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Ammeters and Voltmeters
Batteries and Chargers
Battery Cut-off Switch
Brake Balance and Handbrake Warning
Connectors and Terminals
Fuses and Fusebox
Heated Rear Window
Ignition Warning Light
North American 'Key in' Warning
Seat Belt Warning
Sealed Wiring Junctions
Switches in General
Wire Colours, Terminal Numbering
Wiring Harness Replacement
Won't Switch Off!
The sectioned MGB at the British Motor Museum, Gaydon
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.
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:
"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
Most older-style ammeters are of the 'local shunt' type and need you to interrupt the brown wires at the solenoid and run two very heavy gauge wires capable of taking at least 45 amps up to the ammeter. As well as these two new connections which can corrode, those on the back of the gauge can come loose as well, 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 they are likely to be even higher, which means the normal range of needle movement is compressed into a tiny section of the middle, moving-iron meters have unstabilised i.e. trembling needles, both of which 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.
Updated November 2010:
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 about 14v, reducing towards about 12.5v 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 could happen 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 than the alternator and battery voltages, even worse on a digital voltmeter where owners get paranoid about tenths of a volt. I've also seen one digital voltmeter that displayed half a volt less than two other instruments connected to the same point! Beware of voltmeter vendor claims that the instrument will tell you the 'strength' of your battery. All it does is tell you system voltage, which for 99.9% of the time will be charging voltage not battery voltage, and in no way does it tell you anything about the cranking ability of a battery.
Both ammeter and voltmeter will tell you if the battery is being charged or not in their slightly 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 were 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 this recently, 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. In this case a voltmeter would have immediately and clearly shown the problem. 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 not using sat nav, just how I would describe to the AA (in my case) exactly where I was!
There were 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) was 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 was 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. This is a '2-wire' alternator. Clausager states that a new version of the 16ACR with modified regulator and surge protection was provided in March 72.
Possibly because of problems with low battery voltage, in 1973 the alternators seem to have reverted to battery sensing again (Clausager states the 17ACR was fitted from February 73) now with an additional thin brown in the alternator plug wired back to the solenoid as before, and this seems to have remained the case up to and including the 77 model year at least. 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 wires 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 shows this diode pack for the 18ACR which is the 3-wire battery-sensing type, and I have seen the harness plug on a 78 which does match that.
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:
'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.
Alternatives May 2016
Ignition Warning Light (aka 'Idiot Light') and Charging Theory Schematics
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.
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.
On a dynamo system the warning light 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.
Dynamo Control Box April 2013
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.
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:
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:
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.
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.
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:
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.
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.
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
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
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.
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):
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.
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:
The voltage regulator should have the following wires:
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
Batteries and Chargers
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.
Late North American spec cars had an inertia switch as standard that cut power to the fuel pump - part number C41220. You may be able to find one of those by Googling, but unless you are replacing a factory item and want to keep the car original you would be better off with one for a modern car (should be standard on anything with fuel injection) and either be new - or only a few years old if from a scrapper. If you Google 'car inertia switch' you should find loads. Regardless of where you physically mount it the easiest place to connect it is in the same place as is recommended for a fuel pump fuse i.e. where the white wire in the rear harness joins the main harness in the mass of bullet connectors by the bulkhead on the off-side. Connect the two in series. Incidentally North American spec cars also had an inertia fuel valve in the fuel line to shut off the flow directly, but they have a reputation for leaking!
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
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
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
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) 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). 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. 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, but they are also available in a more modern construction with internal links and a single fill cap.
Leacy seem to have two types: GBY3031D and GBY421D at £72 and £65 respectively. Dry charged i.e. no acid and so suitable for mail order but they don't seem to list acid packs. They are showing GBY3031W and GBY421W at £77 and £79 for collection only. No capacities listed for any.
MGOC are showing a 'standard' 6v at £69.95 inc VAT and a 65 amp at £79.96, both dry charged, and three acid packs will be added for each battery at £3.50 each. Wet 'standard' for collection only is also £69.95.
Moss Europe list a 56Ah at £69.95, and a 63Ah at £77.95, add acid at £27.95 (enough for both batteries). Wet versions for collection only are the same price but no acid needed of course.
Lincon are showing a 421 59Ah at the 20 hr rate, a 421/11 73Ah, and a 421/13 88Ah, all the same package size of 169mm wide by 174mm long by 218mm high. Supplied dry charged with acid packs, price only seems to be available on enquiry.
Battery Megastore (aka Performance Batteries) have a 57AH at £73.50 inc VAT and an 'activation pack' which is the acid. Also free delivery, so a good deal. They give the battery size as 170mm wide by 175mm long by 225mm high. These are maximum dimensions, the base is smaller, but due to the different case design of this battery compared to others there may be an issue getting it to sit properly on the rubbers that should be in the base of the cradle. There were also no instructions on filling the cells with acid, more info on both these aspects here.
B J Banning in Birmingham have an Exide 80AH 600 CA at £90 inc VAT collected. Getting on for double the minimum performance specified for the MGB, but at a price.
Halfords list the same battery as suitable for an unmodified chrome bumper MGB and a rubber bumper, so obviously incorrect.
421 seems to be the designation used by a lot of suppliers, quite a few possible suppliers in this Google search.
Leacy are showing GBY072W as wet for collection only, £71.70 inc VAT, no spec.
MGOC are showing a dry battery at £89.95 inc VAT, and six acid packs will be added for each battery at £3.50 each i.e. £110.95 in total. A wet for collection only is listed but not priced.
Moss Europe show a dry at £96.95 to which must be added £27.95 for acid, or wet for collection at £96.95. No capacity indicated. They also have a 'heavy duty' at £109.94 which is wet for collection only. Cheaper options for use with a 'battery box', presumably smaller.
Lincon 291/9 and 291 seem to be the closest in size that should fit (251mm by 165mm by 224mm using the Halfords size as a comparison), at 58Ah and 73Ah at the 20 hour rate respectively. Again prices only on enquiry.
Halfords list HB072 with a 3 year guarantee at £84.99, 68Ah, 550 amps 'starting power' (looking at Yuasa specs for one of Halfords Yuasa batteries 'starting power' seems to equate to CCA) and HCB072 with a 4 year guarantee at £94.99, 70Ah, 570 amps 'starting power'. Both are sized at 261mm long by 175mm wide by 220mm high.
Battery Megastore (aka Performance Batteries) lists several 57024 Alphaline 70Ah 570CA 268mm long by 170mm wide by 224mm high at £52.50 inc VAT, E24 Varta 70Ah 630CA 261mm long by 175mm wide by 220mm high at £63.50 inc VAT, both 4 year guarantee, and AX D26R ABXAGM 75Ah 750CA 260mm long by 172mm wide by 220mm high, 5 year guarantee, at £99.50 inc VAT.
All prices correct as of October 2014.
12v for a chrome bumper? 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. MGOC recently advertised a conversion kit including battery, acid, clamps etc. At the moment there is no price given (no longer available?), but originally it was given as a very pricey £130. The battery is also only 50Ah, so even less capacity, and no cranking amps given.
Note that 12v batteries have the posts down one long side and can have the polarity either way round. Rubber bumper cars require the posts to be adjacent to the tunnel and the +ve towards the front of the car when installed. Type 069 and 072 batteries seem to have the correct orientation and size. You need to check carefully, REVERSE CONNECTION WILL DESTROY THE ALTERNATOR AND BURN WIRING. I've seen type 063 recommended but these seem to have +ve and -ve reversed.
About the only good technical reason for converting from twin to single concerns access to the fuel pump. 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 left-hand box, and made the cable long enough to reach that, so that he can move the pump up into the right-hand box making access to that far 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 finally... The batteries, particularly the single 12v in a rubber bumper, are 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.
Battery Drains January 2014
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.
Battery Types Added December 2008:
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!
Plastic battery bins
Fused Battery Connector January 2014
Battery Chargers Updated November 2007
Update February 2014:
Another important thing to remember, from the Lucas Fault Diagnosis Service manual:
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:
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:
Battery Cut-off Switch
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. 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
|GLU101||Headlamp||Sealed Beam||60/45||101-187210 RHD|
|GLU106||Headlamp||Sealed Beam||75/50||187211-360300 RHD and CB V8|
|BFS415||Headlamp||Bulb||50/40||101-360300 LHD except Europe and North America|
|GLB410||Headlamp||Bulb||45/40||101-360300 LHD Europe except France|
|?||Headlamp pilot||Bayonet||?||101-360300 LHD Europe except France|
|GLB411||Headlamp||Yellow bulb||45/40||101-360300 France|
|17H9472||Headlamp||Sealed Beam||60/45||101-410000 North America|
|GLU123||Headlamp||Sealed Beam||75/50||360301-410000 RHD and RB V8|
|5||360301-410000 RHD and RB V8|
|GLU114||Headlamp||Sealed Beam||?||360301-410000 LHD except France, Germany and North America|
|5||360301-410000 LHD except France, Germany and North America|
|GLB411||Headlamp||Yellow bulb||45/40||360301-410000 France|
|GLB233||Headlamp pilot||Bayonet||BA9||4||360301-410000 France|
|BHA5387||Headlamp||Sealed Beam||?||360301-410000 Germany|
|GLB233||Headlamp pilot||Bayonet||BA9||4||360301-410000 Germany|
|GLB472||Headlamp||Halogen||60/55||410001 on RHD|
|GLB233||Headlamp pilot||Bayonet||BA9||4||410001 on RHD|
|17H9472||Headlamp||Sealed Beam||60/45||410001 on North America|
|GLB989||Parking||Bayonet||BA9||5||101-360300 Not North America|
|GLB380||Parking/Flasher||Offset bayonet||BA15||6/21||All North America|
|GLB207||Number plate||Bayonet||BA9||5||101-339964 and V8 to 1247 except as below|
|5||187211-219000 North America|
|GLB989||Number plate||Bayonet||BA9||5||339965-360300 and V8 1247-2100 except North America|
|GLB233||Number plate||Bayonet||BA9||4||All RB except North America and Germany|
|GLB254||Number plate||Festoon||Festoon||6||339095 on North America|
|GLB233||Number plate||Bayonet||BA9||4||339095-410000 Germany|
|GLB989||Gear light||Bayonet||BA9||5||Automatic only|
|GLB273||Reverse||Festoon||SU8, 5-8||21||101-410000 and V8 except as below|
|GLB270||Reverse||Festoon||SU8, 5-8||18||268698-410000 North America|
|GLB254||Load space||Festoon||Festoon||6||GT and V8|
|GLB254||Interior/Courtesy & Boot||Festoon||Festoon||6||219001-410000|
|GLB989||Side marker||Bayonet||BA9||5||187211 on North America|
|GLB921||Switches and controls||Screw||LES||1.2||410000 on, use GLB280|
|Note 4||410000 on|
|AAU3857||Main-beam tell-tale||Note 4||410000 on|
|AAU3857||Indicator tell-tale||Note 4||410000 on|
Note 1: I have seen the dash harness for 77 and later UK models with wedge-type bulb holders.
Note 2: Generally the 3-digit number following the GLB code is the industry code for the bulb style, fitting and output.
Note 3: It should be noted that LED bulbs are not always road-legal.
Note 4: Holder with bulb, spade connectors.
Clocks February 2016
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.
Connectors and Terminals:
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.
Translucent multi-plugs can be tested by pushing a probe in along side the wires, again test both sides with the connector fully assembled. However black and grey multiplugs are usually moulded on and only the pin-side can be tested by slightly parting the connector.
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.
Sealed Wiring Junctions Added November 2009
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.
Fuses and Fusebox
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 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: 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.
As far as I am aware all the MGB fuses except one of the two for the sequential seat belt system are 17 amp continuous rated, 35 amp blow rated. The odd man out is 500mA. 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. If you fit a 35 amp rated fuse then in the event of a fault the current may not be enough to blow the fuse, but may damage the wiring. 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.
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.
Apart from the earliest cars some additional in-line 17/35 amp fuses protect various circuits. Watch out for the one for the hazards which has brown wires on both sides of the fuse. Also watch out for the electric fans on late cars - although these appear to be on the green circuit they have their own in-line 17/35 amp fuse which comes directly off the white/brown circuit (ignition relay), i.e. it is not associated with the usual green circuit which is fused from the second fuse up in the four-way fuse block. UK cars from 1977 have yet another subdivision of the green circuit with another in-line 17/35 amp fuses supplied by the white/brown (ignition relay) circuit feeding things like heated rear window, indicators, heater fan and tach, which leaves the original green circuit fuse (2nd one up in the four-way fuse block) feeding things like reverse lights stop lights, washers, wipers, and circuits associated with the seat belt warning lamp and time delay buzzer.
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.
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 circuit with the short is connected. If it's out, or just dim, you know the circuit with the short is not connected. 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 fused supply wire, 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
Before the 1973 model year the heated rear window was always optional. Prior to 1972 (possibly 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 1972 (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.
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 had a dedicated relay and powered the HRW from the purple circuit, but 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 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.
When powered from the 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 much less than that at the element, the rest being 'lost' elsewhere. Even with a (fused) relay direct off the brown circuit I'm still only getting about 10 volts at the connections to the harness under the rear cant rail, 8v at the element contacts, and a measured 8 amps. Even so that's about 80 watts, and the relay has made a significant improvement to screen clearing.
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:
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.
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. However I'll have to wait until the weather gets warmer and dryer before I can try it on the screen.
I also found this Loctite 'Rear Window Defogger Tab Adhesive', and a couple of other repair possibilities from Permatex.
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.
Horns Updated September 2013
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. Clausager says the horns moved to the inner wing in 1963, and that from 1970 'all cars' now had twin horns, implying that some specifications i.e. 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.
The Workshop Manual and Haynes indicate that from the 77 model year 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. However the Bentley diagrams show 2-wire horns being used until the end of production, but this is an error.
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 sprung wire (69 and earlier) and 'pencil'/sprung rod (71 to 76) that bring the column earth to the horn contact is 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 a brass cylinder on the column (69 and earlier) or on a slip-ring on the back of the wheel (71 to 76). 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, pencil, and horn push all rotate as a unit. The sprung wire (69 and earlier) and the pencil (71 to 76) simply connect the output of the horn button to the purple/black wire which goes to the horns. They 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).
Horn Mounting September 2013
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 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.
There were five different types of horns over the years, and three different brackets. Originally a flat bracket mounted the horns on the edge of the slam panel, before changing to an angled bracket mounted on the inner wing. Clausager says the move to the inner wing was in January 1963, although the arrangement where the harness only had one horn tail near the right-hand headlight, and there was a sub-harness to extend the wires across to the other side if you had the optional second horn, continued until the end of the Mk1. 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 (both Bee's and Vee's are like this) rather than forwards. This 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.
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
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. 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 turn 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 cabin as well.
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.
Instrument Voltage Stabiliser
Calibrating the gauge
Electric Temperature Gauge
Electric Oil Gauge
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:
Description: There have been two types of electronic tachometers - the earlier inductively-coupled type (which came in two versions - positive earth and negative earth) and the later 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 i.e. it passes through it twice, and the direction is critical. The 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.
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:
|1968-72||138401-294250||Not North America||Negative||Current||Internal||RVI/2430/00|
|1972||258001-294250||North America, Sweden, Germany||Negative||Current||Internal||RVI/1439/00|
|1973-74||294521-360069||not North America, Sweden, Germany||Negative||Voltage||N/A||RVC/2415/00AF|
|1974.5-76||360301-409401||not North America, Sweden, Germany||Negative||Voltage||N/A||RVC/1410/00AF|
|1973-76||294251-409401||North America, Sweden, Germany||Negative||Voltage||N/A||RVC/1410/00AR|
|N/A||N/A||V8 (all models)||Negative||Voltage||N/A||RVC/1810/00|
In the RVI current-operated tach coil current flows via the inductive pick-up on the back of or inside the tach, so the tach responds 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! 1964-67 types had the pickup mounted on the outside of the case and the later one (68-72) had it mounted inside the case. For the earlier type a continuous loop of white wire comes out of the main harness from the coil SW terminal, through the pickup as indicated in the top picture on the left (click to expand), then back into the harness again towards the ignition switch. This is for positive earth cars. If the car is converted to negative earth the wire must be removed from the pickup and routed through in the other direction. If fitting a new harness I'm not sure if the ends of the loop are marked to indicate which is coil end and which is the switch end, if not you will just have to try first one way then the other. It won't hurt the tach if you get it the wrong way round. The earth wire for both the electronics and the instrument lighting is under one of the knurled wheels holding the tach into the dash.
For the later type with internal pickup (negative earth cars) the ends of the loop of wire through the pickup are brought out to male and female bullet connectors mounted on the back of the case, see the lower picture on the left. 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.
See here for the circuit diagram and functioning of the 2430/00 RVI tach by Herb Adler, and here for a later version using two transistors from Mark Olsen's Sunbeam Tiger pages. However neither of these 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 goes direct from the switch to the coil, and a tapping off the 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. Both types have a threshold above which any voltage or current pulse will register on the needle, and the relative sensitivities of the tach electronics and the engine are such that under normal circumstances weak ignition pulses will affect the engine before they affect the tach reading.
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 to the case is probably missing.
Updated May 2009: Theo Smit's Tiger pages discuss replacing the Smith electronics with a circuit based on a 555 timer. 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 -ve instead of sensing the current pulses in the white wire going to the coil SW or +ve.
Another DIY replacement circuit can be found here. Don't be put off by talk of square wave generators and oscilloscopes, 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.
Updated 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. 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 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.
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.
Early vs late systems
Internal and external seals
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 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. This has several components inside the gauge which is technically quite complex and expensive. The later system uses an external (to the gauge) voltage stabiliser and a much simpler and cheaper method that uses the heating effect of the current flowing through the tank sender, which varies with fuel level.
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.
The early system requires the gauge to be earthed both in order to indicate the fuel level and for the night-time illumination. On Jaeger gauges at least the bulb in its holder is attached to the fixing bracket i.e. external to the gauge, which has holes round the edge of the case to let the light in. Later gauges have the bulb pushed into a socket on the back of the gauge. The later system uses an earthed gauge as well, but that is purely for the night-time illumination, not for indicating fuel level. Both types should have an earth wire under the knurled nut that secures the gauge in the dash. If this earth is missing from the early system the gauge will not register anything when the ignition is turned on, unless the parking lights are turned on as well when you will get an incorrect reading.
Another difference between the two types 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 (30 ohms or so) increasing towards 300 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 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 two connectors and their spades on the tank unit are clean and bright and making good electrical contact.
Non- or erratic operation: 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)?
Yes - looks like the stabiliser is faulty.
Switching - looks like the gauge is faulty.
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?
Yes - bad connection back towards the main earthing point on the back panel of the boot.
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. 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 and the gauge changed to Smiths BHA4470. The gauge changed again with the Mk2 in 1987 - to one with a shrouded pointer and internal illumination, and again for the 1977 year when the needle pointed upwards instated of downwards, and those for North America and Japan gained the legend 'UNLEADED FUEL ONLY'.
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 are 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. The manufacturer also has this improved version, which David has purchased but not yet fitted.
For details of tanks, senders and gauges see here.
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 rear RHS corner of the car - use an axle stand. 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 by alternately tapping on the thin end of a couple of the tapered sections of the locking ring with a hammer and drift. 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 tank unit on top of the seal, and finally the locking ring on top of the tank unit, rotating it in a clockwise direction and tightening with the hammer and drift.
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!)
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.
'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!
Updated July 2010:
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. 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 their Mk2 stabiliser at least has the 'TOP' markings, all of which implies they are the original thermal devices. But from what I can see they don't have the adjuster studs, which begs the question as to whether they are 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.
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?
Is my stabiliser working?
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:
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.
Electric Temperature Gauge: Added November 2008 Schematic
The electric temperature gauge is very 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. During a thread on high temperature gauge readings on the MG Enthusiasts bulletin board Ralph from Germany contributed the following temperature/resistance data, which may prove helpful in determining whether high readings (for a correct coolant temperature) are due to the sender or the gauge:
|85||185||82||'N' (Normal), about the temperature of a typical thermostat|
|95||203||62||About the point at which the later electric fans cut in|
However note that there were several electric temperature gauges and senders, and in some cases it seems possible to get a mis-match which gives incorrect readings. Ralph's measurements relate to the pre-77 sender. From the Leyland Parts Catalogue and Clausager the senders and gauges changed as follows (only North American spec cars got the first two variants in each case, all models had the final variant):
|Date||Chassis numbers||Part number||Notes|
|Nov 67||138401-368081||BMK 1644||Almost certainly red insulator.|
Start of MkII to 74 1/2
18GF 101 to 18V 672/673
|Dec 74||368082-410000||88G 580||Probably red insulator.|
75 and 76 model years
18v 836/837 to 18V 801/802
|Sep 76||410001 on||13H 5602 |
or 13H 9715
|Almost certainly black insulator.|
77 model year on
18v 883/884 on
|Date||Chassis numbers||Part number||Notes|
|Nov 67||138410-258000||BHA 4686||Start of MkII to 71 model year|
|Aug 71||258001-410000||BHA 5090||72 to 76 model years|
|Sep 76||410001 on||AAU 3030 (BT 2231/01)||Start of 77 model year|
Dates corrected May 2008, error spotted by Holger Beck
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.
However two sources indicate that the sender changed from having a black insulator between terminal and body to a red insulator, which does seem to indicate they would have different characteristics. One (BBS contributor) 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.
Electric Oil Gauge: Added November 2008 Schematic
The situation with the electric oil gauge is a little more complex. For the first year (67) of this gauge 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. 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.
Dual oil pressure/temperature: This gauge is mechanically-operated. The temperature part consists of a sealed system containing a gas or fluid. It is usually the temperature part that fails first, and usually due to a fractured tube. In this case you will have to get an exchange unit. Information on internal and external gauge seals can be found here.