsequent events. The battery was not of the correct size and was only resting on the battery tray - it was not secured. In the extreme circumstances of the heavy impact, the battery was able to move and short out on the metal body of the car because of the lack of secure fixings. Now remember the petrol pump was still running and pumping fuel out of the fractured fuel lines and tragically the arcing between the battery and metal body ignited the petrol vapour.

    Now I do not have to go into the details but suffice to say the death of a conscious person by burning is one of the worst fates you could imagine and I have had the unfortunate experience of witnessing three such deaths in my service.

    The moral is clear - secure your batteries properly (gravity is not enough!) and if possible fit an inertia switch as found on the current MG Efi models which would cut the power off from the fuel pump in events of a violent nature. Together these precautions would have prevented the death of this driver. Enough said. Incidentally the fire was so intense that most metal items in the area of the seat of the fire actually melted - including a whole spare wheel.

    Clamping arrangements:

    • MGB batteries were always held down with a strip of angled metal as a retaining bar along one edge of the battery (plastic packing strip to clamp deleted in December 1977), and two hooked rods down from that to holes in the cradle, secured with double-nuts (GHF200) above the retaining bar. The batteries sit on rubber buffer strips (AHH6351) along the front and rear angles of the cradle base.
    • CB cars have the retaining bar (AHH6353), with a strip of plastic packing (AHH5169) underneath, along the rear edges of the batteries. The rods (AHH6750) run down and forwards with hooks going through holes in the sides of the cradle. Care needs to be taken with the positioning of the retaining bars and battery connectors as with posts in diagonally opposite corners one of them on each battery will always be very close to the retaining bar. No problem with the earth terminal, but the other three are all 'live'. Originally there was an option of a larger battery that required a rectangular frame (AHH7109 - NLA) to sit on top of the battery and be clamped down by the rods.
    • RB cars have the retaining bar (34G2065) and packing 37H3743 running fore and aft along one long side of the battery. 12v batteries have the terminals along one long side, and are normally positioned to put the -ve terminal towards the rear of the car. This means the terminals can be nearer the tunnel (Type 072) or nearer the off-side of the car (Type 075) depending on which type of battery you get, and the norm is to have the posts nearer the tunnel. The retaining bar goes on the opposite long side of the battery to the posts, i.e. normally towards the off-side of the car. The rods (BHH1586) run down almost vertically to one pair of two sets of holes in the angle pieces that form the base of the cradle. In theory the rods could be angled across the short sides of the battery to the other pair of holes, but they would need to be longer, and the ends of the retaining bar would also be at an angle to the rest of it, as they are on CB retainers, but RB retainers are the same shape all along. Physical battery size can impact on the security of the retainer.
    • The physical size of both 12v batteries and 6v batteries can vary, and with the smaller ones you should make sure that the base is fully supported by the front and rear angles of the cradle base. My latest set of 6v are slightly smaller than those I've had before and I've had to add a strip of metal to the rear angle of the base to support the battery properly. With 12v batteries and the rods going straight down there is the additional problem that on a smaller battery they will be angled away from the battery when in the holes, which will pull the retaining bar off the battery. I've had to add some packing between the other side of the battery and the side of the cradle to move the battery further across. Yet another problem is where the battery is shorter top to bottom than the rods can accommodate, and you run out of threads. Many years ago I had to bend new 6v rods into an S-shape to take up excess length, and put thicker packing on top of the 12v battery. The latest 6v batteries are even shorter but I have dies now so have been able to cut more thread.
    • Fitting the retainers can be a fiddle. I put the rods in the retaining bars with one nut started, then fit the assembly over the battery. By twiddling the upper end of the rods you can turn the hooks towards the holes and hook them through. The trick is to have the nuts on far enough so that the hook only just goes through, then tighten that a few more turns so it can't come out, while you fit the other rod in the same way. Slide the plastic packing under at that point, before tightening the first nuts and adding the second nuts, tightening the two against each either to lock them without applying too much pressure to the case. The irony is that this process is much easier with smaller batteries than larger as you can get your fingers down the sides more easily!

    When doing any work involving any battery terminal, or the battery cable at the starter solenoid, or any 'always live' wiring such as brown wires, always remove the earth/ground cable from its battery post first and replace it last. This is regardless of whether the car is negative earth/ground or positive earth/ground, twin 6v batteries or single 12v. Many sources of automotive information say to always remove the negative connection first before the positive, but they are only thinking of 'modern' cars, not classics. I repeat, always remove the earth connection first and replace it last, regardless of polarity, and that applies to all cars i.e. moderns as well as classics. The reason for this is that if your spanner should happen to touch the body whilst it is also touching the earth/ground post of the battery nothing will happen. Once the earth/ground connection is removed it is now safe to undo the 12v (aka 'hot' or 'live') connection, because if your spanner should happen to touch the body while it is on the 12v post still nothing will happen because the earth/ground connection has already been removed. If you remove the 12v connection first and the spanner touches the body whilst doing so, you will generate a large spark which can ignite any battery gases that may be present, or maybe even cause the battery to explode in your face! So it's always earth cable off first, and back on last, regardless of polarity.

    Car batteries contain a large amount of energy and can discharge it very rapidly under the wrong conditions, generating large sparks, toxic fumes, even exploding and showering corrosive liquid around. Not all batteries have polarity markings, on classic cars battery cables are not usually colour-coded for polarity, and the battery terminals and connectors do not usually have insulating covers. Great care must be taken to ensure the 'live' or 12 volt terminal does not accidentally contact the car body or a large spark can be generated which can ignite battery gases, and tools or other metal parts can become welded and glow red-hot.

    Many MGBs and other classics of the era have two 6 volt batteries instead of a single 12 volt battery, and the two 6 volt batteries are connected together with a link cable. Both ends of this link cable must be considered as being 'live', as well as the 12 volt terminal, and need the same care to prevent accidental contact with the car body.

    Another warning is never to run the engine without a battery connected, especially on alternator equipped cars. Although the alternator has a voltage regulator it actually generates pulses of AC which are then rectified to DC before being regulated, and will output pulses of high voltage if a battery is not connected. The battery acts as a very powerful smoothing device which prevent the high-voltage pulses reaching other circuits on the car. Without a battery the high-voltage pulses can blow bulbs and damage electronic circuits.

    Twin 6v Link Cable

    Chrome bumper MGBs (and others of the era) originally had two 6 volt batteries connected in series with a link cable so as to deliver 12 volts. Here to indicate polarity to others I've wrapped a strip of insulation - a spiral cut off a red battery cable - around the main cable that feeds power to all the circuits on the car, as well as round the positive end of the link cable. Also shown is an armoured link cable available from various sources such as Leacy Classics. This cable goes over the prop-shaft and it is essential that it is clipped to the back of the heel-board as high as possible. The armouring prevents the cable chafing in the clip, if this cable shorts out even though only one battery will be affected fire can still destroy the car.

    My roadster came with a single 12v which needed replacing in 1994, but it was too big to lift out through the hole in the shelf. Looking underneath I noticed that the carrier had been modified to take the bigger battery, and I wondered if they had welded it up after getting it in from underneath! But then I had a brain-wave and found that if I turned it over to lie on its end (it was sealed) I could just get it out from the top. It had just been loose in the cradle, so along with the new 6v batteries and interconnecting cable I got two clamp kits. It was apparent that originally the interconnecting cable went through some flexible armoured tubing, but with the clamps already crimped onto the end of my new interconnecting cable there was no way I was going to reuse that. It was rotten anyway so I pulled it out and just put the cable through on its own, installed the earth clamp in the other box, installed the batteries and clamps, checked the volt-drops, and away we went. Not too long afterwards I had occasion to remove the interconnecting cable, can't remember why, and saw with horror that it had been hanging down and rubbing on the propshaft! It had marked the insulation but fortunately not rubbed through. So I installed my own tube to support it up out of the way.

    Added April 2009

    When I fitted the battery cut-off switch to Bee last year I found a mass of corrosion around one of the terminals on the right-hand battery, where the link cable attached, and the fluid level in the cell closest to it was well down. I cleaned it all off (it made a terrible mess of the drive and is only now starting to fade 12 months on) and found it had eaten away quite a bit of the clamp, so bought a new one, this time of the correct armoured type (BHA4348 currently at Moss Europe and Sussex Classic Car). These are supported by a clip at the top of the heel-board directly under the battery shelf between the two batteries. I found this clip is held by a bolt that goes into a welded nut in a box-section I didn't even know was there. Quite a bit of surface rust up there, and restricted space between the battery boxes and above the prop-shaft, but the bolt came out very easily. Almost certainly the first time it had done so since being fitted at Abingdon, so I treated it with due respect! The new cable went in quite easily with the clip and bolt, but when I went to attach it to the battery posts I found it was shorter than the old one, and would only just reach its post by lying tightly across the other clamp - not a good idea. I tried turning the battery round 180 degrees but then the main battery cable was nowhere near its post. Turned round just 90 degrees both clamps fitted, but one of them was very close to the clamping strip, which in any case was hard up against the cover over the refilling ports, again unsatisfactory. Fortunately because I had fitted the cut-off switch in the main battery cable I was able to remove the short length between the switch and the 12v post, remove the lug and connector, and transfer them over to a suitably longer length cut from the old link cable which would reach its post with the battery rotated 180 degrees.

    I've seen a couple of comments from people who have flattened the battery, then charged it up in reverse, which seems a really iffy process to me, if not downright dangerous if someone else should go by any + and - markings for reconnection, boosting or even charging. Also some sources stating that +ve and -ve plates are made of slightly different materials which aid battery performance, which would work against you if the polarity is reversed.

    Battery Cover December 2014

    These have a seal around the opening, although it's probably more to do with preventing the lid from rattling than acting as a seal against fumes etc. When I restored Bee 25 years ago either this wasn't available, or I didn't bother getting it, I just used some white foam draught proofing, although of course it never looked right. But when buying several items from Leacy for attention over the winter I decided to add that to the list. I was surprised to see it was a sponge rope seal, I had expected a flat self-adhesive, but looking closely at the picture in Clausager on page 98 you can see it is a rope seal. I was planning to fit the seal to the cover so it isn't damaged by replacing batteries etc., but because of the shapes of the two surfaces this seal would have to be fitted to the shelf - incidentally only on twin-6v cars going by Clausager photos. This would need very careful application of adhesive so as not to squidge out from under the seal and look messy, so in fact I opted to use black flat self-adhesive draught excluder that is 1.5mm thick by 10mm wide that I had previously bought to pad out the bubble-seal on some UPVC garden doors, on the cover. Although 10mm wide it went round the corners quite well, the trick is to press down a very short length of a few mm each time.

    Replacement Batteries

    It is vitally important from a safety point of view to securely clamp the batteries in the cradles.

    Twin 6v in CB
    12v in RB
    12v in CB?

    From 1st July 2018 it has become illegal to supply sulphuric acid to members of the public without the appropriate EPP licence. So whereas mail-order supplied a dry-charged battery and a set of acid packs, in future and depending on the retailer, either only the dry-charged battery will be supplied and you will have to find a supplier of acid locally and get them filled there, or collect wet-filled batteries over the counter. July 2019: At least, that's what advice at the time seemed to say, but wet-filled batteries seem to be available for delivery by courier from some sources now (see below) including the MGOC.

    June 2019: Another sudden failure, and it's a case of "déjà-vu all over again"!

    October 2014: Sudden failure of one of Bee's batteries and subsequent replacement of both.

    Twin 6 volts in chrome bumper cars, single 12v in rubber bumper. Some years ago two 6v were only slightly more than a 12v for a rubber bumper, but now a single 6v is nearly as expensive as a 12v. Nevertheless I intend to stay with 6v. It is frequently said that modern batteries benefit from more recent technical advances but why shouldn't that also apply to 6v as well as 12v? And at least one company offers 59Ah, 73Ah and 88Ah versions in the same package size. Certainly mine last well enough averaging 10 years for each set, and that with very little use for several months over winter with no recharging. The single 12v in the V8 doesn't last any longer much less in fact since I stopped the daily drive to work (after the 2nd battery failed in as little as 18 months I fitted a battery cut-off switch and it has been fine ever since). If you do opt to replace the twin 6v with a single 12v then you have the potential to move the fuel pump into a far more accessible position as Peter Mayo did.

    The Workshop Manual quotes the original battery capacity as 51 Ampere-hours at the 10-hour rate or 58 Ampere-hours at the 20-hour rate. These days automotive batteries are also often quoted in 'Cold Cranking Amps' (CCA equivalent to SAE) or 'Cranking Amps' (CA) as starting an alternator equipped car is its main use and not a continuous discharge over a period (unless you regularly park with lights on). Can also be SAE, IEC, EN and DIN and this chart converts between them. However for a dynamo equipped car Ampere-hours is more of an issue as at idle or low revs, especially with lights, wipers etc. on, the dynamo won't be charging and you will be discharging the battery, theoretically an issue if you drive in heavy stop-start traffic. CCA represents cranking at 0 degrees F/-18C, for warmer climates CA is more applicable as it represents cranking at 32F/0C. Divide CCA by 0.8 to get CA. One source (unverified) quotes that 6v batteries for the MGB should be 66Ah and 360 CCA. If you regularly start the engine in temps below freezing you may need 360CCA. If normally started above freezing you only need 360CA i.e. a 288CCA (or the next one up) battery. There is little benefit going for a battery with a higher CCA or CA than this unless you have a high-compression (i.e. higher than even the factory high-compression) engine, although the higher the capacity the longer (in years) they are likely be usable. Incidentally this also indicates that modern 6v batteries have benefited from modern technology increasing performance in the same package size, and not as some aver. For complete originality I think the original 'tar top' batteries with exposed links can still be obtained, although two sets I have had have cracked round the posts and suddenly failed some time later. They are also available in a more modern construction with internal links and a single fill cap. Exhaust heat has been mentioned as a factor against staying with twin 6v, and as both my sudden failures of tar-top have been on that side maybe that is indeed a factor.

    6v batteries for chrome bumper: July 2019:
    Several places are doing 77 and 80Ah 600CCA i.e. significantly more powerful than the originals at about 65, some including delivery, delivered filled, with variously 1, 2 and 3 year guarantees.

    MGOC have D004W 80Ah 600CCA at 65, concealed links, single fill channel, wet, 2yr guarantee, delivery 15 for a pair; and D003 65Ah at 90 (!) plus P&P, supplied dry so you have to source your own acid at additional cost.

    Brown & Gammons have GBY3031DZ at 99, concealed links, dry, acid GBY3031ACID listed at 32 but shown as 'Request stock alert' on one page and 'collection only' on another. GBY3031WZ at 90, wet, exposed links, collection only and GBY3031HD 'heavy duty' at 90, concealed links, apparently wet as collection only.

    Leacy show two types: GBY3031D dry, no acid at 92 and GBY421D dry with acid pack also 92 but showing out of stock, and the acid packs are also showing as out of stock, so perhaps no longer available anyway. They are showing GBY3031W at 83 wet collection only but out of stock, and GBY421W at 86 wet, 'original style' (exposed links?) and apparently available for delivery. No capacities listed for any.

    Moss Europe list a 57Ah at 92 and a 63Ah 'heavy duty' at 102, both dry but no acid listed. Wet versions of both (although the cheaper one is shown as 56Ah) at the same prices for collection only.

    Alpha Batteries are showing a pair of 421 77Ah at 135 including delivery, wet, concealed links and single-fill, 2 year warranty.

    This eBay seller has Exide 421 80Ah 600CCA at 68 including delivery, concealed links and single-fill, but no indication of wet or dry.

    Halfords don't list a battery for a chrome bumper MGB.

    421 seems to be the designation used by a lot of suppliers, quite a few possible suppliers in this Google search.

    12v batteries for rubber bumper: December 2021
    Note that the original batteries have the terminals on one of the long sides, not diagonally as on the 6v, so terminal layout is important. As originally installed the terminals were closest to the tunnel, with the positive post closer to the front of the car. The generic UK (at least) designation for this battery is 072. 075 would appear to fit but have the terminals the other way round i.e. positive towards the rear of the car when they are closest to the tunnel. If the wrong type of battery is used the original way round, or the right type but the wrong way round, you will destroy the alternator and damage wiring.

    Leacy are showing GBY072W as wet for collection only, 86 inc VAT, no spec.

    MGOC are showing wet for shop collection only at 100.

    Moss Europe show a dry at 122, nothing about acid, on back-order, or wet 122 collection only, 450CCA 60Ah. They also have a heavy duty 550CCA 70AH at 106 (cheaper!) which is also wet for collection only.

    Halfords list HB072 with a 3 year guarantee at 99, 68Ah, 550CCA, sized at 261mm long by 175mm wide by 220mm high, and Yuasa HSB072 Silver with a 5 year guarantee at 129, 75Ah, 640CCA, sized at 269mm long by 174mm wide by 223mm high (this will be a struggle to fit).


    18th December 2021: Vee's battery has needed the lithium jump pack a couple of times while not being used as much in the poor weather. Cranked measuring the voltage on the battery terminals and inititially it was 9v (ideally 10v), faded quite rapidly with more cranking checking the voltage on the clamps and the body earth, dropping to 6v with the meter back on the terminals. So time to replace, but fitted in 2004 so not a bad life at 17+ years! Halfords still showing 99 for their HB072, Euro Car Parts showing Lion 031 600CCA 70Ah 266mm long 222mm high 172mm wide 3 year guarantee at 74 which is a 45% Christmas bonus off the normal price of 135. Note Euro almost always have some kind of discount but it varies over time, for example a couple of weeks later it was down to 40%.

    Listed for the RB I noticed it was classed as '031' and not '072' and it's 5mm longer than the Halfords 072. This battery table lists all the UK battery types and 031 and 072 are extracted below:

    Battery CodeAmp - HourVolt (V)CCALength (MM)Width (MM)Height (MM)Weight (KG)Terminal

    However that shows 031 as 7mm shorter than 072 and 20mm higher, and neither the Halfords nor the Lion meet those specs exactly except for the voltage and the Lion Ah! But given the listed size of the Halfords HCB072 a couple of years ago (but no longer listed, 269 x 174 x 225mm) which is even bigger it shouldn't be a problem.

      But it's a whopper compared to the one that was in before which was 230mm long, this is 255mm long so I wouldn't like to have to do the Yuasa HSB072! It only just fitted through the hole, with the cables pushed down out of the way and teased up afterwards, and I had to move the clamp struts from the middle holes to the end holes. That has put the front strut under the lip of the shelf and it was a fiddle fitting the nuts, and the +ve clamp is partially covered as well as limiting how it can be orientated to tighten the nut (clamps are tapered to match the posts so should not be turned over even if they are the moulded-on type without screws). But I get it all in and do voltage tests that gives 12.8v open-circuit, and without charging since receipt cranking a cold V8 (with the fuel pump and ignition disabled) gives 11.6v on the battery posts and 10.6v between the solenoid and engine earth which is very good, and it's really whizzing round. Fuel pump and ignition connected it starts almost instantly, but then it had been sucking some fuel in during the battery tests even though the choke was fully home. This is sealed but the old battery has cell caps and there is more than 2" of headroom above the seperators, so a 'small' battery in a biggish case, but still rated at 590CCA so basically the same (originally) as this 600CCA

    Due to the cramped installation the 12v cable is resting against the clamp plate which means it could eventually short out. Black +ve cable from the factory, but as I have a cut-off switch on the heelboard I'm thinking of getting a red cable from there to the battery, longer to give more options for routing the cable, and a different moulded clamp that can be fitted either way up. I can't find any ready-mades like that, but Halfords have an adapter (HEF366) so their cable with eyelets both ends can be used. However their cables are 9" which isn't really as long as I need to make it worthwhile or 18" which is too long! Whether it came with the V8, the roadster or somewhere else I can't remember but I have an old red rubber boot to go over the cup-type connectors which I slid onto the 12v cable years ago to indicate to anyone else that it was +ve, and have slid that back down the cable a little to act as another thick layer of insulation. I was also a little concerned about the clamp plate sliding off the edge of the battery as the struts are going straight down, they aren't long enough to go diagonally, so have wedged a couple of bits of wood between the clamp plate and the side of the cradle to prevent that happening. All in all, as it's in and working well I leave it as it is. A side-benefit is that whereas the near-side HID wouldn't strike unless the engine was running and the alternator charging it does so with just a couple of flashes now.

    But knowing what I know now I wouldn't go for either an 031 or an 072 again. Looking at that battery table and whittling it down to the same terminal type that are smaller than those but with a high CCA I get the following possibilities :

    Battery CodeAmp - HourVolt (V)CCALength (MM)Width (MM)Height (MM)Weight (KG)Terminal

    The 075T is 590CCA which is actually higher than both the 031 and the 072, albeit at 60Ah, but unless you leave it with parking lights on that's not going to be an issue. Yuasa YBX5075 is an 075T at 640CCA which is even better, 4 or 5 year guarantee, several sources at around 74 so the same as mine but easier to fit. Ditto various 075s a bit cheaper but around 540CCA.However this type have the terminals the other way round and you would have to extend the +ve cable (significant job without a cut-off switch) and maybe the earth (easier) or position the battery with the posts away from the tunnel instead of beside it. On my second replacement under guarantee all those years ago - under protest saying "Don't come here again" I noticed immediately that he had given me one of these (I suspect deliberately so I would burn out my alternator ...), so I did have to install it the other way round. But that meant the clamp plate had to be nearest the tunnel, pushing the battery towards the outside of the car. There is a large gap that side so the battery was tilting, and I had to put a block of wood in there to keep it square in the cradle. As I've chosen to do that anyway with the 'correct' battery that's neither here nor there and fitting an 075/075T would have been much easier! Next time ...

    12v for a chrome bumper? October 2020:
    MGOC are showing a conversion kit including battery and clamps but WITHOUT acid for 120. 50Ah so quite a bit less capacity than the original 6v or RB 12v, and no cranking amps given.

    Bottom Gear Bob on the MGOC forum mentioned HB/HCB202 which is a direct replacement (including the clamp) for a 6v battery at 175mm x 175mm and 190mm high and around 80. Check the cranking capacity though as some are lower than the original 6v at 300 amps as opposed to 360 amps, and note the 550 and 570 amps quoted for an RB 12v and 600 amps for MGOC 6v. Ampere-hours tends to be about 20% lower with the HB202 but that's only an issue if you need to leave it parked with the lights on.

    January 2022: Another type 202 is this Yuasa YBX5202. 440 CCA, 45Ah, 175mm x 175mm x 190mm high, which is pretty-much the same as an old 6v I have so the same clamp should fit. 5 year guarantee and under 50 so a lot cheaper than the Halfords above, less than half the price of the MGOC kit, and less even than a single 6v. Whilst not as powerful as current 6v it's about 30% more poerful than the original 6v, which were always 'adequate' when new, so it's something I'll be thinking about the next time I have to replace them. The posts are down one side instead of diagonal, so for safety with the clamp on the opposite side to the posts that would put the posts towards the front, and the +ve towards the tunnel. Make sure you get the connections the right way round.

    Americans often quote 'group 26' as a suitable 12v battery for a chrome bumper MGB and this site quotes the dimensions of that as 208mm long by 175mm wide by 197mm high. The Performance Batteries site lists no less than 14 batteries that might squeeze in, but the most powerful is only 52Ah which is quite a bit less than the original 6v, albeit it at up to 520CCA (624CA). The dearest of these is 62 inc VAT. Some batteries come with one or two lips on the bottom edge for clamp brackets on modern cars, which can make the difference between fitting and not fitting. I've seen it suggested that these can be cut off, which may well work, but remember you have probably nullified the guarantee by doing so.

    Note that 12v batteries have the posts down one long side and can have the polarity either way round. The convention for rubber bumper cars was to have the posts adjacent to the tunnel and the +ve towards the front of the car when installed. Whatever battery you fit you need to check carefully - REVERSE CONNECTION WILL DESTROY AN ALTERNATOR AND BURN WIRING.

    About the only good technical reason for converting from twin to single concerns access to the fuel pump by fitting it in the near-side which would mean extending the 12v cable. Peter Mayo said that when fitting a geared starter it needed a longer 12v cable from the battery, so at the same time he moved his 12v battery to the near-side and made the cable long enough to reach that. He went even further by moving the pump up into the off-side box making access to it even easier. Make sure you use the link cable bracket at the top of the prop-shaft tunnel to hold the cable up out of the way, and always fit a clamp to hold it down.

    I've seen one person doing this had cut the lid in half, which on the face of it might seem to make sense making it like the RB cover, and only having to remove half to either inspect the battery or access the storage bin, but it left one half of the cover with only two fasteners on one end.

    And finally... The batteries, particularly the single 12v in a rubber bumper, can be a tight squeeze through the hole. If your new battery doesn't come with a handle it is a good idea to put strong cord or webbing around the battery before you fit it, and leave it in-situ. The next time you come to remove the battery you will congratulate yourself on your foresight. There are 'battery lifters' available, some with eyelets that fit over the posts (don't worry, they are connected by webbing ...) that would work, but others that grip the short sides of the battery with a scissors clamp wouldn't work on an RB as one end of the standard battery is under the lip of the shelf.

    Battery Drains January 2014

    This is when an otherwise good battery seems to go flat when the car is left parked for some time. Ordinarily the MGB should go for a couple of months without being used, and still crank near normally. Less so if you have certain types of alarm as these draw a continual current from the battery. It should only be in the order of 10 or 20 milliamps, but unless you use the car almost everyday the continual slight discharge every time it is left for a week or two will shorten the life of the battery significantly. In my case when I stopped using the V8 every day I had to change the battery twice in three years. Then I fitted a battery cut-off switch, and the present battery has now been in there 10 years.

    But back to drains when you don't have an alarm, which could be from many causes, and may even be from the battery itself. However that can be determined by disconnecting the battery earth strap after a decent run, leaving it until you would normally expect problems, then reconnecting the earth strap and trying to start the engine. If it still won't start, then it's the batteries (or maybe the connections ...) but if it starts just fine you have a drain.

    To diagnose this it's best to use a voltmeter, but connect it like an ammeter - very confusing to the uninitiated! Disconnect the battery earth cable, and connect the voltmeter in its place. Why not use an ammeter? An ammeter gives a very low resistance path to current, so if you have a high drain a high current will flow. If more than the typical 10A max of a hobbyists meter it could damage the meter, but more importantly create a spark when it is connected and disconnected, which could ignite battery gases. Yes, I know that disconnecting the earth strap might also create a spark, but there is little you can do about that unless you have a cut-off switch, and just one spark is better than several. By contrast a voltmeter offers a very high resistance path, so negligible current will flow even if there is a very large drain, and no spark. For an early positive earth MGB the positive of the meter goes to the earth terminal of the battery, and the negative terminal goes to the car body. For later negative earth MGBs, and those early cars converted to negative earth, the meter is connected the other way round. Digital meters don't usually mind being connected round the wrong way, and will just show a minus sign in front of the reading. If there is any current drain at all, you will see a reading on the meter. There should not be any drain at all on a dynamo-equipped car, unless you have added a modern radio with stations stored in memory (and not all of those), a car-powered clock, or an alarm (in most cases but not all). With an alternator equipped car there will always be a very slight drain, which is from the reverse leakage current of the alternator diodes. This is in the order of micro-amps, and a battery should be able to support this for several months. If you use an analogue meter, the alternator drain may register anything up to 12v on the meter, and you can eliminate this as a cause by unplugging the wiring from the alternator, and an analogue meter should drop to zero. But a typical digital meter is very much more sensitive than a typical analogue meter, and will almost certainly register some voltage even with the alternator unplugged. As long as this is less than 12v then it is an insignificant insulation leakage that can be ignored, a little dampness or dirt across the solenoid or other 'always live' component can be enough to cause that.

    But if registers 12v then it could be significant, and if you don't have an analogue meter to compare it with (which should show zero volts) you will need to switch your meter to current. If that shows something in the tenths of a milli-amp or higher then it really ought to be investigated, not so much that it will flatten the battery (although it will over several weeks) but in case it is from damaged insulation on something that might suddenly get worse and cause a catastrophic short-circuit.

    You can use a test-lamp in place of the volt-meter, but as well as even a low-wattage bulb causing a spark on a high drain, a small but still significant drain may not be enough to make even a low-wattage test-lamp bulb glow. A high-wattage bulb should not be used while looking for a drain as it will generate a significant spark on connecting and reconnecting. However a high-wattage 12v bulb in place of the earth strap is very useful when testing a replacement wiring harness as it protects the harness from damage if there should happen to be a short-circuit on any of the unfused circuits, but will allow most circuits to function to some extent.

      If you have a battery cut-off switch then you can do these tests without disconnecting the earth strap, which makes things easier as well as safer. Turn the switch off and remove any bypass fuse.

    • If your cut-off switch is in the 12v cable going to the starter, then connect the meter between the 12v post of the battery and the brown at the fusebox, and continue as above.
    • If your cut-off switch is in the earth lead from the battery going to the body, then connect the meter between the earth post of the battery and a good body earth elsewhere.

    Battery Types Added December 2008:

    Battery technology and hence selection is getting ever-more complicated with technical advances.

    Originally there were just lead-acid or 'flooded' types with screw caps on each cell or lids covering all cells. These need periodic checking and topping-up to replace the distilled water lost through gassing and evaporation, which occurs during cranking and charging i.e. normal use. These must be operated in a well-ventilated space and you should not make the last connection of first disconnection or a charger or jump-leads directly on a battery terminal as it can ignite the gasses. There are 'sealed' version of these which are nominally maintenance free i.e. you can't top them up, but they can still gas. You definitely should not operate these in the plastic so-called 'battery boxes'. Some modern cars have the batteries within the passenger compartment or boot and these should have a vent tube leading to the outside, which must always be connected to the battery, so it follows that batteries used in this situation must have the facility to connect the vent. These are often sold with a little red angled tube taped to the top of the battery.

    There are so-called 'sealed' versions of flooded which no longer have a removable lid to check and top-up the electrolyte, but they still gas and again must be used with the same precautions in ventilation, charging or jump-starting, and if inside the car must be used with the vent connected.

    'Calcium' variants of the above have a higher capacity for the same physical size, at about 12% more expensive than lead-acid. They are probably all 'sealed' but can still gas so the warnings above still apply.

    More recently 'gel' batteries became available, in which the electrolyte is a jelly rather than a liquid. These cannot leak, even when the case is damaged. Under 'normal' use they do not gas, so are safe for use in enclosed spaces, which is why they are used as backup batteries in the event of mains failure for burglar alarm systems and the like. However they must be charged at a lower voltage/current than flooded types or voids can develop in the gel which reduces capacity, conventional automotive chargers will damage them unless they have a special 'low-rate' switch position. They also have higher internal resistance so are not as effective as conventional flooded cells when used as starter batteries, so are rarely used in automotive applications. In hot conditions they can still lose water (somehow), which can limit battery life to as little as 2 years. They can stand heavy discharge better than flooded, which is why they are often used in charge/discharge applications like golf carts.

    Following gel batteries Advanced Glass Mat batteries were developed. These have liquid acid, which is kept in place by a fibreglass mat which acts like a sponge. They have a higher cranking capacity for a given physical size compared to flooded. However because they need a higher acid concentration than lead-acid they need to be charged at a higher voltage, which may have implications on MGBs with standard alternators and high electrical loads, and especially dynamos. The big draw-back is that they are 4 to 5 times more expensive than flooded types so are hardly a practical proposition in conventional automotive use.

    Some gel and AGM batteries are described as 'Valve Regulated Lead Acid' (VRLA) types which means they have a valve to the outside that maintains a positive pressure inside the battery, which normally prevents any gas escape. However under very high charge rates gas pressure will build up to open the valve, so again these should really be used in well ventilated spaces. They are also more sensitive to high 'ripple' charging currents, which is another reason why conventional automotive chargers cannot be used. And if you take into account the very high and peaky voltage and current output that can be seen from an alternator when the battery is disconnected (never run an engine with the battery disconnected) it seems to me that these shouldn't be used in automotive applications either!


    Driver Technology - Battery Types
    Arizona Wind And Sun

      Plastic battery bins

    I can't see the point in these, in fact I think they are being mis-described and mis-sold. If you have converted a chrome bumper car from twin 6v batteries to a single 12v and use one in the empty space as semi-secure storage that is fine. But other than that it seems to me that:
  • buying a box to put in the hole,
  • buying a battery small enough to fit in the box (particularly an issue if you are trying to put a 12v battery in a 6v battery box),
  • cutting holes in the box for the cables and ventilation (battery fumes are explosive and will come up into the cockpit otherwise, particularly an issue in GTs, and the high concentration above the battery will accelerate corrosion),
  • sourcing insulation for the underside of the cover to stop the batteries jumping up and shorting out (I've heard of people being advised to wedge thick foam between battery top and cover, i.e. across the terminals, which when the foam absorbs any moisture will trickle-discharge the batteries) or cutting more holes in the box for the clamp rods and cutting the clamp plates to fit,
  • is an awful lot of bother when the upper half of the battery stays pretty clean anyway. And even if you do decide to fit the clamps I have been told that the boxes do not sit on the cradle but are supported by the lip, which seems to me like quite a lot of load on a bit of plastic, even worse if you are considering using the boxes because your cradle has rotted away.

    Fused Battery Connector January 2014

    Someone on the MG Enthusiasts Bulletin Board asked what the starter current was. He'd apparently witnessed an unattended MGB burst into flames, over the back axle, possibly from the battery cable shorting to the body, and wondered about fitting a fuse. AES advertise these fused battery connectors in ratings up to 300 amps. However recommended batteries are 290 and 360 cold cranking amps which might be a bit marginal, especially if you need to crank the car out of a dangerous position if the engine has failed, or maybe if someone tries to crank it in gear with the clutch pedal released, both of which will increase the starter current above that for 'normal' cranking. Then again, a modern geared starter would reduce the 'normal' and 'crank out of danger' currents at least. Given information about the car, the starter motor and the recommended batteries AES thought the 260 amp would be suitable, but agreed it was difficult to be sure as there are so many variables, even oil viscosity. You would also need two for the 'live' terminals of twin 6v batteries in case the link cable shorted-out, although maybe you could get away with of negative earth cars - if AES do a clamp for the negative post.

        Battery Chargers Updated November 2007

    I've just seen an episode in the new series of The Garage on Discovery last night about the 'English Mobile Mechanics' in Spain. They had a BMW with a flat battery so connected a boost charger direct to the battery in the boot (even though my son's BMW has jump-lead connections in the engine compartment). These boost chargers put a very high voltage to the battery and hence a high current, a lot of boiling and gassing. They got it started and the boss told one of the mechanics to disconnect the charger, which he did - by disconnecting one of the battery clips without turning off the charger first. Big spark from breaking the high current, ignited the gases around and inside the battery, and the battery exploded. The mechanic was very lucky in that he only got acid up his arm and not in his eyes, nor any injuries from plastic 'shrapnel'. Whilst I suspect that most of us have smaller chargers that would result in less gassing and a smaller spark, I wouldn't want to be the one to find out just how small a spark and an amount of gas would cause an explosion. This is also why the last connection to be made when using jump-leads should be made remote from the battery.

      Update February 2014:
    Another important thing to remember, from the Lucas Fault Diagnosis Service manual:

    If the battery should become fully discharged, it should not be left on the vehicle in the hope that it will become fully recharged by the vehicle's charging system. Unless the battery is charged by an external source it will probably never become more than half-charged, and even though it appears to be working satisfactorily, the plates will harden and the life of the battery will be considerably shortened. Modern cars with their continual small discharge from ECU and alarm systems can also suffer from battery degradation if not used on a near daily basis, and is why I've fitted a cut-off switch to my ZS. Some years ago I read somewhere that Mercedes were having to select alternator regulators at the higher end of the output tolerance because of the number of complaints about failing batteries.

    I'd take this further and say that if ever the battery becomes discharged enough to need a jump or a bump start, i.e. not fully discharged, an external charger capable of outputting more than the 14.3v to 14.7v of an alternator should be used to fully recharge the battery. It should be quite safe to recharge up to 15.5v with the battery still in the car as this voltage can be reached by dynamo systems under normal use. But a higher voltage boost charger should be used with the battery out of the car and in a well-ventilated space (see above). Again from the Lucas manual, external charger current should be limited to one tenth of the ampere hour capacity of the battery at either the 10 or 20 AH rate. For the original batteries this represents about 6 amps, but replacements from some sources can be 75AH and even 88AH (see above), giving charge current of 7.5 amps and 8.8 amps respectively.

    I have battery cut-off switches on all three of my cars, but normally the ZS is only turned off in the summer as in the winter it gets used more. However for various reasons this winter it's only been doing a few miles a week, and I have noticed the cranking speed gradually getting lower. So I took the battery off the car and charged it on the bench using my high-output charger. Initially taking just under 6 amps and showing just over 13v, after several hours the current had dropped to about 4 amps and the voltage risen to 15.5v. Put it back on the car (and checked it would start) and left it over night with the cut-off switch not turned off. Next morning, i.e. with the overnight load of the alarm and ECUs, it was cranking much faster than it had previously. I'll have to start turning the switch off in winter as well now.

    One of the perennial questions is "How do I charge two 6v batteries". The answer is you treat them as if they were a single 12v. Forget any 'buts' about different current or voltage in each, it is exactly the same as having six 2v cells in a 12v battery - they are all charged in series so get the same current. Yes, they may exhibit different voltages as they age, but that is down to individual cells and applies equally to 12v batteries as to 6v.

    As to how to connect the charger it would be easy to get confused by the interconnecting cable and end up connecting a 12v charger across just one of the batteries, which wouldn't do it a lot of good, and it is a right faff getting the battery cover off anyway. If your car has a cigar lighter you can forget the batteries, just buy a cigar lighter plug and connect the wires from the charger to it - observing the correct polarity! The +ve wire from the charger must be connected to the +ve circuit in the car, and the -ve to the -ve. This applies to both +ve earth and -ve earth cars. The cigar lighter was an option from the beginning and standard from the 1973 model year, but was wired differently over the years, each needing a different approach as follows:

    • Up to and including 1968 it was connected to the brown circuit at the ignition switch hence not fused inside the car. In these cases it would be advisable to add an in-line fuse at the lighter (or at the ignition switch if you can identify the correct wire). A standard 17amp continuous, 35amp blow will suffice.
    • In UK 1969 and 1970 cars it was connected to the 'accessories' contact of the ignition switch. Not only was this unfused as on earlier cars, but also needs the ignition key to be in and turned before one can use it for charging. This is likely to be a problem so rewiring the cigar lighter to the purple circuit (fused, always on) as on later models might be the best option.
    • In UK 1971 cars it is wired to the green/black circuit, and although this does have an in-line fuse it is still on the accessories circuit so needs the ignition key to be in and turned as above. Again rewiring to the purple circuit may be the best option.
    • From 1972 (UK) and 1969 (North America) all cigar lighters were wired to the purple circuit which is fused in the main fusebox and doesn't need the ignition key.

    There are two types of cigar lighter plug - fused and unfused. The fused type might seem the safest option but the current travels through a very fine spring to get to the fuse which makes the plug get quite warm during charging. A better option is the unfused type which has a higher current carrying capacity. With a fuse in the car and another in the charger you are quite safe using an unfused plug.

    Another option is to use a different plug and socket with the socket in, say, the engine compartment connected to the purple fuse and earth and the plug on the charger wires, but still needs the bonnet to be opened and closed to connect and reconnect. One thing to be aware of is that cranking the engine whilst the charger is connected may blow the charger and/or cigar lighter fuse.

    Trickle chargers supply current to the battery continuously (not the same as 'constant current'). As long as they don't raise the battery voltage over 15v you should be OK to leave them on overnight as an exception, but not for long periods or regularly every night. This level of charging, even though it is the same as when the car is being driven, will cause the battery to 'gas' (incorrectly called 'boiling') to some degree which will cause the distilled water in the electrolyte to evaporate lowering the electrolyte level. Also while in a garage, or even out in the open unless there is free air circulation around the battery i.e. lid off and windows open, you will get a build-up of gas in the battery compartment which could cause an explosion and corrosion of metal parts. For chargers that raise the battery above 15v this evaporation will occur to a much higher degree and could even damage the battery i.e. warp the plates. For long-term battery maintenance e.g. when the car is not being used for some months get a 'conditioning' charger. These sense the battery condition and vary the rate of charge accordingly over time.

      Added July 2009: My son is in the market for a charger to keep the battery in his 'occasional use' classic BMW topped-up and looked at the Halfords Fully Automatic Charger. On the face of it this charger is intended for extended connection and has a 'maintenance' mode with reduced current, but if you read the Customer Q&A two posts state that the battery must be disconnected from the car for charging, also its maintenance charge level is 1.5A which is too much in my opinion. In fact if you look at the Q&A for all the Halfords chargers they say not to use them with the battery in-situ or connected to the car. Utterly pointless, for an intermittent-use classic car if you are going to disconnect them to charge them you might as well just disconnect them anyway, they will hold their charge for more than two months like that, remember when you buy them they have been sitting on a shelf at least that long. If you want a charger that you can leave connected all the time, while the battery is still in the car and connected, then one of the few suitable ones seems to be the Accumate battery optimiser (cheaper from the MGOC though) which charges down to less than 200mA and is specifically designed to carry the load of alarms, radios/CDs etc. Note that the Accumate can be switched between 6v and 12v, but the twin 6v batteries of the chrome bumper MGB should always be changed in series on the 12v setting. This Optimate 5 charger (also from Accumate) additionally has a recovery mode for deeply discharged batteries. Two others are recommended by AutoExpress here. However for an MGB at least I think a cut-off switch (without bypass fuse!) is much cheaper, more convenient, has an immobilising function and more importantly an emergency disconnect function in the event of an alternator fault or short in the many unfused wires. More recently I have come across this Battery Brain which can be used to disconnect the battery more conveniently than undoing clamps, and will disconnect itself if the battery drops below 12.1v. Still less preferable to a cut-off switch in an MGB, but a definite possibility for my ZS which gets little use in summer and where the fitting of a cut-off switch is more involved and the use less convenient, although I did fit one to that as well).

    A warning on optimisers though. These are designed to be left permanently connected, to keep the battery topped up. I think that is fine for long lay-ups without using the car, but if you continue to use it when the car is being used on a regular basis you are never going to know when your batteries are on their last legs, or if the charging system is down on output. Go for an overnighter somewhere, without the optimiser, and it may not start next morning. So put it away whenever the car is being used more than, say once a month.

    Another question is 'Can I charge two batteries at the same time?'. Firstly if we are talking about twin-6v batteries in an MGB then the answer is that is how they should always be charged, and in series as described above. As far as charging two 12v batteries at the same time then it all depends on the charger:

  • If you are using a 12v battery conditioning charger that varies its current over time according to battery condition, which is intended for long-term use when the car is laid up for example over winter, then apart from the case of twin 6v batteries being charged in series you should never use one of these for charging two 12v batteries either in series or parallel. Also long-term connection of a conventional trickle charger is not a good idea, even at very low currents.
  • If you have a high-voltage charger that delivers at least 25v or 26v then you can use one of these to charge two 12v batteries, but in series only. Charging in parallel with such a high voltage will probably damage the batteries and possibly the charger. It must also be done off-car or at the very least completely disconnected from the cars electrics.
  • If you are using a conventional 12v charger then you can charge two 12v batteries in parallel. A conventional charger works by delivering a voltage in excess of a fully charged battery voltage. The difference in the charger and battery voltages is what drives the current through the battery, the higher the difference the greater the current. Home use chargers usually can't deliver a constant voltage, and if connecting them to a deeply discharged battery a relatively high current will be drawn, which will tend to pull the charger voltage down. As the battery charges, its voltage will rise, the charging current will decrease, and the voltage will increase. Connecting two half-charged batteries in parallel to a charger is much the same as connecting a single deeply discharged battery. Both will charge, but it will take longer than if only one of them is being charged at a time. When the charger is switched off the batteries should be disconnected from each other, or if one is good and the other a bit leaky, the leaky one will discharge the good one.

  • Deep charging: Alternators are designed to charge at about 14.5v, whereas dynamo systems can charge at up to 15.5v. Whilst the alternator voltage regulator controls things to a much tighter range than the dynamo control box can there is still some variation. And I have seen a report that some at the lower end have been found to not charge at a sufficiently high voltage even in normal use, i.e. not even having flattened the battery by having left the lights on, for example. Therefore it is no bad thing, on alternator equipped cars, especially if they are used little over winter, to give them a deep charge at about 15.5v say, once a year, for a couple of hours. If you have flattened the battery then it will need longer, and charge up to about 17v. If you have a charger with an adjustable output measure the voltage after an hour or so at each setting (measuring immediately after connection will not give an accurate reading) to determine which one to use. If your charger's highest range isn't high enough, or it is fixed at a lower voltage, then hard luck. Also some 'boost' chargers designed get enough charge into the battery to crank as soon as possible may deliver too high a current to be used for other than a very short time, they should not be used for longer periods.

    Battery Cut-off Switch

    I've had one in the V8 for a number of years, installed because the alarm was flattening the battery when I stopped using the car on a daily basis, and I was having to replace said battery every 18 months or so. I don't get the flattening problem in the roadster - it will go more than a month and still crank well with only a very slight reduction from 'normal'. But something started nagging me to fit one a couple of months ago for its safety aspect - that of being able to disconnect the power when it is left in the remote garage, and even more so if there should be a short on one of the several browns (all unfused) when driving - I wouldn't want to have to jump out, get a screwdriver and spanner out of the toolbox, remove the cover, undo the clamp ... all while the harness was burning merrily!

    Got a shock when I removed the cover as one battery post and connector was completely obliterated by a 'growth'! A real surprise, because whilst I only check the batteries once a year these are now several years old and there hasn't been any sign of this in past years. Checked the electrolyte levels and the other battery only needed a drop in one cell, but this battery had all cells down and one needed quite a bit. I guess that even these are on the way out, even though they show no reduction in cranking power. As you can see these are 'modern' versions of the original tar-top batteries with separate cell filler caps. I bought a pair of tar-tops in 1994 (14 years ago) to replace an under-sized single 12v on the drivers side, and although I have mislaid the record of when I bought these it must be at least 7 years ago.

    As on the V8 I installed the switch on the heel-board behind the drivers seat where I can reach it quickly in an emergency. The +ve lead passes right by here so that is the most convenient place to put it without buying new and extending the cable run. Some argue that it should be in the earth lead, but it makes no difference electrically and as far as cutting the power goes (but see below). One slight advantage of having it in the earth lead is that this is the lead that must be disconnected first (and reconnected last) when working on any of the battery connectors - twin 6v or single 12v. However unless all you are going to do is tighten one of the other posts, when you would need to remove the earth cable even though you may have a switch in the 12v cable, there is no advantage. If you are going to remove the battery you will have to disconnect the earth cable anyway, regardless of where any switch might be. As far as working on anything else goes i.e. the solenoid or anything with brown wires having the switch in either lead is equally safe, as its immobilisation function, and it's emergency disconnect function. The main advantage of being in the earth cable is that it avoids cutting the 12v cable and fitting two lugs. One slight disadvantage of having the switch in the earth cable but positioned on the (left-hand in this case) heelboard is that with single-12v battery cars you will need a longer cable to get from the battery to the switch, but the original cable could be used to get from the switch to earth. However the major disadvantage is that if you want to keep power to a clock and/or radio (but isolate power from everything else e.g. alarm) you would have to completely insulate said radio and clock from the car's body - which is not going to be easy - and run separate earth wires from the battery-side of the switch. In that event putting the switch in the 12v cable and running a fused bypass circuit to said clock/radio is the only way to go. Update January 2014: In a Bulletin Board discussion on this topic a pal mentioned another benefit, and this is if something goes wrong with your work, and it shorts to the body, then better the earth cable than the 12v cable. And a day later someone elsewhere posted that he had seen an unattended MGB burst into flames, over the back axle, so probably the battery cable shorted close to the battery. If that was the last few inches before the battery terminal, then a cut-off switch in the 12v cable at the heelboard would be no use, but one in the earth cable would be - if you could get to it. That person wondered if a battery terminal fuse might be even safer. But the biggest advantage of fitting it in the earth cable is that you just swing the existing earth cable round from the body to the switch, and provide a new one from the other side of the switch back to the body, instead of cutting the 12v cable. Much easier, and reversible should one wish. For twin-6v systems this means the switch is behind the passenger seat, but still easily accessible for the driver. End of update

    All pretty straight-forward, I made a cardboard template for the complex hole in the heelboard, and cut it out using a combination of drills, little grinding wheels and a metal cutter. However in hindsight although the switch is designed to be mounted on the front of a panel, it makes sense to mount it on the back as only a simple circular hole is required, and there is a lot overlap between switch and panel that can be filled with sealant to prevent any water ingress. Mounted on the front of the panel a long slot is required in addition to the hole, the ends of the slot are very near the edges of the switch, resulting in very little overlap and a greater chance of water ingress. I orientated the switch so that when the handle of the 'key' is more-or-less vertical the switch is on, and when it is horizontal it is off. As well as having a certain amount of logic in that the key handle points in the same direction as current flow down the cable when it is on, and across the cable when it is off, it also means the two connections on the back are equally accessible rather than having one on top and one underneath. This also means the cable coming up from below can be left a little longer which makes it easier to work on in-situ when stripping, tinning and soldering the terminal. Only when I could get the switch into the panel did I mark and drill the holes for the fixing screws. Before mounting the switch I laid the heel-board carpet back over the hole and made two cross-cuts where the barrel of the switch would be sticking out i.e. where the hole in the carpet would be required.

    Cut, stripped, tinned and soldered the end of the cable leading to the starter in-situ. A bit cramped but easier than trying to remove the cable from the car or at least the rear brackets to be able to get the cable out the side, the last time I tried undoing any of those they all sheared. When I bought the switch and terminals at Stoneleigh last month the seller recommended some rubber 'boots' which were actually quite a bit larger than the terminals and would have been quite loose when fitted. He had some smaller one that I reckoned I could fit on, and indeed I was able to fit them after soldering the terminal to the cable, as I didn't want the heat from the blow-lamp to damage them by slipping them over the cable end first and they fit the terminals really snugly. Not only will they resist dislodging and possible shorting, but also water ingress and corrosion. I used a wet cloth wrapped round the cable insulation leaving just the bare end free, and arranged some pieces of metal around the battery compartment to protect the switch, fuel pump and wires etc. while I had the blow-lamp in there (my wife's cook's blowlamp!). By comparison the battery end was a doddle as it could be done off-car. Bolted to cables to the switch, daubing Vaseline around the connections before fitting the boots for protection against dampness and corrosion.

    In hindsight it would have been easier to install it in the earth cable, as both ends could have been dealt with off-car. You would need at least one new earth cable, plus perhaps a longer replacement for the original. In that case CB cars with twin 6v batteries would have it on the left-hand heelboard, with RB and CB cars converted to single 12v on the right-hand side, being careful to avoid the 12v cable. In this latter case if you still have the armoured sheath between the battery boxes you could run the (longer) earth cable through it and have the switch on the left-hand side. But if not, and you consider running the cable through the holes and being unsupported between them, be aware that it can rub on the prop-shaft and wear the insulation away. Not a huge problem on the face of it as both are at earth potential anyway - except that if you then need to turn the switch off for any reason power may still be connected.

    Cleaned the 'growth' off the batteries, connectors and clamping plate and reinstalled. I will replace the interconnecting cable in due course as its bolt and nut were badly corroded and parts of the connector have been eaten away, but it is OK for the time being. Liberally daubed the battery posts and connectors with Vaseline (before fitting) which really does help keep corrosion at bay (normally!).

    I've got into the habit of turning Vee's switch off every time I put her in the garage so shouldn't have much trouble getting to use Bee's. I just hope I never have to use it 'in anger', but I shall be ready. In fact it has become so much of a habit that a couple of times I have turned Vee's switch off before the engine, which is something you should never do. Even though the alternator has a voltage regulator it still needs the battery to be connected for it to work correctly, without a battery you can get very high voltages which can blow bulbs and possibly damage the alternator. I've been lucky, I've seen my coolant level warning (green glows all the time when the level is OK) get brighter and flicker when I've done this, but no lasting damage.

    Update April 2009: Very glad I had done this, and put the switch in the 12v cable instead of the earth, as when I replaced the link cable the new one was too short to go between the posts in their existing positions. The only possible way to make it reach comfortably then meant the 12v cable was too short. By having the switch in the 12v cable it was a relatively easy matter to remove the short piece between 12v terminal and switch, turn that battery round, then make up a new, longer cable to go between the new position of the 12v post and the switch.

    Bulbs Added July 2009

    Bayonet bulb holders

    Part No.LocationTypeWattsUsage
    GLU101HeadlampSealed Beam60/45101-187210 RHD
    GLU106HeadlampSealed Beam75/50187211-360300 RHD and CB V8
    BFS415HeadlampBulb50/40101-360300 LHD except Europe and North America
    GLB410HeadlampBulb45/40101-360300 LHD Europe except France and Germany as below
    GLB233Headlamp pilotBayonetBA9457028-59462 Germany
    GLB411HeadlampYellow bulb45/40101-360300 France
    17H9472HeadlampSealed Beam60/45101-410000 North America
    GLU123HeadlampSealed Beam with pilot window75/50360301-410000 RHD and RB V8
    GLB501Headlamp pilotWedgeT10
    5360301-410000 RHD and RB V8
    GLU114HeadlampSealed Beam?360301-410000 LHD except France, Germany and North America
    GLB501Headlamp pilotWedgeT10
    5360301-410000 LHD except France, Germany and North America
    GLB411HeadlampYellow bulb45/40360301-410000 France
    GLB233Headlamp pilotBayonetBA94360301-410000 France
    BHA5387HeadlampSealed Beam?360301-410000 Germany
    GLB233Headlamp pilotBayonetBA94360301-410000 Germany
    GLB472HeadlampHalogenH460/55410001 on RHD
    GLB233Headlamp pilotBayonetBA94410001 on RHD
    17H9472HeadlampSealed Beam60/45410001 on North America
    GLB989Front ParkingBayonetBA95101-187170 North America
    101-360300 Not North America
    GLB382Front FlasherBayonetBA1521101-187170 North America
    All, not North America
    GLB380Front Parking/
    Offset bayonetBA156/21187170-on North America
    GLB323Front FogBulbP36s48101-187210
    GLB185Front Long-rangeBulbP36s48101-187210
    GLB380Stop/tailOffset bayonetBA155/21All
    GLB207Number plateBayonetBA95101-339964 and V8 to 1247 except as below
    GLB501Number plateWedgeT10
    5187211-219000 North America
    GLB989Number plateBayonetBA95339965-360300 and V8 1247-2100 except North America
    GLB233Number plateBayonetBA94All RB except North America and Germany
    GLB254Number plateFestoonFestoon6339095 on North America
    GLB233Number plateBayonetBA94339095-410000 Germany
    GLB382Rear FogBulbBA15211980 UK models
    GLB987Map lightScrewMES E102.2101-258000
    GLB989Gear lightBayonetBA95Automatic only
    GLB273ReverseFestoonSU8, 5-821101-410000 and V8 except as below
    GLB270ReverseFestoonSU8, 5-818268698-410000 North America
    37H 1547ReverseFestoonSU8, 5-8?France, possibly yellow
    GLB254Load spaceFestoonFestoon5GT and V8
    GLB239Interior/Courtesy & BootFestoonFestoon5219001-410000
    GLB989Side markerBayonetBA9S5187211 on North America
    GLB987InstrumentsScrewMES E102.2
    GLB987Ignition warning,
    main-beam and
    indicator tell-tale
    ScrewMES E102.2Tin dash, chrome bumper, not V8
    GLB643Indicator tell-taleBayonetBA9S2Early padded dash, claw holder
    GLB281Ignition warning,
    main-beam and
    indicator tell-tale
    BayonetBA7S2Other padded dash, all V8, all RB. Push-in holder with spade connectors
    GLB921Switches and controlsScrewLES E51.2410000 on, use GLB280
    GLB643Cigar lighterBayonetBA92.2 
    GLB280Brake warningScrewLES E51.5 

    Note 1: Unless otherwise indicated image are from Moss Europe.
    Note 2: I have seen the dash harness for 77 and later UK models with capless/wedge-type bulb holders.
    Note 3: Generally the 3-digit number following the GLB code is the industry code for the bulb style, fitting and output.
    Note 4: It should be noted that LED bulbs are not always road-legal.
    Note 5: RHD RB optional H4 headlamp - some suppliers describe this as 'flat glass', but whilst mine are shallower than the sealed beam, they are nowhere near flat.
    Lucas Bulb Catalogue and Application Guide

      Bayonet bulb holders:
    Bayonet bulbs use two pins one either side of the bulb base to push down into channels in the bulb holder, then turn a few degrees clockwise and come back a little way to lock the bulb in position. Dual filament bulbs have offset pins to ensure the bulb can only be installed in one orientation and the correct filament illuminated. With these even though the bulb can be pushed into the holder either way round it takes a particularly ham-fisted or brutal person to turn it and lock it in position, which has been known! If it doesn't turn easily then withdraw, rotate 180 degrees, and try again.

      MES/E10 bulb holders:
    The bulbs can be a fiddle to fit as the claws get in the way of fingers trying to turn the small globe, but there is a technique to make it easier.

      Clocks February 2016

    A clock was standard equipment for the 77 and later models, powered from the purple circuit, in the centre console on UK cars but in the main instrument panel on export models. I don't know the 'technology' of the original clocks i.e. whether it is modern electronics taking a brief pulse of current every second, or the older escapement-style that wind themselves up with a motor every few seconds.

    For both factory and after-market clocks, if you find it works with the lights off but not when the lights are on, then the earth supply to the clock is probably missing.

    A PO had fitted a clock (electronic) to the V8 by cutting a hole in the extreme left hand end of the dash, which doesn't sound very convenient for the driver but is surprisingly easy to read. Originally powered from the purple circuit, when I fitted the battery cut-off switch I soon got fed-up with (not 'of' ...) having to reset the clock each time, so ran a wire from an in-line fuse attached to the battery cable connector direct to the clock, disconnecting the original purple feed. Why didn't I fit a switch with a bypass fuse? Because the switch was to prevent the alarm from flattening the battery when I stopped using the car every day. I did a similar thing when I fitted a cut-off switch to the ZS for the same reason.

    I wouldn't contemplate cutting a hole in Bee's dash for a clock, nor wanted one in a separate bracket, and for years struggled with pushing my sleeve back - often being a coat and a jumper to see my watch. Eventually a pal with an interest in wood-turning mentioned he had made a holder for a watch insert and case that fits the cigar lighter socket, so I got him to make me one as well and get me an insert from Stiles and Bates (so he could make the holder a snug fit for the insert), which is a company he uses for wood-turning tools and materials, that just happens to sell clock and watch inserts! I chose one with a black face and bezel with bright hands as being most in keeping with the rest of the dashboard, and sprayed the 'holder' with several coats of satin black as well. 4.85 for the insert which isn't bad, but another 4.75 for delivery. However with the 'clock' installed I found that the bright hands reflected the black seat covers and I couldn't see the time! So I took the insert out of its case and painted the hands with a thin smear of Snopake. Much better, but having painted the seconds hand as well, I found I was having to look at it for two or three seconds, or several times in that time, in order to see where the other two hands were pointing to get the correct time! Also at about that time the insert stopped working, replacing the battery didn't help, which was very annoying. As well as having 'defaced' the insert if they wanted it back, I didn't want to put my pal to more trouble and expense in getting it replaced and posting it up to me, so ordered one direct at another 10, then complained that one didn't work. In the event they didn't want the old one back and sent me two more (one the same and one with a bright blue face in a bright surround) as well as two replacement batteries. So good service, but it would have been cheaper for my pal to complain then me pay him to send them up to me. Any road up, with a working clock, I painted just the hour and minute hands white, and finally I can see the time at a glance ... in the day time anyway.

    May 2020: Herb Adler has sent me some information on a classic Hillman clock that he tried to make use of, but in the end gave up. It's rather complex mechanically and electrically and the info document from the Hillman Car Club of South Australia recommends servicing one particular component every couple of years! No wonder Herb eventually installed an 80s VDO mechanism in the case.

      Connectors and Terminals:

    Sealed Wiring Junctions
    Earth/ground Connections

    When checking voltages at components check the component terminal i.e. its spade as well as the terminal on the end of the wire that slides onto the spade as corrosion can develop between the two. Factory wires are usually spot-welded to the spade connectors so pretty robust and the least likely to fail.

    Test the voltage on each side of bullet connectors as either bullet could be making a poor connection with the connector sleeve. Bullets are crimped onto the wires in factory harnesses, usually OK, but at the front of the car you can get corrosion running under the insulation for several inches. In one case I've had the conductor strands corrode right through where the insulation was damaged on an unsheathed headlight wire under the wing.

    Multiway Connectors:

    Translucent multi-plugs can be tested by pushing a probe in along side the wires, again test both sides with the connector fully assembled. But moulded-on types can only be tested by parting the connector just enough to get a probe in, and even then only the pin-side.

    If not moulded-on the connector pins and sockets can be teased out of the connector blocks using a bit of tubing the right size. The pins and sockets have two little sprung tabs that stick out and latch behind a flange on the connector block as they are pushed in, I've used a piece of brake pipe to get them out. For projects or modifications Autosparks (and maybe others) sell plug, socket and connector kits in various sizes and shapes.

    With PO wiring or re-terminations also check the connection between the wire and the connector if you can. With PO 'repairs' using dodgy components and techniques or if the car has been abandoned in a field for years bad connections can develop that don't occur in normal use. When adding any wiring I really don't like those blue ScotchLok connectors, I've found on a number of occasions that after a while even in the cabin the bifurcated blade loses tension on the copper strands and they start to cause problems. If you are near a bullet connection then substitute a 4-way for a 2-way and put a bullet on your new wire, or if there is already a 4-way with 4 wires in it I'd rather make up a couple of inch length with bullets and add a second 4-way, but you can get 6-way connectors. If you want to tap into a wire going into a multi-way connector then really there is nothing for it but to cut the wire, put two bullets on the end, and use a 4-way. However if you are near a spade connection then piggy-back spade connectors are a good way of tapping into these.

    The fusebox often causes problems in a number of places. The most obvious is where the fuse ends are clipped into the holders, they don't grip very tightly and corrosion can burrow underneath. There are also the spade connections, and on the 4-fuse type a rivet underneath the fusebox that connects the spade terminals to the fuse holders can corrode. Last but not least the fuse itself can corrode internally, even though the element appears to be whole. Generally speaking (but see below) problems in the fusebox will only affect the circuits fed by it i.e. the purple, green and red circuits, note that nothing to do with the starter or ignition goes through any fuses. However the white feed from the ignition switch may go onto one spade, and come off another spade for the coil on some years, and only in this case could fusebox problems cause ignition or starting problems. Any white circuit that gets its 12v supply from the 'other' spade on the fusebox could be affected by corrosion on the spades. More information on fuseboxes here.

    There are a couple of areas that can cause problems even in regularly used and cared for cars. Horns, lights and electric fans can all suffer from poor connections as their connectors tend to be exposed to the worst of the weather - spade connections of horns, bullet connectors for all the front lights, and two-pin connectors for electric fans. Chrome bumper indicator/parking light units earth through their physical fixings to the wing, and these are exposed to the worst that the front wheels can throw at them by way of water and salt. And even though rubber bumper indicators have a wired earth it comes via a rather flimsy bullet-type connection on the light unit that is also exposed to all the weather. Rear light clusters can also develop earthing problems as they also rely on the mechanical fixings, but being in the boot and protected from weather they should be less likely.


    I always assemble bullet connectors and fan plugs/sockets at the front of the engine compartment using Vaseline which makes assembly easier as well as providing a seal against moisture. Even so it can take some force to push bullets right home into the connectors, so I modified the handles of a pair of pliers. I subsequently discovered there is a specialised tool for this, but at 20 I'll stick with my modification, thank you very much. However replacement connectors seem to be rather inferior to the originals, not retaining bullets firmly enough, allowing one to be pushed in too far so it's opposite number isn't pushed in far enough, and plastic sleeves that slide all too easily to expose metal parts.

    Sometimes it is necessary to replace electrical connectors, or you may be fitting additional electrical components. There are after-market, crimp-on spades and bullets available, male and female in both cases, colour-coded for current carrying capacity - red (5amp), blue (15amp) and yellow in increasing capacity and conductor size. There are also brass solder bullets. Some are shown in the picture on the left, click to enlarge. The only places the small red female spades fit on the MGB that I am aware of is the fuel tank sender (without fuel feed pipe) earth, some tach earths, and the 'boost' contact on rubber bumper solenoids that provides a full 12v to the ignition coil on cranking (however many rebuilt starters seem to have the medium sized spade for both the solenoid 'operate' connection and its 'boost' connection). The medium sized blue spade is correct for virtually everywhere else on the MGB. The large yellow female spade may fit the large output spades on alternators but I have not used them. Use of male spades attached to wires is very rare on the MGB, I can only think of the handbrake diode on later MGBs with male on one side and female on the other to ensure it is connected the correct way round. Similarly with female bullets on wires, possibly only the signal input on the later electronic tach. Female spades come in two varieties - fully insulated and partly insulated. On the face of it the fully insulated are best for anything other than earth connections, but that precludes soldering the wire (see below). Bullet connectors are a problem. The red males are too small for the standard bullet connector on the MGB and the blue ones are slightly too big. They can be forced in, but this distorts the connector and weakens it. Once a blue bullet has been forced in the connector will have opened up such that a standard bullet is now a loose fit, and crimping it tighter just weakens it still further. The brass bullets are the correct size for the standard connector, and are themselves too loose a fit in the blue females (confirming that the blue crimp type are the wrong size for the standard bullet connector). I only ever use crimp bullets for new work if I have the opposite gender on the wire it is connecting to, for anything connecting to existing wiring I always use a standard connectors and brass bullet. There are also crimp connectors for 'permanently' joining two wires together. I never use these, preferring to solder and heat-shrink - remembering to put the heat-shrink tubing on first and sliding it away from the heat of the iron!

    I don't trust the mechanical strength of crimped connections, even when using the proper tool and doing a double crimp, so I always solder as well as crimp, using the semi-insulated female crimp spades or cutting the insulation off female crimp bullets which are only available fully insulated AFAIK. Some claim that heat and solder wicking affects the strength of the copper conductors causing it to fracture about 1/4" from the end of the connector, but I always use heat-shrink tubing over a soldered crimp connector and about the first inch of wire to stop it flexing at the connector anyway.

    Spade adapters:

    These piggy-back and 'Y' adapters are very useful for wiring modifications.

    Branching connectors:
    To connect several additional circuits to the some source i.e. 12v or earth these WAGO221 branching connectors are ideal.

    Sealed Wiring Junctions Added November 2009

    There are a number of sealed junctions in wiring harnesses, where a number of wires of the same colour come together in a sealed and permanent junction rather than a multi-way bullet connector. This seems to have started in the 1970 model years, and there could be one in the brown, black, green, red/white (instrument lighting) and maybe the green/orange of a late 1980 UK model, and as far as I'm aware all are behind the dash i.e. where there are the greatest number of components in close proximity. Understandable in the brown circuit because of the high currents you can get and the need for low contact resistances, and where there are five or more wires coming together, but for other circuits where there are four or less wires in the junction it doesn't seem to make sense. Also 68 to 72 North American models were making use of 6-way bullet connectors in the white circuit at the same time as sealed junctions with six or less wires on other circuits - most odd.

    The wires are crimped into a brass 'staple', for want of a better word, the wires and staple then being soldered. A heat-shrink end-cap is fitted over the junction first, then a length of conventional heat-shrink tubing over that, the two being shrunk over the soldered junction and that is all there is to it.

    The chance of a fault developing inside one of these connections is highly unlikely bar severe abrasion of the insulation and hence shorting to metalwork, which is more likely to happen to a length of wire anyway.

    Earth/ground connections:

    by Felix Weschitz

    Two types - a wired earth, and an earth derived from the physical mounting of the component. Although useful for other reasons the Dan Masters simplified drawings do not differentiate between the two and neither do they show which components share earths with other components, and some of those shared paths use bullet connections for branching. Earth faults can cause some very strange interactions between components and the Leyland/Haynes drawings will be essential to work out where the actual fault may lie.

    Wired earths: From the Leyland schematics for the 62 to 64 MGB there seem to be quite a few wired earth points in various places:

    • Headlights
    • Wiper motor and instruments
    • Dynamo control box
    • Wiper motor switch and indicator switch (tell-tale contacts)
    • Heater fan
    • Fuel tank sender
    • Fuel pump
    After that there are generally only three covering all the above - one in the engine compartment for stuff at the front, one in the cabin above the wiper motor for stuff in the middle, and one (or two) on the rear panel for stuff at the back. Variations on this were:
    • Early Mk2 and the 68 model year look like the engine compartment and cabin earth points were combined into one.
    • Although the fan motor is in the engine compartment it always seems to have used the cabin earth point.
    • North American Mk2 and rubber bumper cars used wired earths whereas tin-dash cars pick it up from their mountings.
    • From around 1971 or 72 although there were only a couple of wires on the cabin earth point one of them fed a sealed junction behind the dash that could have many more wires, and some of those could be daisy-chained off various components and be branched in a series of 4-way bullet connectors with three or four wires.
    • The schematic show that until 1972 models with the seat-belt warning the fuel tank sender used a wired earth and a local earth from the tank after that. RHD cars are shown as using a wired earth until the start of the 77 model year, but in fact all RB cars also used a local earth even though the sender still had the earth terminal.
    • From 1974 North American cars with the sequential seat-belt warning system used a wired earth for the number-plate lights on the number-plate backing panel, before they when they were in the overriders they picked it up locally. RHD cars gained wired earths with rubber bumpers for the same reason.

    Physical earths: A number of components pick up an earth for their electrical operation from their physical mounting:

  • Starter motor and starter solenoid
  • dynamo and alternator
  • Distributor
  • Chrome bumper front parking lights and indicators
  • All rear light clusters
  • Overrider-mounted number-plate lights
  • CB (not V8) ignition, main-beam and indicator warning lights
  • Horn button where it is wheel-mounted
  • Overdrive solenoid
  • Factory fog/driving lamps
  • Early map light
  • Instrument voltage stabiliser
  • Courtesy light switches
  • GT load space light
  • Electric oil and temperature gauge senders
  • Brake balance failure switch
  • RB and all V8 fuel tank sender
  • North American ignition switch with 'key in' warning
  • North American anti-runon system oil pressure switch
  • North American induction heater
  • North American TCSA solenoid
  • Horns 1977 on
  •   Fuses and Fusebox

    Fusebox Connections April 2016
    Fusebox Mounting June 2016
    Fusebox Replacement May 2016
    Blowing Fuses
    Fused Battery Connector January 2014


    Originally a 2-fuse unit was used, with one fuse protecting 'always powered' circuits like the interior light and horns and the other certain ignition-powered circuits. This fusebox is 'handed' in that one fuse has four spades at each end, and the other has four spades at one end and six at the other. It seems that the main harness for 1969 has five individual green wires at the fusebox, even though the Workshop Manual schematic only shows four, and in this case the fusebox needs to be orientated such that the six spades are pointing at the bulkhead and the five greens go on these, with the whites on the front of this fuse.

    In 1968 and 69 in-line fuses were added to protect the parking lights - one for the front and one for the rear, simply added to the bullet connectors where the red wires came out of the main harness for the rear harness for the rear lights, and back into the main harness for the front lights.

    In 1970 this was changed to a 4-fuse unit with the additional fuses protecting the parking lights, one fuse per side. This fusebox only has two spades per fuse end, and to accommodate additional wires two were sometimes put in one spade connector.

    The 4-fuse fusebox is also 'handed' but in a different way to the 2-fuse, in that the front of the top two fuses are connected together as part of the splitting of the parking lights into two separate circuits with one fuse for each side. This link can only be seen from the rear, as shown here, but if you have some funny electrics and think you may have fitted it the wrong way round (which will put the linked pair at the bottom rear) you can check from the front by looking for the terminal numbers. These are quite small and easy to miss (circled on this image). In fact the Lucas Part No. and week of manufacture are easier to spot ('rectangled') and these should also be at the top of the fusebox when fitted to the car the correct way round.

    Also shown are the riveted connections on the rear of the fusebox, which can suffer corrosion and bad connections. You may think that a solid connection here would be preferable, but the rivets allow the external spades to move from side to side while fitting the wiring connectors without twisting the fuse-holders, which would mis-align them with the end caps of the fuses. This could result in very small points of contact, so limiting current and resulting in volt-drops and hot-spots, which as well as affecting the performance of the electrics connected to that fuse this can also cause premature fuse failure. This image shows typical corrosion that can develop on the copper fuse holders.

    The terminal numbers count from 1 at the top front to 8 at the bottom rear, slightly illogical when you consider that the bottom two fuses are the originals carried over from the 2-fuse fuseboxes. If you are wondering what the three circular holes in the fusebox are for and have lost your cover, then this image shows that the middle hole is for the cover locating peg and the two outer holes for spare fuses.

      Fusebox connections: April 2016 It's a common misconception that all wires at the fusebox are fused. Only the ones towards the rear are fused - outputs to the purple circuit (horns, interior lights etc.), green circuit (fused ignition stuff like instruments, brake lights, reversing lights etc.) and on fuseboxes with four fuses red circuits (parking lights one fuse per side). The ones towards the front are the unfused supplies to the fusebox - brown (powered all the time), white or white/brown (powered with the ignition on), and red/green (powered with the lights on).

    It's also confusing as to why there can be two or more browns, whites or white/browns on the front of the fusebox. This is because the fusebox is being used as a branching point as an alternative to using a multi-way bullet connector with three or more wires. The power comes in on one of the wires, and goes out on the others as well as going through the fusebox. This happens elsewhere where there are two or more greens for example on a component - again one is bringing power in and the other is daisy-chaining it on to another component.

    Prior to 1977 a white wire from the ignition switch supplies power to the fusebox, and there can be anything from none to three other white wires at the fusebox. These other whites are feeding things the coil, fuel pump, overdrive and heated rear window at various times. But where there is only one white wire on the fusebox, the ignition switch is feeding a bullet connector by the bulkhead, and further wires in that bullet connector are then feeding the fusebox, coil and fuel pump. There are many subtle differences over the years, and you have to be looking at the right diagram for your car to work out what is going on for diagnostic purposes.

    In the case of the white/brown on 77 and later cars with the ignition relay, the feed is from the ignition relay to the fusebox, but after that there are differences between 77 and 79, with the change being made some time in 78.

    It started off with there being three other white/browns at the fusebox - one to the coil ballast, one to an in-line fuse for the cooling fan, and one to the usual bullet connector by the bulkhead for the fuel pump and overdrive - see this schematic.

    After the change there were only two other white/browns on the fusebox - one to the in-line fuse for the cooling fan, and the other to the bulkhead bullet connector. Now the coil ballast is fed by the ignition switch, but using a white/brown wire on the same ignition relay terminal as the white wire! This goes to the coil ballast then branches off to a new in-line fuse under the fusebox, which feeds things like the heater fan, indicators, and heated rear window - see this schematic.

    If that weren't enough both these in-line fuses have white/brown one side and green the other, making three separately fused green circuits in all. Not only that but the way the wires run and the fuse holders have been connected, it's possible to connect both the white/browns together, and both the greens together (as a new harness that came to me was), which makes things very confusing indeed.

      Fusebox mounting: June 2016 The first thing to say is make sure you fit the 4-fuse fusebox the right way up! These have a link between one end of two of the fuses, and this must go at the top and the front or when everything is connected up the ignition will be permanently powered.

    Pondering replacement of Vee's fusebox as well as Bee's I thought I'd take it off and have a look at the back. However the lower screw was seized and the cross-head slots stripped. Eventually I cut a slot across the head with a junior hacksaw (space limited for drilling given the V8) which got it turning with a large flat-blade screwdriver, until it sheared. Fortunately it did so leaving a stub long enough to get a Mole wrench on, and with the previous application of WD40 and Shock and Unlock it unscrewed, so a new screw needed - and that's where the interesting bit started, in that the 73 roadster and the 75 V8 have quite different mounting arrangements, let alone earlier and later models.

    The 73 roadster fusebox is screwed to a plinth, with nylon sockets pushed into it, and pan-head self-tappers.

    The 75 V8 fusebox is screwed to welded nuts on the inner wing, with a sheet of insulating material under the fusebox, then spacers between that and the wing. I'm not sure why the insulating sheet or the spacers were felt necessary, there is nothing loose under the fusebox and the electrical parts are spaced away from the base. The roadster doesn't have either, and the surface of the plinth is solid just like the inner wing.

    Moss Europe and MGOC say the screws are SE910201 (3/16" UNF x 3/4") up to chassis 456250 (Feb 78), with AB610081 (No.10 self-tapper pan head 1") and hence the nylon socket after that. Brown and Gammons says they are PMZ316 (3/16" UNF but 1" long) from 70 to 77, with AB610081 (as above) from 77 on. Note that my 73 has the self-tappers and nylon sockets despite the above saying these weren't used until 77 or 78, and whilst they are 1 1/4" from tip to top of the head, they project from the nylon socket by about 1/4", so a 1" should be OK. The screws on the V8 are 3/16" UNF, but they are 1 1/2" long rather than the 3/4" or 1" mentioned above. When fitted the end of the screw just reaches the far face of the nut so they are not over-size, a 1" would not allow the spacers to be used. Since a nylon socket was used in the 73 and avoids problems of corrosion either causing the screw to seize or the panel to rust, it seems odd that the 75 should have the less desirable arrangement. No mention of spacers or insulating sheets by the above three suppliers. The Parts Catalogue has no information at all on screws, nylon sockets, spacers, or insulating sheets.

      Fusebox replacement: May 2016 I decided to replace Bee's fusebox as prior to the MOT the horn seemed a bit iffy, the fuse holder springs were well tarnished, and it isn't easy to clean them. But the new one doesn't grip the fuses anywhere near as tightly as the old - I had to tweak the springs closer together, and the neither does the cover hold the spare fuses as tightly as the old. One of the spares does seem to be a little undersized and isn't gripped at all, but was held by the old cover. Never mind, I'll just use the old cover on the new fusebox ... only to find that the new fusebox is a couple of mm longer and the old covers won't fit! No 'Lucas' branding or part number on the cover, so obviously after-market despite the OE part number being used by suppliers.

    As mentioned above I decided to replace Vee's fusebox as well (at 10 from Moss it's not worth trying to clean up corrosion) but one of the screws sheared as they go into welded nuts on the back of the inner wing and hence are open to water and salt from the wheel arch, and none of the usual suspects show the correct screws i.e. 3/16" UNF x 1.5". But I got a pair of stainless from Stig Fasteners at 3.12 shipped.

      Fuses: As well as the two or four fuses in the fusebox there were a number of in-line fuses at various times. Unless otherwise stated these were always 15 amp rated,35 amp blow.

    • Firstly Mk2s prior to the 1970 model year had two in-line fuses where the rear harness joined the main harness for the parking lights - one 10A fuse for the front and one for the back. These were deleted at the fitting of the 4-fuse fusebox where the top two fuses are for the parking lights.
    • From 1971 to the end of chrome bumper production the so-called 'accessories' circuit - so-called because it powers the wipers and heater fan (and North American screen washer) as well as the radio - had an in-line fuse under the fusebox with white/green one side and green/pink the other. All V8s i.e. both chrome and rubber bumper had this system, which also powered the electric washers.
    • 1973 North American cars gained an anti-run system utilising an electric valve fused with an in-line under the fusebox.
    • The 1974 models gained hazard a flasher and these are powered with an in-line with brown wires both sides. Originally behind the centre console (with the flasher unit - hardly convenient). The fuse moved to under the fusebox on rubber bumper 4-cylinder cars, remaining behind the console on V8s.
    • For 1974 only North American spec cars had the sequential seat-belt system with its own 500mA line fuse with brown one side and brown/purple the other, location unknown.
    • 1977 models had a second separately fused ignition circuit from an in-line under the fusebox with white/brown one side and green the other.
    • At some time in 1978 RHD cars gained a third fused ignition circuit, from another inline under the fusebox and also having white/brown and green wires. Someone must have been having a laugh with these as the fuseholders and the wiring is such that you can connect the two white/browns together and the two greens together, which is how a new harness came to me.
    • One of these seems to have been replaced by a thermal cut-out - probably the cooling fan fuse. Clausager says that between January and March 1978 "Thermal cut-out switch instead of line fuse to eliminate fuseholders overheating". He indicates RHD as well as LHD, but I've not heard of them being found in the UK, just a couple of occurrences from America. As North America had twin fans but the UK 4-cylinder only a single it does make more sense if it were LHD only. Certainly the first fuseholder I fitted to the V8 (twin fans) melted in normal use.
    The tubular glass fuses used on the MGB usually show quite clearly when they are blown, especially if they have a slip of paper inside, and it can be confirmed with an ohmmeter. But it is not unknown for a bad connection to develop between the fuse wire or tape and the end cap, and appear to be sound. An ohmmeter may also show good continuity, but ohmmeters are not a reliable indication of a bad connection. The fuse needs to be in its holder, and the circuit powered and drawing current from one or more of the components it is supply, then to test both sides of the fuse with a voltmeter. There can also be bad connections at the fusebox between the end caps and the fuse holders, between the fuse holders and the spade terminals, and between the spade terminals and the spade connectors, so each of these points need testing as well, more info here.

    With the exception of those specified in the above list MGB fuses are glass 17 amp continuous/hold rated, 35 amp blow rated, GFS3035. The Parts Catalogue specifies 35 amp i.e. the blow rating, and some replacements are only described with one rating so you would have to be sure that relates to the blow rating and not the continuous rating or you could find that in a short-circuit situation the fuse wouldn't blow and the wiring would be damaged. Most of the usual MG suppliers only give the 35A rating, Moss in some places gives the continuous rating as well but in others only the blow rating. One source (who I refuse to use anyway) states 35A but shows 25A! It's claimed that generic American fuses only state the continuous rating, so you would need to use a 15 amp or 17 amp, not a 30 or 35 amp. Also these generic American fuses are said to be physically slightly longer than UK fuses at 1.25" as opposed to 1.17" so at first glance look the same and will fit the fusebox, but could cause a problem with in-line fuseholders. However US MG parts suppliers such as Moss, Victoria British and LBCarCo do supply the correct 17 amp continuous/35 amp blow fuses. Fuses can come with a slip of paper inside giving the rating, or it is screen-printed on the outside of the glass, or stamped into one of the end-caps. The first two can give either one or both values, the third one only one of the values.

    There has also been some unnecessary worriting about the voltage rating of MGB fuses. Automotive fuses seem to be rated for '32v', or 32 volts, even though our cars are 12v. This seems to be simply because some august body has decided every electrical component must have a voltage rating, and (presumably) because automotive applications don't usually go above 24v they have decided on 32v! An MGB owner was concerned that as his system was 12v, should he be fitting a lower rated 32v fuse instead of a 35 amp? The answer of course is 'no', amps are amps and depend on the voltage and resistance in the circuit they are testing, not some notional maximum safe voltage which is what the 32v represents. But even that notional safe voltage is ridiculous - voltage ratings are supposed to represent the maximum (plus a safety factor) voltage the fuse can break without arcing occurring between the end caps so allowing current to continue to flow. 250v fuses are half the length or less of MGB fuses, as are modern blade-type fuses. The concept of something higher than 32 volts jumping between the end caps of an MGB fuse when the fuse blows is ridiculous, even 20kv HT voltage wouldn't jump that, and 250v fuse are less than half the length. The bottom line is that as long as you fit a fuse with the correct current rating, ideally one specifically for British cars of the era i.e. 17 amp continuous/35 amp blow and not a modern generic item of a similar physical size that seem to be available in America, you will be fine.

    You would be well advised to add fuses to the fuel pump and overdrive circuits, as both these have been known to short out and cause major harness damage. See Pump Fusing and Overdrive Fusing.

    Blowing Fuses
    This can be a bit of a beggar, especially if it's intermittent. Even if it is constant you don't want to keep blowing fuses while you are diagnosing the problem, and I'll deal with this first. Temporarily replace the fuse with a high-wattage bulb. You can get away with a 21w indicator or brake light bulb, but a headlamp bulb (e.g. one with one blown filament but one good one) is better. Why not use a meter? If you used an ammeter, that has a very low resistance to current flow, which will still allow a very large current to flow. If in series with a fuse the fuse will still blow, if not it could damage wiring, or the meter. Solder a couple of wires to the bulb, and use it to replace the blowing fuse. Then while switching things on and off, waggling wires around, or parting and joining connectors, watch the bulb. If the bulb glows at full brightness you know the short-circuit is present. If it's out, or just dim, you know the short-circuit is not present. On any of the fused circuits if only one circuit at a time is powered the test bulb - especially if it is a headlamp bulb - won't glow at full brightness. It will get closer to full brightness if you turn all the circuits on the fuse on at once, like brake lights, reversing lights, heater fan, wipers etc., but not otherwise. The bulb is limiting the current that can flow to a safe level, and you aren't shelling-out for dozens of fuses. However don't think you can operate the car like this, as in the case of an intermittent fuse blow, as the bulb will be taking some of the voltage away from the circuits normally fed by the fuse, so either they won't work properly or they won't work at all. It's simply a diagnostic tool. The fault could be on the wire leading from your test bulb, in which case the bulb will be bright even though all circuits fed by the fuse might be switched off. In this case you will have to study the circuit diagrams and work out where the branches are, i.e. at bullet connectors, so you can isolate various branches to isolate the offending one. Unfortunately while pulling the wiring about to find, disconnect and reconnect these bullets you may well cause the fault to disappear, to appear again at the most inconvenient and inopportune moment. If the test bulb only goes bright when a particular circuit is switched on, then the fault is between that switch and the component it is powering - which should be easier to find.

    For intermittent fuse blowing you have to be a bit cleverer. Get a couple of in-line fuseholders, with, say, 10 amp fuses in them, and use those to subdivide the fused circuit into separate sub-sections. It's then a matter of waiting until a fuse blows. As the factory fuses should all be 35 amp blow, your 10 amp sub-section fuse should blow first, leaving the main fuse intact. This does mean you will have to replace sub-section fuses as you go, but it's about the only way if waggling wires with the test bulb as above doesn't help by bringing the short on. If there is more than one spade used on the fused side of the fusebox (as is often the case), you can put one or more sub-section fuses on each of those first of all. Then by seeing which circuits work and which don't when the sub-section fuse is blown, and consulting the diagrams, you should be able to work out which 'branch' of the circuit has the problem, and so which parts of the branch to move the sub-section fuses to next, i.e. at bullet connectors. There are quite a few branches at bullet connectors in the green circuit, however some parts are daisy-chained, with two green wires in a single spade connector, meaning your sub-section fuse can isolate just one component or circuit at a time, and not a group of them. Hopefully, short of accident damage, only one circuit will be the cause, and it doesn't happen very often anyway.

    Heated Rear Window

    Adding a Relay

    Before the 1973 model year the heated rear window was always optional. Prior to 1971 an additional wire was run in to the front of the car. For Mk1 North American spec and prior to the 1972 model year elsewhere, a switch and warning light was provided on a separate small panel somewhere. For Mk2 North American spec and 1972-on for other markets, the switch moved to the centre console. From 1971 the wiring for the HRW seems to have been part of both main and rear harnesses (including roadster main harnesses, it not being worth producing a separate harness minus that one relatively short wire), even though prior to 1973 the HRW was still optional on all cars. From 1973 on HRW was standard on UK cars. In December 1974 production of the GT for North America ceased, and for other LHD markets in June 1976.

    For most of the time there seems to have been a white tell-tale warning light associated with the switch, integral with pull-on switches, beside or above it with toggle/rocker switches. Mk2 non-North American cars had a pull-on switch until 1971 changing to separate switch and warning light in 1972, Mk2 North American cars had separate switch and warning light until 1973 when it changed to a pull-on type with integral warning lamp. Clausager says that for the 1977 model year until December 77 the switch had a built-in warning lamp - quite why is unknown as there was a blank position beside it. In December 77 the switch was changed to one with an external warning lamp - beside the switch, but the diagrams always show this switch with an external warning lamp. In 1980 the switch was changed back to one with an internal warning lamp, as there was now a rear fog lamp switch which took the place of the external warning lamp.

    Up to 1970 these were powered off the white circuit via their own in-line fuse near the fusebox, the remainder of chrome-bumper cars were powered from the green circuit, sharing this with many other components.

    Rubber-bumper cars up to December 1977 (but not V8s) had a dedicated relay and powered the HRW from the purple (fused) circuit. After that it reverted to no relay and was powered from the green circuit again as there was now an ignition relay which took the load off the ignition switch. However that was only for one year, apparently there were problems with the relays sticking on and leaving all the ignition powered stuff running even with the key in your hand. So some time in 1978 on RHD cars things like the ignition, heated rear window, indicators and heater fan were moved back to the ignition switch, but the fuel pump was still left on the relay, see the ignition schematics for more info. At that point the HRW (plus indicators, heater fan and tach) were powered from one of two inline fuses under the fusebox that connected a white/brown wire to a green wire. One of these has thick wires but this is for the cooling fan, the HRW etc. are powered from the other one with the thinner wires.

    When powered from the main green circuit not only does this have a tortuous route through many components and connectors in the brown, white and green circuits but because of the very heavy drain of the HRW it reduces the voltage to these other components and results in a low voltage at the rear screen, only about 7 volts in my case. Most of the other circuits aren't that bothered by the lower voltage but the indicators are very sensitive to it and use of the HRW can stop them flashing at idle if headlights, fans etc are also on. Whilst this is usually due to one or more (probably several) bad connections, tired flasher unit, tired bulbs etc. even good connections result in low voltage at the rear screen. My flashers didn't stop with the use of the HRW but they did slow down so I decided to add a relay to remove the load from the green circuit and boost the voltage to the HRW at the same time

    Updated September 2015: On my 75 V8 the window element measures about 1 ohm at the contacts on the sides of the glass, so from Ohm's Law with the engine running and the system voltage at about 14v one could expect about 14 amps to flow in the circuit. However as mentioned above taking its voltage from the green circuit, the long run, and the ageing connectors I was getting about half that at the HRW connections, the rest being 'lost' en route.


    There often questions about the wiring and connections to the HRW on the tailgate itself. There were two types of HRW - the earlier embedded wire element type and the later surface-printed element type, I can only speak for the latter. On my V8 the wires exit from a hole in the rubber seal surrounding the glass very near the hinges, and terminate in bullets. The wires run down under the seal to the element connection points which are about mid-way down each side. The rubber seal is pretty hard to lift, and I don't want to damage the connections, so I've been unable to determine what lies beneath, but word is that it is a small spade connector (October 2011: it is, see here). You should be able to test at these points with a voltmeter to see if non-functioning of the HRW is due to a break in a wire feeding the element, or a problem with the elements themselves.

    Note that due to the high current drawn by a working screen some voltage drop in the long wiring run and connections from the front of the car is inevitable, and will result in something less than full system voltage being seen at the screen spade contacts. Bad connections will result in significantly higher volt-drops leaving progressively less voltage to be measured at the screen spade contacts. But the more horizontal tracks that have failed the lower the volt-drop will be, and if the screen itself has completely failed (as opposed to just one or two tracks), or the earth connection on the other side of the screen is missing, you will see full system voltage at the right-hand spade connection with respect to a good body earth.

    As well as the connections at the front of the car, and above the rear cant rail, there is another connection by the right-hand rear light where a 2-wire sub-harness with white/black (HRW) and purple (load-space light) joins the main rear harness.

      Note also that any bad connections in the earth connection will result in something higher than zero volts being measured at the spade connection in the middle of the left-hand side of the screen, with respect to a good body earth, as well as a higher voltage than normal at the spade connectors on the right-hand side.

    For the earlier embedded element type that's about all you can do, but for the later surface-printed type you can go further, and should be able to measure voltage anywhere along the tracks. Connect the negative of your meter to a good body earth, then use the positive probe lightly (to prevent damage) on the exposed surface of the track. Right by the spade connectors at the side of the glass in the middle you should see the full voltage that is reaching the screen.

    Assuming the screen is working, testing at various points up and down the main track at the right-hand side of the screen will show a small drop in voltage as you move away from the spade connections in the middle, in the order of a couple of tenths of a volt at the ends of the vertical track.

    Moving along any horizontal track should show a gradual reduction from the voltage measured above, towards zero (but see above), being about half the voltage in the middle. Note that if you have a broken track, the voltage will NOT gradually reduce along that track, but will suddenly drop to zero as you cross the break.

      Adding a Relay:

    A added a fused relay direct off the brown circuit and that increased the voltage to 10 volts at the connections to the harness under the rear cant rail, 8v at the element contacts, and a measured 8 amps. Despite that being quite a bit less than the theoretical 14v it's still about 80 watts and has made a significant improvement to screen clearing.

    It is convenient to interrupt the white/black at the bullet connector where the main harness joins the rear harness near where the firewall joins the right-hand inner wing. Mount the relay near the fusebox so there is a short run of thick brown wire between the two. Use a relay with an integral fuse or an in-line fuse close to the fusebox. Pick up the earth from the physical mounting, then run two wires from the relay - one to the existing bullet connector still on one of the wires and a new connector on the other. It would be preferable to use the new one on the wire to the rear harness as that carries the greatest current, and clean up the bullets. You could add a thick purple back to the fusebox instead of a fused relay or separate in-line fuse, although I used a brown as I was not aware of the factory relay arrangement at the time. Also make sure the connectors and earth at the back of the car are clean and sound.

    The ignition, via the HRW switch as before, controls the relay which draws a very low current whereas the high current is carried by the relay and the short run of thick wire back to the purple (or brown) at the fusebox, and ensures that the HRW is only powered when the ignition is on. This increased the voltage at the rear screen from about 7 volts to about 10 volts. If you mount and insert the relay at the connector at the back of the car and run the thick brown direct to the battery you can get an even higher voltage, but even with my arrangement the screen clears noticeably quicker than before.


    Peter Ugle reports that after trying various repair paints for the surface tracks without much success he obtained a custom-made replacement kit from DS Demist which works really well. The link I had for DS-Demist now displays a web page that will infect your computer if you click anything on it, and you have to force close the browser or shut down the computer. On no account click anything on a page like that. The same link is displayed on Citroen DS car sites, so take care there as well. April 2016: One such paint is from Bare Conductive, but from their data sheet you can see a 70mm strip 3mm wide has a resistance of 473 ohms. Now this is much longer than one would hopefully need to repair a track, but even if it were only 1mm long it would have a resistance of nearly 7 ohms. And if one kept to the track width of about 0.75mm it goes up to 28 ohms, although increasing the thickness will reduce it. As my screen measures about 1 ohm, which represents 9 elements of 9 ohms each, you can see that even one 'repair' is going to leave the track virtually useless for screen clearing. There is also this HRW kit from Holdens, which consists of lengths of self-adhesive foil that can be cut into narrow strips and stuck to the glass. Expensive at 60, it's intended for providing full HRW rather than a repair, and I don't think I could reasonably cut anything as narrow as my strips, and there is the problem of connecting the ends to the existing connectors. They say it is suitable for front screens as well, but getting it thin enough not to cause annoying if not dangerous obstructions would be difficult to say the least.

    August 2016: The above paint is not aimed at HRWs, but another specifically for HRWs is Granville Electro Connector. They couldn't tell me the resistance of a typical bead of product, saying a typical repair is a 'thin bead to a preferred short length of 3-5mm although it is possible to mend longer breaks'. At the time of writing the typical price from Amazon and eBay is in the order of 15, but amazingly Halfords have it for 11.49! Mostly negative reviews, but one gave a detailed description of how he made a successful repair, so I thought it was worth a punt. The ZS has lost an element, testing with a meter located the fault but there is no visible break, so hopefully it is hairline and stands a chance of repair by bridging it. It took three goes ordering online before they managed to find it in the store, by which time they had reduced the price to about a tenner for my trouble! A single-coat test section about 1mm long and the same wide exhibits a resistance of about 1 ohm, which is significantly less than the other product. I had to wait until the weather was warmer and dryer before I could apply it, then wait for damp weather before I could test it! It didn't work, and despite re-testing and finding the break right at the end of the repair, and applying a longer repair, it still didn't work. Annoying, as I hate seeing that dead track in my rear-view mirror. Even more annoying is a second dead track a year later.

    I also found this Loctite 'Rear Window Defogger Tab Adhesive', and a couple of other repair possibilities from Permatex.

    March 2019: Graham Moore bought an unused NOS screen that was 24 years old. However it measured about three times the resistance mine does and two of the tracks were defective, when tested using steam from a kettle. Close examination shows copper oxide on parts of the tracks, i.e. corrosion, so I suspect the tracks have 'thinned' making them higher resistance than they should be, to the point of going open-circuit altogether on the two defective tracks.

    As well as the usual considerations for front and rear screen glass replacement there are the electrical considerations for the HRW. The original has two small spades on each side of the element, one pointing up and one pointing down, concealed under the rubber seal. Wires run from these under the rubber seal to exit at the top near the hinges, for connection to the vehicle wiring, which is very unobtrusive. However replacement screens seem to have a completely different connection arrangement that is very much less than ideal.

    There is only one spade connection each side of the element which is fair enough, but it protrudes across the face of the glass in line with the elements, quite a long way. As the wire connector has to push onto this spade, the wire will protrude even further, and be quite unsightly. Anything catching on the wires could well rip the connection off the glass, and you would be back where you started. It might be possible to bend the spade back on itself so it is pointing at the seal instead and the wire could come out from the seal straight onto the spade, but great care would be need to hold the 'glass' end of the spade steady while you did so. Probably the best bet is to use a right-angle wiring connector instead, the wire making another right-angle from that to go under the seal. You will still need to take great care when pushing the wiring connector onto the spade, I'd advise trying it on another spade elsewhere several times to make sure it slides on easily.

    But even better - and brave - is Joshua Taylor's approach which was to bend the spade over (while keeping the glass-end clamped to the screen top prevent it breaking off!) back on itself so the connector was under the seal.

      Horns Updated September 2013

    Wheel-centre Horn Push
    Fault Diagnosis
    Adding a Relay


    Originally the horn push put out an earth from the steering column to one side of the horns, the other side being wired back to the purple fuse, these are known as '2-wire horns'.

    For the 1979 model year on the horn push was fed with 12v from the purple fuse, and thence to one side of the horns, the horns picking up a local earth from their physical mountings. These are known as '1-wire horns' and as well as saving about 3 feet of wire they eliminate the problem of the poor earth connection through the steering column mentioned below. There is conflicting information from various sources as to when this change occurred - all rubber bumper cars; the start of the 77 model year; a few months into the 77 model year; but Clausager dates it to chassis number 471001 in May 78 for the start of the 79 model year.

    Also originally there was only one pair of wires for the horn as the second horn was optional. This came out by the right-hand headlight and went to the horn mounted on a flat bracket on the slam panel. This harness also only had one set of bullets for the front lights, which reached the middle of the grille, the tails from the light clusters reaching that point also. If you had the optional second horn there was a sub-harness between the two. However it seems that the indicator and headlamp tails currently available have shorter tails for the later harness that went right across the car, so you will need to make up extenders to reach the to the middle for the earlier harness.

    Clausager says the horns moved to the inner wing in 1963, and that from 1970 'all cars now had' twin horns, implying that some markets e.g. North America may have had them earlier. I've not found that itemised in his book, but the Workshop Manual schematics do show twin horns for North America with the start of the Mk2 in 1969.

    Note that with the early harness where a sub-harness is needed to power the second horn, you need a horn with two sets of spades on each terminal for the right-hand horn at least, these horns were BHA4514 and BHA4515. Later horns only have one spade per terminal, so there is nowhere to connect the sub-harness. On cars with twin horns from the factory the wiring to the right hand horn has two wires in each spade connector, so horns with only one spade per terminal can be used. You can use a male-male-female spade adapter on a later horn with the early harness.

    Originally the horn push was in the centre of the steering wheel. For North American MkII models and in 1970 for other markets it moved to the end of the indicator column stalk but was unpopular and reverted to the centre of the wheel for all markets in 1971. It remained there until the 77 model year when all markets moved back to the indicator column stalk until the end of production. (NB. Either arrangement is far preferable to that on the ZS, which has two little buttons at the edge of the large centre boss. This means that not only are they several inches away from fingers and thumbs when holding the wheel in the correct '10 to 2' position, but the buttons also move position as the wheel is turned. With their small size and changing position you have to look to see where they are before you can sound the horn, and you need to use a finger-tip rather than the palm of your hand, hardly ideal when you need to give an urgent warning of approach!

      Wheel-centre horn push: Updated October 2009

    The various arrangements for connecting to the wheel-centre horn contacts can be seen by clicking the thumbnails to the left. Note that North American Mk2s, and other markets for the 1970 model year, moved the horn push to a column stalk. But this was not liked and it moved back to the horn centre for all markets for the 1971 model year. However it moved to a column stalk again for the 77 model year to the end of production.

    The sprung wire (69 and earlier) and 'pencil'/sprung rod (71 to 76) that connects to the live side of the horn contact are often described as a 'brush' but that is incorrect. A brush is something that typically provides a rubbing contact between a fixed component and a rotating one, e.g. on the commutator of a dynamo or motor or the slip-rings of an alternator. Up to 76 MGB centre-push horns do have a brush, but it is a contact fixed to the column that as the wheel and column turns either rubs on an insulated brass cylinder on the column (69 and earlier) or on a slip-ring on the back of the wheel (71 to 76). This brass cylinder and slip-ring are connected to the sprung wire or pencil, which then connects to the horn push. Some think that the pencil rubs on the back of the brass ring that is on the rear face of the wheel as it is turned, but a moments thought and a turn of the wheel with the horn push removed will reveal that the wheel, sprung wire/pencil, and horn push all rotate as a unit. The sprung wire and the pencil simply sit between the horn contact and the column or wheel slip ring, under spring tension, but rubbing against neither. They are spring-loaded to absorb the movement of the wheel centre when the horn is sounded, as well as press on the slip-ring and horn switch to make a good connection between them. The pencil in particular seems quite a complicated component for what it does, the same thing could have been achieved (without detachability) by soldering a wire between the slip-ring and the horn contact. Detachability could have been achieved by having a pull-apart connector in this wire. Some after-market wheels dispense with the instant detachability of the factory 71-76 horn-push. The Moto-Lita wheel on my V8 has a wire soldered to the back of the slip-ring which connects to a threaded stud on the back of the wheel centre with a nut, removing this nut allows the horn-push to be detached from the wheel, and the fitting of pushes with different logos as required. The 71 to 76 pencils can be fitted either way round and the horn will function, but the correct way round is with the long hex brass rod pointing at the slip-ring on the back of the wheel and the end with the insulating sleeve facing the horn push. This way round means that the brass ends of the pencil can't come into contact with the frame of the horn push, which is at earth or earth potential (and would sound the horn) when any of the springs are touching the wheel, when refitting the push to the wheel or if the push is rotated once fitted. The factory horn push has four bosses which fit between the heads of the bolts securing the rim and spokes to the hub, and these only allow the horn button to be rotated a small amount in either direction which in practice keeps the pencil away from anything at earth potential even if it has been fitted the wrong way round, as well as keeping the logo centralised (once fitted correctly in the first place).

    April 2019:
    I discovered during a very rare need to sound the roadster horn that it needed to be pushed in a certain way, whereas pushing anywhere on its periphery (tilting it) or pushing it straight down should work, so an opportunity to inspect it. Three screws connect the earthed base to the rubber push via springs, and another screw attaches the 'live' contact to the rubber push, and both that and the copper ring round the hole in the earthed centre were pretty manky. A bit of fine wire wool cleaned both up, and reassembled and refitted it now works as it should.

    August 2019: At least I thought it had. Something made me try it while driving, and again some positions worked and some didn't. Back home I tested the horn push with an ohmmeter (not ideal) and all the way round it was about a couple of ohms which is nothing with the relay I have fitted. That relay is powered from the purple circuit behind the dash i.e. for the courtesy lights and headlamp flasher, whereas I discovered just the other day that of the three purple wires at the fusebox, two are standard-size in one spade connector and power the courtesy/boot lights and headlamp flasher, and a thicker wire is in it's own spade connector and goes direct to the horns. With that wire pulled off and the other two connected I realised I could hear the relay operating without the horns blaring out. And working round the edge of the horn push I could tell that the relay was operating better in some positions than others, and it varied if I turned the wheel a little. So this time use a voltmeter on the horn brush behind the wheel (after removing the cowl) and find the 12v is only dropping to 2 or 3v instead of zero volts. So off comes the wheel and I clean up the brass slip-ring on the back - which is pretty manky - with Solvol Autosol, and the stud on the brush. Peering into the horn-push pencil hole the back of the slip-ring looked pretty manky as well, so removed the slip-ring and its carrier from the wheel, and bent back the three tabs to remove the brass ring from the carrier. Tried cleaning that with Solvol Autosol but it is pretty pitted from having carried horn current for many years before I fitted a relay. So polished up another section and refitted it to the carrier rotated 120 degrees to use a 'new' area. At 17 for a new slip-ring (BHA5042) it was worth a few minutes of my time. Fitted the carrier to the wheel (has to go in one of two possible ways for the pencil holes to line up), the wheel to the column, and the horn-push to the wheel, tested the voltage on the brush and now it's dropping to zero volts all the way round and the relay sounds much 'stronger'. Checked the headlamp flasher and indicators to make sure I hadn't disturbed anything else, tightened the steering wheel nut and refitted the cowl - all hunky-dory now ... until the next time maybe.

      Horn Mounting September 2013

    There were five different types of horns over the years and two different brackets. Originally a flat bracket (57H5309) mounted the horns vertically on the edge of the slam panel, before changing to an angled bracket (originally 17H8641, now GGE110) basically identical to the earlier flat bracket but with the bend, on the inner wing in January 1963 which positioned them horizontally. Also originally the harness only had one horn tail and there was a sub-harness to extend the wires across to the other side if you had the optional second horn. In late 69 dual horns and hence two tails in the main harness became standard. Both locations - when used with the appropriate brackets - position the horns horizontally, but going by the illustration in the Leyland Parts Catalogue you have very little choice about orientation - they have to point forwards or the spades will foul the top of the bracket. It's true that being horizontal water and dirt can't fall down inside the trumpet, but it will still be driven inside. However at least two horn types have the spades in a different position and so allow the horns to point across the car towards each other rather than forwards. Vee's are like this and it protects them to some extent from driving rain and dirt, whereas the position of the spades on at least one other type means they have to point forwards for the terminals to be accessible.

    I bought new horns for Bee (why I forget) and as the existing ones didn't have brackets I bolted them direct to the inner wings as before, which positioned them vertically instead of horizontally. Originally I fitted them facing backwards to keep water and dirt out, and with the spades uppermost for accessibility. But they were never as loud as the ones I subsequently fitted to Vee (which needed a relay from the start or they didn't work at all), and thinking this was partly due to them facing backwards I turned them round to face forwards, which meant to keep the spades at the top again for accessibility they changed sides. Several years later and the horns seem to be getting worse if anything, so I do the voltage tests indicated below and find that whilst I'm only losing about 0.5v in the purple feed I'm losing about 3v in the earth feed i.e. the tortuous path back through the rack and steering column to the wheel centre button, so fitted a relay to Bee as well and that made a huge difference.

    I had wondered whether I could 'tune' the horns to be louder, some have an adjuster screw and locknut in the centre of one side, so took one of mine off. Nothing as simple as a screw and locknut, just a plastic stud under a rubber cover, so I decided to leave that alone rather than risk damage. However while I was turning the horn over I became aware of a rustling noise, and when I tapped it on the bench all sorts of rubbish started falling out! Much tapping, shaking, turning, and poking a length of stiff copper wire up the trumpet of both horns extracted quite a pile of dead flies and debris that wouldn't have helped at all.

    I then started thinking about the orientation of the trumpet, which curves around the edge of the horn body. My dual Mixo horns i.e. one high note and one low note are mirror images of each other. The construction is such that installed as mine were facing forwards with the spades uppermost, the curve of the trumpet is downwards and so any water, dirt, dead flies etc. that find their way in will remain lodged inside. They need to be mounted such that the curve of the trumpet points upwards, with the trumpet itself angled downwards to some extent, and both aspects will naturally resist stuff going in and getting stuck. The downside is that the spades now point downwards, but one can't have everything. The upshot is that my low note Mixo has to be installed on the left as you look at the front of the car, and the high note on the right.

      Fault Diagnosis

    Two-wire horns:
  • If the fuse is blowing or the horns sound continuously with the purple/black connection removed check for continuity between the horn terminals and the body. Any continuity here is a fault, as Steve Penkethman found in June 2021 with his 1970 which was blowing fuses but only when mounted.
  • Check the voltage on the purple at the horns with the horn button both released and operated. If there is no 12v at all, or you see 12v with the button released but significantly less with the button pushed, then check the purple fuse (bottom in the 2- or 4-way fusebox) and the wiring back to it for broken or bad connections.
  • Check the voltage on the purple/black with the horn-push released. If you don't see 12v at all then the horn itself is bad.
  • If you see 12v then measure again with the button pushed. This should drop to almost 0 volts (earth). If it does but the horn doesn't sound then again the horn is bad.
  • If the voltage doesn't drop, or doesn't drop very far, check the wiring back towards the horn button for broken or bad connections, and test on the connector going to the brush, the brush itself, and the brass ring on the back of the wheel. If you get the same 'less than zero' voltage at the body of the wheel then the column earth is bad, fit an earth strap or relay.
  • (Updated May 2008)If the wheel shows zero volts, but the voltage on the brass ring on the back of the wheel is higher than this, then there is a problem inside the wheel and horn push. There are several possible places this could happen - the pressure contact between the back of the brass ring on the wheel and one end of the pencil, the braided wire between the two brass ends of the pencil, the pressure contact between the end of the pencil and the brass contact inside the horn push, the connection between the brass contact and the copper ring attached to the horn push frame (this is the operative part of the horn switch), the copper ring where it is attached to the horn push frame, where the springs are attached to the horn push frame, and these springs and the wheel when the push is fitted. Unless these springs have been bent and are slack this last is very unlikely unless the wheel is badly corroded.

    One-wire horns: Check the voltage on the purple/black spade with the horn button pushed. If there is no 12v or something less than 12v check the wiring and connectors back to the horn button for broken and bad connections the horn button itself, and 12v on the purple wire feeding the button. If there is a good 12v on the purple/black measure the voltage on the horn body. If you see more than 0v then the horn earth is bad. If you see 0v (earth) then the horn is bad.

  •   Repair September 2014

    This horn is from a TR4, but the principle should be the same for the MGB as the originals were electro-mechanical as this is. This horn was completely dead, but another fault could be that it just clicks when power is applied. Both could be down to adjustment, so before opening the horn up try twiddling the adjuster screw that is usually present. On this horn just a quarter-turn of the screw is enough to change the fault from 'completely dead', through sounding to some degree, to 'just clicking'. If adjusting the screw doesn't bring it back to sounding normally then you might as well open it up and see if it can be fixed, you have nothing to lose. Note that if it just clicks, leaving the power connected for any length of time will probably burn the internal coils out, the horn will get very hot in the process.

    The casing is usually in two halves, with the 'trumpet' in one half and the active stuff in the other, with a diaphragm clamped between them. The two halves are usually riveted together, in this case with six rivets, one side usually being easier to get at all six than the other as some may be in the mouth of the trumpet. Use a drill the next size up from the head of the rivet and drill the head off - it may be easier to start with a small drill to drill a pilot hole part way through first, then use a nail punch to punch the remainder of the rivet out. If the two halves haven't parted by now, carefully lever them. This horn had a paper gasket either side of the diaphragm, you may be able to separate the halves and remove the diaphragm without ripping the gasket as I did, if not it's no big deal to cut new ones out of thin paper.

    Inside you should find a couple of electromagnet coils, and some kind of interrupter contact, in series with the two spades (or the single spade and the body of the horn in the case of 77 and later MGB horns). The principle of operation is that current flows through the coils and attracts the diaphragm towards it. This diaphragm has a 'crinkle' in it, which means that rather than it moving gradually as the magnetic field builds up (electromagnet coils are effectively inductors and there is a finite period over which the magnetic field builds to its maximum, it is not instantaneous) the field has to reach a certain level before the diaphragm starts to move, then it moves suddenly as the force overcomes the resistance of the crinkle. This is exactly the principle used in the 'D-Day Cricket' used by paratroopers in WW2 to tell friend from foe when they couldn't see someone, rather than risk showing themselves. The crinkle makes the diaphragm move further and faster with a snap action than it otherwise would, which greatly amplifies the sound over a simple flat diaphragm. However, unlike the Cricket, as soon as the diaphragm has moved it's operating pin moves a lever which opens a contact, which breaks the electrical circuit through the coils, so the diaphragm is released, again with a snap action. As soon as it has released the diaphragm the operating pin releases the lever, so the contacts close again, re-energising the electromagnet, attracting the diaphragm again, and so on, vibrating the diaphragm.

    The adjuster screw acts on the assembly that holds the contacts, moving the contact lever closer to or further away from the operating pin of the diaphragm, to get the most effective movement of the diaphragm, and hence the loudest noise! Whilst there is some pitch change as adjustment is made, the primary difference between high-note and low-note horns is in other aspects of the design.

    With the innards exposed the first thing to do is check the continuity of the coils, because if one of those is open-circuit you may not be able to go any further, it should be easy to see where the ends of the coils are terminated. If those test OK - typically 4 or 5 ohms - then test the continuity of the contact. This one was open-circuit, possibly through oxidisation during a long restoration of the car. A little bit of wiggling and manually opening and closing the contact was enough to restore continuity in this case.

    The only real way to test is to clamp the two halves together again, as the diaphragm has to be held firmly and at the correct distance from the coils and the contact lever in order to function properly. I used three nuts and bolts in case I had to take it apart again, although I intended to re-rivet when I was happy with the repair. This is when I discovered just how small the operating range of the adjuster screw is - just a quarter turn. With that I was happy with the sound, so fitted three pop-rivets and backing-washers in the so-far unused holes before removing the three temporary nuts and bolts, as I didn't want to disturb the alignment of the two halves and the diaphragm, then fitted the remaining three rivets and backing-washers. Retested, final tweak of the adjuster screw, and returned it to the very satisfied owner.

      Adding a Relay: One fairly common problem with the earlier 2-wire horns, particularly with collapsible columns, is a weak or non-operating horn even all the right voltages seem to be present. This is sometimes caused by a bad earth to the column itself - it relies on its mechanical fixings between the chassis rails, crossmember, rack, UJ and the upper and lower halves of the later collapsible steering columns for this and not a dedicated earth wire. I've even had one where the bad connection was where the UJ clamped onto the shaft at one of the splined joints! This earth path is only a problem with the earlier 2-wire horns prior to 1977, but not on cars with the horn push on the column stalk (1970 model year) as these have a wired earth. I used a relay to 'boost' the earth to operate the horns, but I have heard of others connecting an earth-strap between chassis and rack. Before going to the bother of adding a relay or earth strap check the other connections first.

    Note that this circuit will only get round a poor earth from the column through the switch and onto the purple/black. I opted to install the relay as the column on my V8 had a low-grade earth that would not even operate one horn (I wondered why it came to me with what looked like a moped horn!)let alone two. I mounted it behind the dash on the firewall close by the indicator flasher and voltage stabiliser where all the wires that are needed are close by (on an RHD, at least). The purple/black is cut between the column multi-way connector and the main harness, and will need short extensions to the relay, I opted to use a 2-way 'chocolate block' connector rather than solder bullets or spades. Inside the cabin is also a better environment than under the bonnet. Subsequently when I found the roadster would benefit from a relay as well as it was losing about 3v in the earth path, to avoid cutting this wire I did think about fitting it behind the radiator diaphragm, diverting and extending the purple/black from the right-hand horn back to the relay winding, then extending the relay contact to the horn. However that would only have helped the right-hand horn, I would need to extend the new purple/black to the left-hand horn to benefit both. So I decided to put that one in the Bee's cabin as well.

    February 2021:
    Even more subsequently I realised that for 1970 to 76 inclusive there is a spade connection to the horn brush under the cowl, so with a relay by the column the existing wire can be removed and extended down to the relay contact, and a new wire run from the brush to the relay coil, although on CB cars at least the cowl is pretty snug round the column and harness.

    The relay spades are shown with modern markings and the diagram also shows which pair are the winding and which are the contacts. If you use an older-style Lucas relay the 'W1' and 'W2' spades are the winding and the 'C1' and 'C2' are the contacts. It doesn't matter (on either type of relay) if you reverse the winding pair or reverse the contacts pair, as long as you don't get any of the winding wires on the contact spades and vice-versa.

    The relay is operated from the earth from the horn-push and 12v from the purple (the purple is always hot and fused for safety) and will operate reliably even with quite large resistances in the horn-push circuitry. The contacts push out a good earth, taken from a tag secured under the relay lug, onto the purple/black to the horns themselves.

    Footnote: Some time later I decided to see just where the high resistance connection on Vee was and the results were interesting. I was losing 0.5v between the body and the outer column despite all the fixing bolts, and another 0.5v between the outer column and the inner column. But the greatest loss was inside the horn button itself. As this was a Moto-Lita wheel and the two halves of the switch casing were held together with spire clips on three small plastic pins that always break when you try to remove them, discretion was the better part of valour. Given that, there didn't seem much point in making a better connection between the outer column and the body, even less trying to fabricate another brush to get a good connection between outer and inner columns. The relay has been working perfectly well for a number of years so I left well alone. Subsequently I replaced the Moto-Lita wheel with an original but left the relay in place.

    September 2013: Bee's horns have never seemed as loud as Vee's with the relay, and yet more testing showed an iffy earth through the column. So without any more messing about I fitted a relay to Bee as well, and a noticeable improvement.

      August 2014: I've been helping a pal finish off the restoration of a TR3 and one of the last things is to deal with some electrical problems to get it ready for the MOT. One of those is the horns not working - "should be easy" I thought. The principle is the same as on the MGB i.e. an earth up the column, through the horn button in the centre of the wheel, out to the horns, then back via a fuse to the 12v supply. The TR3 has two steering column UJs, and they are rubber doughnuts, so there is an earthing wire from one yoke to the other, around the doughnut. There is also an earthing wire going direct from the rack to the chassis, even though the rack is bolted to brackets on the chassis and not a removable cross-member like in the MGB. The rack earth wire was broken, the lower UJ earth wire seemed to be missing altogether, and the upper one was iffy being a bodge of wire strands wrapped round the UJ clamp bolt. First job was to replace the rack and lower UJ earth wires. Still didn't work. Must be that iffy upper earth wire, but did some testing, and to our amazement the earth wire was fine, the fault was where the upper yoke was clamped onto the steering column at the splined joint! Removed both UJ clamp bolts with the intention of tapping the yokes up and down the shafts to clean the splines, but the upper one didn't move. Nothing for it but to remove the UJ and the column. Four bolts and the doughnut comes out, and we found bolt-through terminals under one bolt-head each side - for the original but missing earth wire! We'd put the earth wire on the lower UJ between the two UJ clamp bolt heads, as the bodged wire on the upper UJ had been, thinking that was the correct position, but we aren't going to move the lower one now. Also the four yoke bolts screw into the opposite yoke, no nuts, but have locking-wire through the ends of the bolts. There is also a weird clamp with two bolts and an Allen screw and lock-nut on the column shaft. Another difference to the MGB is there is no outer tube as part of the column, it is part of the bulkhead. So the column needs to be removed via the engine compartment as the yoke was stuck on the lower end, but it can only go forwards along the line of the column as the outer tube is fixed. Would there be enough room? We removed the steering wheel, but didn't get very far as something was stopping it going forwards, possibly the indicator cancelling cam hitting the fixed outer tube. But then we found that the column was in two halves - a long section that withdrew into the cabin leaving just a short section to withdraw into the engine compartment, so easier than feared. It took some pounding with hammer and drift to get the shaft out of the yoke, so we could clean up the splines with wire brushes. With it all apart we could see that the clamp with the Allen screw clamps the two halves of the column together, a) to get a good earth going all the way up, b) to position the upper part and the steering wheel correctly in relation to the indicator switch cowl, and c) to set the fore and aft free play of the column shaft in the outer tube that is part of the bulkhead. The clamp with the two bolts goes around the upper half of the column, which has the lower half of the column sliding inside it, and the Allen screw tightens through a slot in the upper column onto the lower column. We weren't sure if the Allen screw had been adjusted correctly before so went to slacken the lock-nut but it was stuck fast, and needed heat before it would shift.

    After that it was 'just' a matter of putting it all back together again. I'd decided to fit the new earth wire to where we had found the original bolt-through terminals, and not where the bodge had been or where we had put it on the lower UJ. That meant the wire could run through the middle of the doughnut rather than being round the outside, and so not scrape on the shelf or various tubes and pipes nearby. Also when removing the UJ it was apparent that because the column and intermediate rack shaft were not directly in line, the doughnut would have to be 'bent' to get at least two of the bolts to line up correctly with the opposite yoke when refitting it. So I decided to assemble the UJ off the shafts, but even that needed the doughnut to be squashed a bit in a vice to get the fourth bolt started. By leaving the Allen screw clamp off I was able to slide the lower half of the column up out of the way enough to get the two sets of splines engaged, then fitted the Allen clamp, setting the fore and aft free-play in the process. Time for a test ... and we have a horn! Actually we should have two as there are twin horns, but one isn't working. More voltage tests on the horn spades and it is apparent that horn itself is faulty, but one is good enough for the MOT. That's taken all morning, so the wipers will have to wait for another day.



    Fuel Gauge
    Tank Sender
    Instrument Voltage Stabiliser
    Calibrating the gauge
    Electric Temperature Gauge
    Electric Oil Gauge
    Dual oil-pressure/temperature
    Gauge seals
    Instrument Lighting

    The smaller gauges are secured in the dash with a small 3BA knurled nut (thumb nut) 17H932 and various U-straps. Early gauges have external illumination with strap AJH5185 (fuel) and AJH5186 (dual) that carries the bulb, later gauges with internal illumination use AJH5187.

    The electric gauges are usually powered from the green circuit (fused ignition), the one exception is the early electric tach from 64-67 which was powered from the white (unfused ignition) as well as having another white coming in to the pickup from the ignition switch and going out to the coil. I have no experience of electric temp and oil gauges in MGBs but the following info on fuel gauges may be of some use in faulting them. What I can give is the wiring colours. All run off the green circuit, either direct or via the voltage 'stabiliser' as follows:

    • Fuel: 62-64: green circuit - fuel gauge - green/black - tank unit - black - boot earth
    • Fuel: 65-on: green circuit - stabiliser - light-green/green - fuel gauge - green/black - tank unit - black - boot earth. From about 75 on for North America, and 77 on for the UK, there was no longer a wired earth at the tank sender.
    • Temp: (North American spec 67-on, UK spec 77-on) green circuit - stabiliser - light-green/green - temp gauge - green/blue - temperature sender
    • Oil: North American 67/68: green circuit - stabiliser - light-green/green - oil gauge - white/brown - oil pressure sender
    • Oil: North American 69-71: green circuit oil gauge - white/brown - oil pressure sender. From 72-on North American spec reverted to a mechanical gauge


    Electronic ignition
    Testing October 2013
    Internal and external seals

    Description: There have been two types of electronic tachometers - the earlier RVI inductively-coupled type (which came in two versions - positive earth and negative earth) and the later RVC directly connected type. The inductively coupled type uses the white wire that goes from the ignition switch via the tach to the coil for triggering. The wire is looped round the tach pick-up and will only work the right way round. The RVC directly connected type was only used on negative earth cars and uses a black/white from the coil -ve (the same terminal as the points wire from the distributor) which terminates on a bullet-type connector at the tach rather than a flat spade. 1962-64 cars used a mechanical rev-counter.

    Secured into the dash with two large 3BA knurled nuts 17H1304 and spring-washers, with a U-strap AJH5176 to September 64, separate 'legs' 17H3744 on each threaded stud from then until July 74, and 17H1339 after that until the 1977 model year.

    1977 and later have a much better arrangement where the tach and speedo have three studs in the instrument case, and by turning them clockwise 30 degrees these studs will align with cut-outs in the dashboard to allow the instrument to be withdrawn. Unfortunately LHD only.

    The tach is powered from the white (unfused) ignition circuit on Mk1 cars, then for Mk2 cars onwards from one of two green (fused) circuits. Until 1978 this was the green circuit in the fusebox, but when the ignition relay circuit on RHD cars was modified sometime in 1978 and a second in-line fuse between brown/white and green wires was added under the fusebox, the tach is powered from one of these - the one with the thinner wires, the other with thick wires being for the cooling fan. For more information see the ignition schematics.

    Tachometers were marked with the original polarity - positive and negative - from inception, at least until they changed from chrome bezels to plastic for the 1977 model year. But bear in mind that a PO may have changed the internal wiring of a positive earth tach and not changed the legend on the dial.

    Serial numbers (on the faceplate) were as follows:

    YearChassis Nos.MarketBattery
    SizeReference No.
    1968-72138401-294250Not North AmericaNegativeCurrentInternal4"RVI/2430/00
    1972258001-294250North America, Sweden,
    1973-74294521-360069not North America, Sweden,
    1974.5-76360301-409401not North America, Sweden,
    1973-76294251-409401North America, Sweden,
    Note 1: There were two arrangements for the RVI tach - Mk1 cars had an external pickup and Mk2 to 1972 had an internal pickup.
    Note 2: The change to 1968 Canadian models occurred part way through the year.
    Note 3: There were gaps in chassis numbers. From November 1967 with the advent of the Mk2, most model years thereafter started at a 'round number' e.g. the last 1971 model was 256646 (a GT) and the first 72 model was 258001 (a roadster).

    The RVI with external pickup either uses a white wire from the harness through the pickup, or may have a short section of white wire with bullet ends already in the pickup. In the former case there may be a long length (4 ft has been mentioned) of white coming out of the harness with a spade on the end, and this goes through the pickup then connects to an 'ignition on' spade on the ignition switch. This makes it easy to determine which way the wire has to go through the pickup. In the latter case the harness may have two separate whites with bullet ends, but it's an easy job to swap them over if at first the tach doesn't work. These tachs will only work with the current going through the pickup in one direction and you have to reverse it if you convert a positive-earth car to negative as well as reversing the 12v and earth connections. You will not harm the tach if the pickup is wired the wrong way round, but may do if the 12v supply is incorrect.

    The RVI with internal pickup has male and female bullets on the back of the case, and corresponding bullets on two white wires from the harness. Note that these seem to be smaller than the standard wiring bullet and connectors at 4.5mm instead of 5mm (Malc Gilliver). The spade for the 12 supply to the tach electronics is close by the bullet connectors. The harness now has separate white wires with female and male respectively bullet connectors, meaning incorrect connection is not possible. Both RVI type tachs sense coil current and respond to the current pulses through the coil as the points open and close. If that circuit breaks the engine stops, if it shorts to earth you fry the harness! The earth wire for both the electronics and the instrument lighting is under one of the knurled wheels holding the tach into the dash.

      See here for a description of the external pickup type from Mark Olsen's Sunbeam Tiger pages, and here for the later 2430/00 RVI with internal pickup by Herb Adler. However neither say much about the thermistor, this thread from The Sunbeam Owners Club of America states the original should have a value of 150 ohms at room temperature, but items in the 200 to 500 ohms should be able to be used successfully. A negative temperature coefficient (NTC) item is required, there are positive temperature coefficient (PTC) items around which are not suitable. Unfortunately suppliers in the UK only seem to stock items in the thousands of ohms, not hundreds of ohms.

    With the RVC voltage-operated type the ignition current does not go via the tach, instead a wire from coil -ve/points connection goes to the tach on a white/black wire, which responds to the voltage changes as the points open and close. If this circuit breaks the tach ceases to register and the engine continues to run. If it should happen to short to earth the engine will stop, but not fry the harness. (In fact this makes a nifty anti-theft device using a hidden normally open switch connected to the wire at the tach rather than in the engine compartment.) Since the current flowing through the coil has a direct relationship with the voltage at the coil CB or -ve terminal it follows that the two types indicate the same thing.

      See here for information about the RVC tach by Herb Adler, Franz Hubl and Rick Astley.

      Problems: Typical problems are sticking, wavering, or simply not working at all. Sticking, where a rap with a knuckle on the glass fixes it, and it only occurs after being parked for a while or at certain times of year, is almost certainly a mechanical problem with the movement itself.

    Wavering or flicking about, if accompanied by changes in the idle speed, have a good chance of being caused by bad connections in the ignition LT circuit that is ignition switch - coil (via tach where appropriate) - points - earth.

    Wavering or flicking about not accompanied by changes in idle speed, randomly dropping to zero for longer periods, or not working at all, could be either the 12v supply to the tach, the connection between coil and tach on the later voltage-operated types, or electronic problems inside the tach itself. From 64 to 67 the tach was powered from a third white (unfused ignition) wire and black earth but after that it was from a green (fused ignition) and black earth for both current- and voltage operated types. In no case is the tach powered from the instrument voltage stabiliser as the output from this is 12v switched on and off about once a second and so is unsuitable for the tach for obvious reasons. Get a multi-meter with an rpm range, connect it to the points-side of the coil, and compare that with the cars tach. If they shows similar variations then there is a problem in the ignition LT circuit through the ignition switch, coil and points. If it is steady when the cars varies, and you have the voltage operated tach, then connect the multi-meter to the white/black at the tach. Variation here but not before would indicate problems with the white/black wire or connections between tach and coil. If that is steady too, or if it was steady at the coil and you have a current-operated tach, monitor the 12v supply and earth at the tach. If these are steady too then the problem must be inside the tach itself.

    If it works normally with the lights off, but doesn't work at all with them on, then the earth supply is probably missing. Early tachs have the earth connection as a tag under one of the knurled securing nuts (as on the other gauges). Later cars have a spade spot-welded to the back of the case, as well as the insulated 12v spade. A similar problem can affect headlights where relays have been installed when the speedo earth is missing and the main-beam tell-tale is in the speedo.

    Very occasionally there are reports of the tach shooting up when the ignition is turned on but before the engine is started, or swinging right round to max with the engine running. About all you can do here is to unplug the illumination bulb and disconnect the trigger wire or wires, and see what happens with just the green 12v supply wire (white on Mk1 cars) and black wires connected. If you have 12v between the spade the green (or white) is connected to and the case, and zero volts between the case and a known good earth elsewhere on the car, and the tach is still registering incorrectly, then it is an internal problem. If it has suddenly happened it is quite likely to be a dry joint or cracked PCB track which may be visible, or a component fault which probably won't be. If you have less than 12v between the green (white) and the case, or any voltage at all between the case and a known good earth elsewhere, then the 12v supply or earth is faulty.

      Updated May 2009: There are several sites around showing how the original tach circuit can be replaced with one using a 555 timer - two examples here and here. This can be done to both RVI and RVC tachs, although in the former case this would effectively convert it to an RVC and would need a trigger wire from the coil CB or -ve instead of sensing the current pulses in the white wire going to the coil SW or +ve. Initial testing and calibration can be done using a basic battery charger as described here, then comparison with a separate instrument such as in a strobe light or automotive multi-meter which are likely more accessible.

    Electronic ignition: July 2011: If your RVI tach (64 to 72) doesn't work with your shiny new electronic ignition system, there are a couple of things you can try, depending on whether you have a positive earth car or a negative. Originally the 'fix' was to try changing the wire going through the pickup from two passes (one turn) to one pass (half a turn) and recalibrating. You will need to dismantle the tach to get at the pick-up on later versions. Subsequently Herb Adler reported that a 123 used with an RVI tach on a negative earth car (67 to 72) caused problems when following the instructions to connect the red (power) lead of the 123 to the Batt (+12v) terminal of the coil. He found that connecting this to an alternative 12v ignition source that didn't come through the tach pickup, e.g. the white at the fusebox, solved the problem. This shouldn't be necessary with the later RVC tachs, but is worth trying if you have other electronic ignition systems and tach problems. Note that this alternative connection is often recommended when putting electronic ignition on rubber bumper cars (i.e. with the ballasted ignition system), as the electronics are then fed with the full 12v and not the reduced and varying voltage.

    Herb also reports that an RVI tach he had modified for 6-cylinders has R6 (see the above Tiger pages) at 820 ohms instead of 470 ohms.

    Positive earth cars: June 2021
    Pertronix/Aldon positive earth Ignitors are wired completely differently to negative earth in that the Ignitor is in the 12v supply to the coil (SW) instead of replacing the points, and the points side of the coil (CB) is taken direct to earth. This means that the alternative source of 12v for the Ignitor to allow the tachometer to work correctly cannot be used, and Aldon for example say that the tach might have to be converted to the later RVC (which at the time of writing is an additional 42). But this Lotus Elan site suggests an additional wiring change which should - in theory! - work. That is to remove the existing white wire from the tach pickup and run a new black earth wire from the coil CB terminal round the tach pickup (in the same direction as the original white wire) and then to the tach earth.

    Updated April 2012: In response to yet another complaint of an RVI tach not working with electronic ignition I dug out an old tachometer adapter that came with my Sparkrite ignition kit in the late 60s to see what it involved. It's no less than four transistors that basically provide a clean signal to drive the tach, probably together with powering the module from an alternative ignition supply. I no longer have the connection information for it but I suspect the adapter gets its input from the coil CB/Points terminal, has a second terminal going to earth, and the output terminal connects to the ignition feed via the tach. And whilst searching for information on connecting it I found these Sparkrite SX2000 instruction sheets. Steps 9 & 10 of the first one set and page 5 e) of the second basically say if you have problems with an RVI tach, the Sparkrite unit has two wires that would normally connect to the coil SW or +ve and you move the red one to an alternative ignition supply, i.e. basically the same thing, so worth trying first. 123 distributor, and the Pertronix, Aldon Ignitor and Powerspark under-cap modules are 3-wire systems that use a connection to each of the two coil spades and an earth from it's physical installation to the block or distributor, suggested wiring of these with the RVI tach is shown here.

    July 2014: I suggested moving the red wire to new owner Bob Warwood, who had installed an Accuspark electronic ignition system, where the tach was reading about 800rpm high after installation ... but not if the lights were on! It seemed to me that he probably had a tach earthing problem, and with the lights on the voltage to the tach electronics was being reduced, and that overcame the problem caused by the electronic ignition. Bob reported that moving the red wire to the fusebox fixed the over-reading problem.

      Testing: October 2013 I've always noticed my tachs give a little pulse of the needle if I turn the ignition on and off without starting the engine. I'd imagined this was from the trigger pulse as a 4-cylinder engine usually stops in one of two positions with the points closed, but a recent topic on the Yahoo MG-MGB list about this jump got me thinking. So I disconnected the coil and tried again, and still got the jump, therefore it's the power being disconnected from the tach electronics that causes the jump. If your tach suddenly stops working in the car turning the ignition on and off and looking for the jump as you turn it off would be the first test, which would indicate that the electronics were working - partially at least, and that the movement is working. It would also prove that the tach had the 12v and earth supply to the electronics. However both my tachs are the later voltage pulse triggered type with an RVC reference number on the dial, not the early current pulse type. A sample of one RVI tach is a little less clear. The first and second times the ignition was turned on and off the tach needle did jump, but on the third it did not. Neither did it jump when the white wire was disconnected from the coil and the ignition turned on and off again. On a subsequent test with the white wire disconnected the tach didn't jump, but when it was reconnected it did jump. As I say it's not clear yet, but the possibility is that the tach only jumps when the trigger circuit is complete i.e. the white is connected to the coil, so would be no help in diagnosing a non-functioning RVI tach.

    You can go further with a bench-test and a conventional battery charger (i.e. not one of the more modern conditioning chargers that are designed to be left connected to the battery long-term, I don't know what results these would give). For the earlier RVI-type tachometer connect the output of the charger to a coil or similar load, with one of the wires going through the tach pick-up in the appropriate direction to give the correct current pulse. Connect a 12v supply from a battery to the tach electronics via the usual insulated spade and case, observing the polarity of both the battery and your tachometer. With the charger switched on a 4-cylinder tach on 50 Hz in the UK (60Hz in the USA it seems, so in the remainder of this section figures in brackets refer to 60Hz mains supplies) should read 3000 (3600) rpm. I've not been able to confirm this as I don't have an early tach, but there are plenty of web sources indicating that this is the case. A 50 (60) Hz mains supply has 50 (60) positive pulses per second and the same number of negative pulses per second. If only the positive pulses are used (half-wave rectification), 50 (60) pulses per second, times 60 seconds, gives 3000 (3600) positive pulses per minute. A 4-stroke engine has two firing strokes per revolution so ordinarily you would have to divide that by two to get the expected reading on the tach, i.e. 1500 (1800) rpm. But most battery chargers should convert both positive and negative half-cycles of the AC supply to positive output pulses (full-wave rectification), which will give 100 (120) positive pulses per second, hence 100 (120) pulses times 60 seconds gives you 3000 (3600) rpm. If you have a charger with low and high settings, you might find that the low setting only has half-wave rectification, hence only 50 (60) positive pulses per second, and in this case you would see 1500 (1800) rpm on the tach.

    The later RVC-type tachometer can be tested in a similar way, but in this case the charger negative needs to be connected to the car body (in-car test) or the tachometer case (bench-test). Remove the wires from the coil +ve and connect the charger to the coil +ve. Disconnect the black/white from the distributor (25D4) or open the points. Turn on the ignition (in-car test) or connect 12v to the tachometer (bench-test) from a battery with +ve going to the insulated spade on the tach body and -ve to the tach case. With the charger switched on you should see the same tach reading as above i.e. 3000 (3600) rpm with a full-wave rectifier in the charger, or 1500 (1800) rpm with a half-wave rectifier. Note that unless the points are disconnected or opened the tach will not register. This test can be performed on rubber bumper cars with ballasted ignition and 6v coils as well as chrome bumper with unballasted ignition and 12v coils.

    This test also gives you a very accurate way of checking the calibration of a tach. V6 engines have three firing strokes per revolution, so the V6 tachs readings would be 2000 (2400) rpm on full wave and 1000 (1200) rpm on half-wave. A V8 has four firing strokes per revolution, so those tachs would read 1500 (1800) rpm on full-wave, and 750 (900) on half-wave.

    Fuel Gauge:

    Early vs late systems
    Fault diagnosis
    Tank sender
    Voltage stabiliser
    Gauge identification
    Internal and external seals

    Originally Jaeger gauge BHA4214E was used with a screwed tank sender, changing to Smiths BHA4470 in October 64 for the remainder of Mk1 production also with a screwed sender but electrically very different. Both these gauges have external night-time illumination with holes round the edge of the case to let the light in, and a fully exposed pointer. Mk2 cars changed to one with a partially shrouded pointer and internal illumination (BHA4736 or BHA4685 according to market with leaded fuel, BHA 5342 for the USA with unleaded fuel). All these have 'hanging' pointers, and use a sender with a locking ring. The final change was for the 77 model year and the plastic dash - to AAU3032 for leaded fuel and AAU3031 for USA unleaded fuel, with the indicator pointing upwards. The sender for these changed to one combined with the fuel outlet pipe, and as the North American electric temp gauge and sender both changed at the same time, and it is known mismatches between that sender and gauge cause incorrect readings, I had wondered if the same would apply to the fuel gauge and sender, but the resistances are very similar. All the Smiths gauges require an earth for night-time illumination, which if missing will not affect the gauge reading (it does with the Jaeger gauges).

    Externally illuminated types are fixed in the dash with bracket AJH5185 which includes bulb holder, the rest with AJH5187. These fit over a central threaded stud and are secured with thumb nut 17H932 with spring washer WL700061.

    Early vs late systems: July 2015: The 62-Oct 64 cars use one system (similar to the MGA), after that it was a completely different system. The two systems use different ways of ignoring changes in system voltage, which can vary between 12v and 15v and would otherwise cause a significant variation in readings. The early Jaeger system uses a kind of Wheatstone Bridge which is a method of determining the value of an unknown resistance where you have an unstabilised voltage supply, by comparing it in a particular way to three other resistances of known value. Two known resistors are connected in series, and another known resistor connected in series with an unknown resistor, then both pairs are connected to the same voltage. A meter is connected between the two junction points of the pairs of resistors, and as the unknown resistor varies (the sender) the meter (fuel gauge) will vary its reading. This has several components inside the gauge which is technically quite complex and expensive and requires the gauge to be earthed both in order to indicate the fuel level and for the night-time illumination. A missing earth will probably vary the reading as the parking lights are turned on and off, and it may not work at all if the panel lights are switched on. The later Smiths system uses an external (to the gauge) voltage stabiliser and a much simpler and cheaper method that uses the heating effect of the current to bend a bi-metallic strip which moves the pointer.

    Another difference between the two systems is that the early is 'fast-acting', i.e. it indicates the fuel level immediately you switch on the ignition. The later system is 'slow-acting' or damped and needs several seconds to indicate the true fuel level. The later system does have the benefit of not swinging about as the fuel sloshes around in the tank, but that is usually controlled by the tank being baffled. However a drawback of the later system is that on a long bend such as on a motorway the fuel level will vary considerably between a left and a right, and take time to settle again once you have straightened up.

    A further difference between Jaeger and Smiths systems is that whilst on both the tank sender is a variable resistance that changes as the fuel level goes up and down, on the early system the resistance is high (about 70 ohms) when the tank is full, and low (close to zero) when the tank is empty. On the later system the resistance is lowest when the tank is full (20 ohms or so e.g. 25 for Vee and 22 for Bee) increasing towards 250 ohms or so as the tank empties.

    A third difference is that on the early gauge the wires must be connected to the correct terminals, but on the later gauge can be connected either way round. The early gauge has one terminal marked 'B' - for Battery, the green wire goes on this. This terminal is on the upper right part of the back of the gauge, and usually has two spades, so you can use it as a pick-up point for a fused ignition supply for something else. The other terminal is marked 'T' - for Tank, the green/black wire goes here, and is in the upper left area of the back of the gauge.

    A quick test of the early system is to disconnect the green/black sender wire from the back of the gauge, and the gauge should register above F. Earthing the sender terminal should cause the gauge to read Empty or just below. Connecting a 70 ohm resistor to the T terminal (tank wire disconnected) should cause it to read close to Full, and a 35 ohm resistance should cause it to read slightly below half. For further information on the early system see Barney Gaylord's web site, the remainder of this section largely relates to the later system.

    From the above it should be obvious that a late sender with an early gauge will not work correctly, nor vice-versa. The correct tanks, senders and gauges are normally available with one exception, and that is for the brief period where the tank still used the screwed sender, but electrically used the later system. This sender is NLA so would normally mean you had to use an early Jaeger gauge, or change the tank to the later type with the locking ring. See here for an alternative to either of those.

    Diagnosis: Problems with the fuel gauge are frequently caused by the connections at the tank unit (they are exposed to a great deal of dirt and spray from the back wheel) or the tank unit itself failing. Problems can be non- or erratic operation, or being wildly inaccurate at E and/or F. You need the ignition on and a reasonable amount of petrol in the tank (i.e. 1/4 tank minimum) for the following tests. You also need to make sure the connectors and their spades on the tank unit are clean and bright and making good electrical contact. Note that CB cars have a factory earth wire going back to a number-plate mounting bolt whereas rubber bumper cars relay on the sender earthing to the tank and the tank to the body.

    Non- or erratic operation (later system): Briefly connect an earth to the green/black wire on the back of the gauge. Does the gauge pointer move smartly towards to 'Full' (remove the earth as soon as you see the pointer moving)?

    No - check for 12v on the light-green/green wire of the gauge. Is it present?

    No - check the same wire on the stabiliser. Is 12v present?

    No - check the green wire on the stabiliser. Is 12v present?

    No - check the green circuit back to the second fuse up in the fuse block.

    Yes - looks like the stabiliser is faulty.

    Yes - looks like the light-green/green wire from stabiliser to gauge is faulty.

    Yes - is it steady 12v, 12v switching on and off 2/3 times per second, or steady 10v?

    Steady 12v (incorrect) - check the earthing of the metal can of the stabiliser, if that is OK it looks like the stabiliser is faulty (giving high readings).

    Switching 12v or steady 10v (correct) - looks like the gauge is faulty.

    Yes - briefly earth the green/black wire at the tank unit. Does the gauge pointer move smartly towards 'Full' (remove the earth as soon as you see the pointer moving)?

    No - disconnection in green/black wire back towards the gauge.

    Yes - with the green/black wire on the insulated terminal of the tank unit connect an earth to the base-plate of the tank unit. Does the pointer move?

    No - looks like the tank unit has failed.

    Yes - bad connection back towards the main earthing point on the back panel of the boot (CB cars) or between sender and tank or tank and body (RB cars).

    Even if you see pulsing 12v (or steady 10v) on the green/black when disconnected from the sender showing there is continuity back through the gauge and stabiliser, it could still be high-resistance from a bad connection. When connected the voltage on the green/black wire between the gauge and the sender will show a voltage that is inversely proportional to the amount of fuel in the tank. With a full tank it will show a low voltage, and as the tank empties the voltage rises. With a full tank and original factory stabiliser I have 2.7v on the green/black, but that's while the stabiliser is still outputting steady 12v i.e. for the short period after turning on the ignition, with the engine not yet started. It would be a bit higher with the engine running and the alternator charging. After that the voltage switches on and off and is more difficult to determine, so you need everything connected and have the meter fairly visible before you switch on the ignition. Once it starts switching on and off you would have to leave the ignition off for a few minutes to 'cool down' before turning on will display a steady voltage for several seconds again. The aftermarket stabiliser that outputs 10v all the time is easier - on a full tank I'd expect that to show ten twelfths of 2.7v, i.e. about 2.25v, so not that different, and all the time even with the engine running. Much lower in either case indicates bad connections, which could be bad enough to stop the gauge moving at all, but still show 12v on a voltmeter when the wire is removed from the sender.

    Tank Sender: Updated December 2015 Schematics


    Early cars up to October 64 had sender BHA 4292 attached to tank ARH 176 with six screws, and was for the unstabilised Jaeger gauge BHA4214E. A stabilised system was introduced over a period from chassis number 47112 to 48767 in October 64 using sender BHA4471E which also secured with six screws and operated Smiths gauge BHA4381E with the same exposed pointer and external illumination as the Jaeger gauge. However the two systems are not compatible i.e. the correct sender has to be used for each gauge or the readings will be completely incorrect. But that only lasted a few months until chassis number 56742 in March 65 when the tank (variously ARH 223, NRP 2 or NRP 1132) secured sender ARA 966 with a locking ring.

      The final change was shortly after the start of the 77 model year in August 76 where it was combined with the pick-up tube, but again with the locking ring. Another change was the deletion of the earth wire from the sender which previously went back to a number-plate bolt in the boot, also for reversing lights and the fuel pump earth. On rubber bumper cars the sender gets it's earth from its physical mounting to the tank and the tank mounting to the body. That only works for the original metal-base sender, current stock are plastic so for rubber bumper cars an earth connection needs to be provided from somewhere reliable. However the Workshop Manual North American diagrams for 1972 with seat belt warning onwards no longer show this earth wire so it may have happened to those cars earlier, and the UK 74 1/2 to 76 diagram still shows it, but going by my 75 (which has what looks like a separate PO earth wire) it had gone by then.

    As the sender and the gauge changed in 1977 at the same time as the temp gauge and sender on North American spec, and if you get those mismatched the readings are incorrect, and I had wondered whether the same applied to the fuel. This was after reading about someone who had been getting strange reading with a late tank and sender and the earlier gauge, but it turned out his sender was at fault cutting out part-way through the range (as my four failures on two cars had been). A replacement OE (i.e. metal) late sender with output pipe measured 20 to 258 ohms before fitting (Colin Tozer), and an after-market plastic without outlet pipe measured 17.5 ohms to 250 (John Holland), although since then two people have posted that their pre-77 plastic ranges between 6 and 250 ohms. Vee's measures 25 ohms with a full tank and Bee's 22 ohms - both OE metal types. Colin was concerned that his late tank with earlier gauge would read incorrectly, but given those figures this seems unlikely.

    November 2019: Colin Parkinson has received a plastic sender from Moss and has measured it at 18 to 185 ohms, so hugely different to the others at the 'empty' end. This could well result in showing a couple of gallons left when it runs out.

    February 2020: As well as the significant variation in readings from plastic senders someone on a forum elsewhere returned a plastic sender to the MGOC saying it was way out, and the response was inevitably "not had any problems on the other 2000 we have sold.... ". Then later on they said they had had a faulty batch!

    Bizarrely plotting resistance and voltage against gauge calibration marks showed that the voltage was linear with the gauge markings, but the resistances weren't. How can that be, Mr Ohm? The resistance wire used in the gauge to heat a bi-metal strip to move the pointer has a positive temperature coefficient i.e. the hotter it gets the higher its resistance goes, and there is also the changing force needed to bend the bi-metal strip along its range of movement, and the system of levers that causes the pointer to move. In the graphs attached I had to use 12v as the supply voltage as I have the original thermal stabiliser which switches 12v on and off to average 10v and not an aftermarket electronic that outputs a steady 10v.

    What follows relates to the later Smiths system, for the earlier Jaeger system see here.

    The sender consists of a length of fine wire wrapped round a former - one end is connected to the insulated terminal on the 'base plate' of the tank unit and the other end is open circuit. As the fuel level rises and falls a float an arm moves an earthed 'wiper' contact across the wrapped fine wire, making a variable resistance The wiper contact is connected to the body of the base plate via the float arm and its pivot in the sender structure. The wound former is tapered in shape and the distance between adjacent turns varies along it in an attempt to make the movement of the gauge indication bear some relationship with the quantity of fuel in the tank ... but it's not very good. On my two cars I get infinite MPG for the first 40 miles, 50-odd MPG for the first half of the tank, about 10mpg from half to a quarter, and something a bit closer to the actual mpg for the bottom quarter. The resistance range is from about 20 ohms for 'Full' to about 250 ohms for 'Empty', although there seems to be quite a bit of variation between tank units and changing them has caused significant variation in gauge readings, see the section on calibrating the gauge.

    Up to September 76 the sender base-plate has two spade terminals - an insulated one to which the green/black wire goes and a smaller, uninsulated one which should have an earth wire on it, which usually goes to the earthing point at the number plate bolt on the boot rear panel together with things like reversing lights and fuel pump. It is possible for the fuel gauge to work without this earth wire but only via several non-electrical metal-to-metal contacts which can be unreliable. From September 76 senders with the integral outlet pipe did not have the earth tag, so does rely on the mechanical fixings of the tank.

    David Jackson wrote to me with a problem he was having with a new Heritage tank ARH176 with the screwed sender for the early Smiths stabilised gauge. When fitting the original sender it seems to foul something internally and does not freely move up and down from Empty to Full. This sender float moves in a different arc to both the Jaeger and the later locking-ring senders, as shown here. He has had to fit the earlier Jaeger sender, but the unstabilised system works in reverse to the stabilised system i.e. Empty is low resistance and Full is high resistance, whereas for the stabilised system Empty is high resistance and Full is low resistance. The upshot is that his gauge was working in reverse. However he found this Spiyder module that as well as allowing calibration of E and F, also reverses the operation to correct the indication on the gauge.

    For details of tanks, senders and gauges see here.

    Sender wiring: December 2020
    CB cars have the 2-wire tail for the sender splitting away from the rear harness in the boot by the off-side light cluster, going out through a hole by the bumper iron, then travelling forwards clipped to the edge of the tank to the sender. It might seem a bit odd to run the sender wires right to the back then forward to the sender, but as the earth wire has to come back to the rear panel it's as broad as it is long. Note that the Leyland diagram for North American 1972 models with seat-belt warning show the sender earthing to the tank and not via an earth wire.

    RB cars have the tail leaving the rear harness by the fuel pump and going above the axle back towards the sender. Prior to 1977 and the change in sender the RHD wiring diagrams show this as being two wires as before - the gauge wire and an earth. However on my 1975 V8 and an American pal's 76 there is only one sleeved wire coming from the harness, which the Parts Catalogue seems to confirm as the rear harness did change part number on all models for rubber bumpers, and without the earth wire it makes sense for the tail to split off sooner rather than go all the way to the back then have to come forwards again. The difference between us is that whereas his earth spade on the sender is unused mine has a local earth wire on it - so almost certainly a PO bodge, perhaps if a replacement tank was too well painted and undersealed and failed to pick up an earth. In which case perhaps that earth is failing and causing the erratic readings after refuelling that I have noticed recently.

    1977 and later have a combined pickup pipe and sender with just one connection for the gauge wire, no provision for an earth wire i.e. the earth is picked up by the sender body from the tank and the car body through mechanical fixings. This means that when fitting a plastic-bodied sender an earth wire has to be provided or the fuel gauge will not work.

    Replacing the sender:
    The sender can be replaced with up to about a 1/4 tank of petrol in the tank if you raise the right-hand rear corner of the car. Early cars had the sender screwed into the tank, from about 1964 it is held in place with a locking ring that locates under three lugs on the tank. The locking ring consists of three tapered sections that locate under the lugs and as the locking ring is rotated in a clockwise direction the tapers cause the tank unit to be pressed in towards the tank making a seal. To remove the tank unit rotate the locking ring in an anti-clockwise direction, having come across a picture of the factory tool after many years I made my own. But by alternately tapping on the thin end of a couple of the tapered sections of the locking ring with a hammer and drift you can achieve the same effect. The use of steel tools is usually quite safe unless the area is wet with petrol.

    Check the new tank unit by connecting it up to the green/black and earth wires and checking the movement of the gauge pointer as you move the float up and down on the tank unit. And before paying for it make sure it makes a smooth and quiet transition from Full to Empty and back again. If the movement is at all 'scratchy' reject it as a sharp edge on the wiper is probably catching on the turns of the resistance wire and will break them in a relatively short time.

    Install the new tank unit by putting a new rubber sealing ring against the tank, then the sender unit on top of the seal, and finally the locking ring on top of the sender unit, rotating it in a clockwise direction to tighten it.

    A new tank or tank unit will often throw the gauge 'accuracy' out, check the 'empty' indication and readjust as soon as possible (personal experience!)

    January 2015:

    It seems that the original senders with the metal base and installed using the locking ring (ARA966, 11H5062) are no longer available and suppliers only have the later 'ceramic resistor' type which have a plastic base (AHU1027 for up to August 76, ADU3218 after that with the integral pick-up pipe) in the UK at last as show both types. The plastic type come with three terminals, the additional one being for a 'low fuel level' warning which the MGB does not have. If the wiring is not connected to the correct terminals the gauge will not work properly, so test the sender through it's full range and check the gauge responds accordingly before installing it. Note that whereas the original pre-77 senders have an under-sized spade for the earth wire, these replacements seem to have standard-sized spades for all three, so the wiring connector will need to be replaced or an adapter made up. However 77 and later cars do not have an earth wire to the sender i.e. there is only one wire from the harness, so you will need to provide an earth wire as the sender won't be able to pick up an earth from the tank. Senders from Moss at least are shown as being provided with this earth wire, other vendors may not include it.

    Herb Adler's experiences with tank senders.

    Voltage Stabiliser: Updated July 2013 Schematics

    Where is it?
    Is my stabiliser working?

    The purpose of the instrument voltage stabiliser is to supply a 'fixed' voltage to the instruments so that the only variation in readings comes from the senders, and not from changes in supply voltage, which can vary between 12v and 15v. Note that MGBs up to October 64 with 18G and GA 3-bearing engines and Jaeger instruments did not have a stabiliser, as the fuel gauge used a different method of compensation for supply voltage variations, see Barney Gaylord's MGA site. Also note that the tachometer is fed with full and varying system voltage, not by the stabilised voltage. The diagrams do show that the North American electric oil gauge used from the start of Mk2 production was fed from the stabiliser, but only for the first year after which it was also fed with full system voltage. This is almost certainly an error in the drawing as the gauge and sender did not change after one year, and the electric sender is of a type used on other makes which has its own method of stabilisation as well as indicating pressure. In any event the electric gauge reverted to mechanical with a capillary tube for the 1971 model year on.

    'Stabiliser', when applied to the original component, seems an inappropriate term since when operating it switches 12v on and off once or twice per second, after a 2 or 3 second 'warm up' period of being on all the time when first switching on the ignition. The relative lengths of the 'on' and 'off' periods change as the system voltage changes - as the system voltage rises the on period gets shorter, and as the system voltage falls it gets longer. The result is that over time, the average output voltage to the gauges is kept at about 10v, and the gauges give a stable reading determined solely by the sender resistance, and not by changing system voltage. Since the gauges it supplies are thermal instruments (the current flowing through a coil heats up a bi-metallic strip which bends and moves a pointer) they are slow to move large distances and so the relatively short on and off periods don't allow the pointer to move very much at all. But if you watch a gauge carefully, when it is showing about 1/2 a tank or normal engine temperature, with the ignition on but the engine off, you can sometimes see the very small movements up and down as the stabiliser switches on and off.

    The stabiliser consists of a bi-metallic strip with its fixed end connected to the I(instruments) terminal i.e. to the gauge(s), and its moving end breaks and makes contact with the B(attery) terminal i.e. the 12v supply. A heating element is wound round the bi-metal strip and connected between it and earth i.e. the case. With the bi-metal strip cold the contact is closed, so system voltage is passed to the gauges and flows through the heating element to earth. The heating element causes the bi-metal strip to bend, which opens the contact, which disconnects system voltage from the gauges. It also disconnects system voltage from the heating element, so the bi-metal strip cools, closes the contact again, which reconnects system voltage to the gauge(s) and the heating element, and the cycle is repeated all the time the ignition is on. With high system voltages the heating current is higher, which causes the bi-metal strip to bend faster, and open the contact sooner. However it cools at a relatively constant rate, so the contact takes about the same time to close again regardless of system voltage, which is how high system voltages result in shorter 'on' times than low system voltages. Pulse-width modulation, if you want to get technical.

    You may wonder about the effects of ambient temperature changes on the bimetal strip in the stabiliser, and indeed the gauges, as they could be fitted to cars operating in temperatures from well below freezing to above 45C/115F. Some have said the ambient changes are much less than the changes due to the heating coil, but even if they were only 10% of the change, a range of 55C/100F in ambient would need the heating coil to be operating at ten times that i.e. 550C/1000F which is ridiculous. Instead the stabiliser and gauges are designed to ignore ambient temperature changes, as described here.


    The stabiliser has two terminals - B(attery) and I(Instruments), and its 'can' needs to be properly earthed (usually screwed to the firewall behind the dash on the RHS of RHD cars for example) for it to work correctly. Because it has moving parts inside original factory items also needs to be mounted the right way up, as indicated by the 'TOP' and arrow markings on various versions. The green wire goes to the B terminal and the light-green/green to the I terminal. The stabiliser changed from Mk1 to Mk2 cars, it looks like Mk1 cars had a stabiliser (12H 7819) with two male spades on each terminal, so it is possible to get the wiring the wrong way round. Mk2 stabilisers (BHA 4602) have male spades on the 'B' terminal and female on the 'I', so it should not be possible to connect the wiring incorrectly. Because each terminal has two spades the unused male spade on the 'B' terminal can be used as a source for a fused ignition supply to an accessory if required. Don't use the 'I' terminal. On original Mk2 stabilisers there is also a threaded adjuster stud for calibration, but remember that with two or more electric gauges altering this will affect all the gauges. If only one gauge is reading incorrectly, because of a replaced fuel tank sender for example, you should recalibrate the gauge, not alter the voltage stabiliser. The position of the stabiliser moved from low down on the firewall behind the dash in front of the (RHD) driver on earlier cars (e.g. Mk1), to hidden away right at the top above the wiper motor, and much more difficult to get at on my 73 roadster. It moved back down again on my 75 V8, and now the indicator and hazard flashers were hidden away. Later on both flasher units may have moved to that position and the stabiliser hidden away again!

    January 2020:
    As far as the 'TOP' marking goes, I was surprised at just how much difference the orientation of the original stabiliser makes to the fuel gauge reading.

    Updated July 2010:

    Incidentally the drawing in the Leyland Workshop Manual is misleading, as shown it couldn't possibly work! The contacts by the B terminal should be shown closed, as this is how they are when first turning on the ignition, to pass current through the winding to heat the bi-metal strip and pass voltage through to the gauges, as shown in this amended drawing. It is only when this has heated up and the moving contact bends away from the fixed contact that voltage is disconnected from the heating coil and the gauges.

      Some (Many? All?) suppliers only have electronic stabilisers available, and these are often polarity sensitive e.g. Moss 128484 for positive earth cars, and BHA4602 for negative earth, so you have to get the correct type. However Moss state the earlier one is for 62 to 67, which is incorrect, MGBs didn't get the stabilised system until 1964. Ironically if you installed an electronic stabiliser to the early cars it probably wouldn't make any difference, but if you connected an original the fuel gauge would go up and down about once per second! Incidentally Moss are naughty as they have retained the original part numbers even though it is not an original component. These stabilisers don't have the adjuster stud. Brown and Gammons (Mk1, Mk2 on showing the correct spades in each case) have also used the original part numbers and also implies the early ones were used from the beginning. Their Mk2 stabiliser at least has the 'TOP' markings, which implies it is the original thermal device. But from what I can see they don't have the adjuster studs, so almost certainly electronic as well (and equally naughty). Googling I can't find any pictures that show the adjuster stud, so maybe they are all electronic these days (you can certainly see the printed circuit tracks in many images). Many of those are labelled 'NEG' or 'POS' indicating they are polarity sensitive, so you must get the correct polarity, as well as one with the correct spades for your wiring. The Moss and B&G items don't show polarity, but the early one will be positive earth and the later negative, so unless they are polarity independent the early one wouldn't work on an early car that had been converted to negative earth.

    Some people are tempted to replace the original stabiliser with an electronic one, thinking it must be better. In fact they are not so good, as they output 10v as soon as the ignition is switched on, whereas the factory stabiliser outputs full battery voltage for several seconds, resulting in a faster gauge rise time.

    Herb Adler describes how he made his own electronic version.

    Where is it?

    On my 73 it is high up on the firewall, behind where the wiper rack comes out of the motor and as such rather inaccessible. On later models (such as my 75 V8) it is on the lower edge below the wiper motor and much easier to get at. LHD cars have it on the other side of the car by the instruments.

    Is my stabiliser working?

    A quick way to check is to see what your fuel gauge registers about 30 secs after turning on the ignition but not starting the engine, and comparing that with what it registers when the engine is running - you do need to have some space in the tank i.e. the gauge not registering Full. There should be no difference, but if your stabiliser is not regulating the voltage it will read significantly higher when the engine is running ... always assuming your charging system is working correctly of course!

    You can use a voltmeter for a simple go/no go test of the stabiliser i.e. is it putting out a constant 12v? Or no voltage? Both of which indicate faults. But if your stabiliser is pulsing but outputting a higher or lower average than it should and hence causing the gauges to read higher or lower than they should, things are a bit more tricky. An analogue meter will probably swing between almost 0v and almost battery voltage, unless the off periods are very short or very long indeed. A digital meter will probably be flicking about all over the place. But you can make a test-rig, as shown here. With this you will get a very slow rise time on the meter, towards battery voltage, until the stabiliser starts pulsing. Once it starts pulsing it will slowly stabilise to the average voltage as experienced by the gauges, which should be about 10v.

    Calibrating the gauge:
    We are in hallowed company, this from a Hawker Hurricane cockpit:

    Whilst it is possible to bend the upper and lower stops on the sender to get a bit more travel (but run the risk of running off the end of the winding at either or both ends), or bend the float arm (which only moves the available travel up or down the range of the gauge to leave an even bigger 'dead area' at one end or the other), the real problem is when the resistance doesn't go low enough at F or high enough at E to get full travel of the gauge needle, so the only real solution is to alter the gauge to compensate for this. Getting at the gauge is also a lot easier (except for 77 and later) than getting at the sender, can be done at any time i.e. with a full tank, and you won't get leaks afterwards! The back of the gauge should have two holes, one by each terminal post (they may be covered by cork plugs), each containing a slotted plate. These slotted plates slide more than twist - they are not like screw adjusters as the slots may imply. Sliding them towards their adjacent terminal post moves the pointer towards the ends of the scale, away from it moves the pointer towards the middle of the scale. Sliding the plate by the terminal at the 'F' end of the gauge will adjust the 'Full' reading, the other adjusts the 'Empty' reading. However it is important to do 'Empty' last as is the more important one to have accurate, and changing one adjuster does have a noticeable effect at the other end of the scale as well. 'Full' is easy, but for the 'Empty' adjustment I ran the tank right out whilst carrying a spare gallon. I put the spare gallon in the tank and only then adjusted to E, to give me a gallon 'reserve'. Make sure you use an implement that is a good fit in the slotted plates, they can be stiff, the plate is only thin, and a poorly fitting screwdriver is likely to 'round out' the slot if you try to move it with a twist movement rather than a slide. This doesn't matter a great deal, but the bigger you make the hole the less you will be able to slide it from side to side before hitting the sides of the access hole in the case. There has been a suggestion that slackening the terminal post screws make the adjustment easier, you will see from the photos that this is not the case, the slotted plate is retained by rivets, the terminal posts are mounted elsewhere on the insulated back-plate.

    1977 and later: February 2020
    For some reason the factory put the temp gauge where the fuel gauge had always been and the fuel gauge in the centre of the binnacle above the steering column. One could possibly understand it if there was still a dual gauge, but not when they are both electric. Or to put the temp gauge out of direct line of vision as drivers of RB cars tended to be paranoid. Any road up, short of dropping the steering column which isn't a trivial exercise and could upset the UJ alignment, the next best thing may be to remove the tach and go in through that hole to release the fuel gauge.

    Update May 2007: Gary Alpern contacted me to say while he was calibrating his gauge he noticed that the pointer moved another 1/8" or so when he tapped the glass, and wondered whether anything could be done to eliminate this. I doubt it, and I think it is a 'feature' of the design - the bimetal and spring strips are connected together by nothing more than what is basically a 'hook and eye' hinge. I think the normal vibration of driving the car will continually 'tap' the gauge, however during calibration you may want to tap it after each tweak of the slotted plates. 'Tapping the gauge' is a long-standing and honourable part of living with machinery of this technology, as anyone who has seen 1940's, 50's and 60's films will know :o)

    Update September 2007: I debunked my long-held theory that the thermal stabiliser was needed with the thermal gauge to eliminate fluctuations caused by ambient temperature variation, click here to see why. However the thermal stabiliser does result in a faster initial movement of the gauge from rest than would be the case with an electronic stabiliser, as full system voltage is available to the gauge for the first few seconds after switching on the ignition with the thermal stabiliser, whereas with the electronic it is limited to 10v from the beginning.

    Gauge identification:

    Updated December 2008 For fuel gauge and sender identification click on this thumbnail.

    Electric Temperature Gauge: Added November 2008 Schematic

    The electric temperature gauge is similar to the fuel gauge, but using a sender on the cylinder head instead of the tank of course. Diagnosis and calibration is the same, substituting green/blue for the wire from the gauge to the sender. There is no earth/ground wire for the temp sender as it is screwed directly into the head.


    The early 180 degree gauges (both numeric and 'C-N-H') were capillary and dualled with the oil pressure, the later 'narrow angle' gauges were independent, electrically operated, and all 'C-N-H'. These gauges use a 'thermistor' (negative temperature coefficient resistor) sender in the cylinder head in which the resistance reduces as the temperature increases, so driving more current through the gauge to give a higher reading.

    There are quite a few senders with different thread sizes and electrical characteristics, denoted by differently coloured insulators containing the spade connection. I have measured three senders that fit the heads used on engines for the MGB and MGC as follows:

    CFResistance'C-N-H' indication
    3086440410594'C' (Cold)
    8518570.054.082.0'N' (Normal), about the temperature of a typical thermostat
    100+212+'H' (Hot)

    Note that there were at least three gauges and three senders (part numbers, may be just two colours red and black) used. The red and white senders give fairly similar readings over most of the range (white starting off higher at low temps, but changing over at about 20C to be lower and hence give higher readings), but black gives lower gauge readings for a given temperature than the other two. For example at 82C (one of the standard stat temps) a red sender may put the gauge in the middle of the 'N', but the black sender would be just below N. Conversely a red sender would show Hot at 90 degrees, whereas for a black sender it would have to be 100 degrees, and that ten degree difference is pretty constant through the range. Also note that like the fuel gauge it is possible to adjust the gauge to some extent.

    Also note that there are other colours, green possibly being for early Minis which did not have an instrument voltage stabiliser, and the temp sender like the fuel gauge sender operated in reverse and so would have a positive temperature coefficient i.e. increasing resistance with temperature.

    From the Leyland Parts Catalogue and Clausager the senders and gauges changed as follows (only North American spec cars got the first three variants, all models had the final variant):

    Chassis numbersGaugeSenderNotes
    138410-258000BHA 4686BMK 1644
    Red insulator
    Start of MkII to 71 model year
    18GF 101 to 18GK
    258001-368081BHA 5090BMK 1644
    Red insulator
    Remainder of chrome bumper
    18V584/585 to 18V 801/802
    368082-410000BHA 509088G 580Red insulator? A couple of suppliers say it is black for all rubber bumpers, but the gauge didn't change between 1972 and 1976
    75 and 76 model years
    18v 836/837 to 18V 801/802
    410001 onAAU 3030
    (BT 2231/01)
    13H 5602
    or 13H 9715
    or GTR101
    Black insulator
    77 model year on
    18V 847 (UK) and 883/884 (North America) on

    Note: The 'BT 2231/01' number for the last gauge is almost certainly the Smiths identification number and is given in the Leyland Parts Catalogue. Unfortunately there is no equivalent number given for the previous two gauges. This number can be found on the earlier 'needle down' gauges as shown in this example of a fuel gauge (on the rear part of the face, tucked up behind the front part of the face, circled) and more easily on the later 'needle up' gauges behind the lower part of the dial. Note the pairs of dots by the C, H and in the middle of the last gauge, these are on all the electric gauges and are used for factory calibration.

    As you can see the only correlation between a sender change and a gauge change is for the 77 model year onwards, which is when UK and other non-North American spec cars got the electric gauge. One source gives the date of change as 71/72 i.e. when the gauge did change and the other (Roadster Factory) gives it as 74 i.e. the same as the Leyland Parts Catalogue when the gauge didn't change. Apart from the Leyland Parts Catalogue the online catalogues of Roadster factory, Moss and Victoria British only indicate two different types of sender - Roadster Factory changing at 74 as previously indicated, the other two changing for the 77 year. September 2015: Further research revealed multiple online parts sources showing pictures of BMK 1644 with the red insulator for 68 to 74 or 75, and 13H 6602 with the black insulator for 1975 on. This gives possible change dates of variously 71/72, 74, 75, and 77. However if there is the potential for a mis-match between gauge and sender, September 76 for the 77 model year (as indicated by the Parts Catalogue) is the most likely date for the change, as the fascias and the gauges changed on all cars at that point.

    There has been another suggestion that perhaps the sender changed at the same time as the thermostat, to keep the needle centred on the gauge, but that doesn't tie in either as the thermostats changed shortly after September 64 then back again in March 69, which doesn't tie in with any of the gauge or sender changes from any source. Neither does it make logical sense when higher and lower stats were available for colder and hotter countries respectively, the gauge is only a general indication, and anywhere from just below the C to just below the H is part of the 'Normal' range depending on ambient temperature and usage.

    Yet another possibility that has occurred to me is that the gauges have basically the same electrical/movement characteristics but the black sender was fitted as that has higher resistance across the range compared to the red, so would give lower gauge readings helping to avoid owner paranoia!

    Electric Oil Gauge: Added November 2008 Schematic
    The electric oil gauge is similar to the fuel gauge, but the connections shown in the Workshop Manual are a little confused. For the first year of Mk2 production for North America the schematics show it was wired the same as the fuel and temp gauges i.e. from the stabiliser, to the gauge, to the sender. After that full system voltage was applied to the gauge as it was the oil sender itself that contained the stabiliser. As there seems to only have been one sender for the electric oil gauge for both wiring arrangements I'm assuming that the wiring for 67 was in error. The gauge reverted to mechanical with a capillary pipe for the 1972 model year for the remainder of production.

    October 2020:
    With the oil sender instead of a continuously varying resistance with changing oil pressure as is the case for changing fuel level and temperature for the other two gauges, the earth/ground signal from the oil sender switches on and off much like the voltage from the voltage stabiliser for the other two gauges. However the duty cycle i.e. the time it is on compared to the time it is off, varies with pressure as well as with system voltage. As it has no connection with the system voltage other than through the gauge it must sense this somehow and vary it accordingly, unfortunately I haven't had one to investigate internally, although my 1989 Toyota Celica had the same system (same physical appearance of the sender) so I was able to determine what the output from that looked like, at least. The sender is attached to a bracket mounting it to the now-unused rear mounting point for the dynamo.

    Dual gauge:

    Oil section
    Temp section

    Information on internal and external gauge seals can be found here.