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 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 ground and being unfused could cause a fire. There are some 'remote shunt' ammeters around 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.

A voltmeter is a much simpler proposition requiring just two relatively thin wires to an ignition white and ground. The only added fault liability is one of grounding of the white, but if the tap goes via an in-line fuse early on this is eliminated.

Both 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.

Types
The first MGB alternators (MkII models in 1967) were the 16AC (remote regulator) and 16ACR (integral regulator) which were rated at 34 amps. It was changed to the 17ACR in February 1973 and finally the 18ACR in about June 1976. There is conflicting information about the output rating of these latter two, some sources say the 17ACR was 36 amps and the 18ACR 45 amps, others say the 17ACR was 43 amps and the 18ACR 45 amps. 43 or 45 amps ought to be sufficient for factory loads, the V8 has a Delco 46 amp alternator and that is sufficient to keep the battery voltage above 12.8v even with headlights, twin cooling fans and the heated rear window running, and at idling speeds. The problem is that people fit voltmeters wired to the green circuit, which can be a couple of volts lower than the solenoid i.e. battery voltage, then get paranoid. It's battery voltage which is important, and any volt-drop between there and the green circuit is down to aging connections, and the best alternator in the world isn't going to cure them, although it may cover them up.

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 normal-sized spade to one side, and another with a single large spade and two standard-sized (or one standard and one medium) spades either side of it although this may be from only one supplier. With first 3-pin type there seems to have been two variations of how the spades were used - on one the central large spade is the output and the other large spade is the battery sense terminal, with the normal-sized spade being the IND terminal, and on the other both large spades are outputs where either (or both together for more current carrying capacity) can be used, and the normal-sized spade is the Indicator terminal.

Where provided the B+ (or BATT+) is a battery voltage sensing terminal 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 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. This is a 3-wire alternator, but can be used on a 2-wire circuit by connecting the third spade to the output spade in the alternator plug.

The final variation was the 18ACR. Clausager says the exact change point is unavailable, but thinks it was from June 76. This is borne out by the Parts Catalogue which shows the 18ACR being used before September 76, but not by the schematics which indicate it didn't happen until the 78 model year. This version seems to have gone back to machine sensing again, with two large gauge brown wires in the alternator plug, and the two large spades on the alternator are both output terminals. This gives increased current carrying capacity and lower voltdrop now cars had electric cooling fans, offsetting the loss in voltage caused by the regulator sense terminal moving from the solenoid to the alternator again. This is a '2-wire' alternator again.

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

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

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

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

Tip: If you carry an alternator as a spare at any time, then it's a good idea to make sure it already has a pulley fitted. The large nut is very tight and makes it very difficult if not impossible to remove the pulley from a failed unit as there is no easy way of holding the rotor still. 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 still charge well enough to get you where you are going.

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

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 ground like the panel 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 ground 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 may 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 disconnect 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 several 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.

Considering the technology of the control box is so old its method of voltage and current regulation is really clever - a form of time-division multiplexing if you want to be technical. Basically when the battery voltage rises i.e it becomes charged the voltage regulator relay will operate, its contacts (normally closed) open, introducing a resistance into the field circuit. This reduces the voltage at the field winding, which reduces the output voltage. But it doesn't operate just once, oh dear me no, it is usually operating and releasing rapidly all the time unless the battery is significantly discharged. When the full dynamo voltage is connected to the battery, the battery voltage can't rise immediately, but takes a period of time. As it's voltage rises so does the voltage across the voltage regulator relay, which eventually operates. This reduces the dynamo current and voltage, but again the battery voltage can't drop instantly, but takes a period of time. It's only when battery voltage has dropped below a certain point that the voltage regulator relay release, so connecting full dynamo voltage and current to the battery again. The really clever bit is that when the battery needs charging, it takes a relatively long time for its voltage to rise enough to operate the voltage regulator relay and reduce charging current and voltage, which results in a relatively high average current. As the battery recharges the time taken for the relay to operate increases gradually, and the time taken for it to release again reduces gradually, reducing the average current over time. The average current as seen on a graph has a relatively steep rise initially when recharging starts, gradually flattening out as it approaches a horizontal 'fully charged' line, until it just touches it, at which point just a trickle charge is being put into the battery. In practice, unless the battery is significantly discharged, the voltage regulator relay operates and releases in milli-seconds, this can be felt as a rapid vibration of the relay armature as it is gently touched, and a continuous electrical arc can be seen at the contacts.

The current regulator relay operates on a similar principle, but it only comes into play when the maximum design current of the dynamo is been reached. The relay operates, also introduces a resistance into the field circuit, which 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 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 conencted 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.

One weakness of this system is that if the battery is very discharged (one source I have says less than half charged) the characteristics of the control box are such that the battery will never recharge.

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 ground, offers a ground-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, hence the requirement for a network of diodes to convert this to pulsed direct current at the output terminals and field windings.

Charging Faults:

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 12.8v, 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, 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 ground to negative ground. Positive ground alternators will probably be very difficult if not impossible to find, a negative ground will be tricky to convert to positive, and the availability of used, rebuilt and new negative ground 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'. See here for Ground Conversion. 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.

Before starting any of the following work disconnect the battery ground strap first, and only replace it as the final step.

Updated September 2007:
The dynamo should have brown/green (field) and brown/yellow (output) wires. At a pinch the old brown/green could be used for the alternator output but will start to drop voltage at higher currents. If you use this it must be removed from the F terminal of the control box and connected to the three browns instead. But it is better to run a new brown (of suitable gauge for the current to be carried) from the alternator output terminal (See here for Lucas 16/17/18ACR terminal arrangements) to the battery cable stud on the starter solenoid, taping back both ends of the old brown/green. The brown/yellow connects to the smaller indicator spade on the new alternator. The two brown/yellows at the control box must be removed from it but connected together, insulated and taped back. This circuit supplies the priming voltage to start the alternator charging, and indicates charge failure. That should leave three browns and one black at the control box. If you are leaving the dynamo control box in-situ the browns and black can be left connected. If removing the control box the browns at the very minimum must be connected together securely (this connection carries the load of all the cars electrics bar the starter), insulate them, and tape them back as before. Similarly insulate and tape back the black (ground) wire. However the higher alternator current will be flowing through these older brown wires and connections and it will be beneficial to move them to the solenoid, which will remove one of the 'choke points' for current as well as the spade connectors which aren't good with high currents. To do this remove the three browns from the control box, and identify which one goes back to the solenoid. This wire is no longer required so its connectors can be insulated and the tails taped back to the harness out of the way. With the two remaining brown wires (one to the fusebox and the other to the ignition switch, lighting switch etc.) replace the spades with bolt-through connectors and put them on the battery cable stud of the solenoid.

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 on its 12v scale in place of the battery ground 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 ground 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 a ground 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 a ground 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 ground. 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 12.8v 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 ground 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.

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 ground 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 ground 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 ground 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 ground 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 ground 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 a ground 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 ground 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 ground 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 12.8v. With everything turned off, and a fully charged battery, and the engine revved to about 3k, you should see a maximum of about 14.5v. With the engine idling again select 4th gear, handbrake and footbrake on, and slowly lift the clutch pedal up so the revs start to drop. The warning light should come on again at about 600 rpm. Dip the clutch again and take it out of gear, slowly raise the revs again and the light should go out again at about 900 rpm. If your normal idle causes the warning light to come on again anyway, it might be an idea to raise the revs a bit so it stays out, that way the alternator will still be charging at idle, rather than the electrical loads of the car draining the battery.

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

'One-wire' Alternators Added January 2010
Some confusion over these. Some people use this term to describe 'one output wire' alternators i.e. where there is also an excitation wire, as compared to the 3, 4 and 5 wire alternators used on MGBs at various times. Others think that any alternator with internal voltage regulation is a 1-wire alternator - very few are. On the other hand there really are one-wire alternators that do not have an excitation or ignition wire, just a single heavy gauge wire from the output terminal to the battery or starter solenoid. These still need excitation, and it is achieved by having some device that either senses rotation, or senses a drop in battery voltage i.e. cranking. In both cases it then internally connects the output wire (which has 12v from the battery) to the field circuit to commence charging. In the former case it can need revving up to 1200rpm before charging commences, and in the latter it triggers before the engine is turning fast enough to charge, or even when simply turning a light on. In this latter case it will be discharging the battery until it senses that the alternator isn't going to charge any time soon so disconnects again. Not only do these alternators cost more, they also have extra things to go wrong. Hot-rodders with an engine bay stripped of every possible thing like them, but for the rest of us they aren't really relevant, and if you don't mind revving your engine soon after starting you can actually use a conventional alt without the warning light wire connected, previously used examples of which will start charging when revved to 2k or so. If you really are going to use a one-wire, i.e. without a warning light, then you are going to need a voltmeter ... which rather negates the loss of a couple of inches of visible wire from the harness wrapping to the plug in the engine bay!

The turn signal circuit, for example, takes an extremely tortuous path: Battery - heavy current cable - brown circuit - ignition switch - white circuit - No. 2 fuse - green circuit - hazard flasher switch - turn flasher - turn flasher switch - turn lamp holder - turn lamp - turn lamp holder - ground - battery. And as well as all the item to item connections implied above there are also a number of in-line connectors in many of the interconnecting wires. In theory the turn flasher and lamps should receive the full 12v, but in practice every connector and length of wire imparts it own bit of resistance. Straight out of the factory these additional resistances are minimal and for practical purposes can be ignored, but as our cars get older these additional resistances increase. As the resistances increase the current through the circuit reduces, and each of these additional resistances has a voltage dropped across it, which means that there is less voltage available for our components. What is the effect of this? Let's assume that there are 25 connections in the above circuit. Let's say that each connection has a resistance of 0.012 ohms (you wouldn't be able to measure this on a typical multi-meter) giving us an additional 0.3 ohms in total. The effect of this is to reduce the voltage available to the turn flasher and lamps by about 10%, no wonder they cause so much trouble!

Another thing to remember is that when we start getting resistances in the heavy current, brown, white and green circuits things will start interacting with each other. Clearly demonstrated where there is a high-resistance shared ground. For example on a rear light cluster, using just one of side light/turn signal or brake light will appear to work correctly, but other lights will glow dimly. But if two or more lights are operated together then none of them in the affected cluster will work properly.

When looking for high-resistance connections we have already seen that using an ohm-meter (of the type available to most of us) is useless. What we have to use is a voltmeter, measuring the voltage at various points in the circuit. However the circuit must have a load as well as a supply - you need the load to draw a current and cause a volt-drop across the fault before you can measure it. The standard multi-meter draws such a tiny current that on its own it will cause negligible volt-drop, and hence not reveal much less than a complete disconnection. A good tip is to measure the volt-drop along a circuit instead of measuring the voltage to ground at several points. We might put the +ve terminal of the meter on the +ve terminal of the battery (assuming negative ground) and the -ve terminal of the meter on the green wire on the flasher unit, for example. With ignition and turn signal on (and hazard switch off) this will reveal the total volt-drop between battery and flasher. Using different points along the circuit will give you the connection or connections that are having the most effect.

Checking the heavy current circuit: The heavy current cables are best checked as follows (assumes negative ground, for positive ground cars change each reference of positive to negative and vice-versa):

Connect the positive probe of an analogue voltmeter (a digital meter may not 'latch' with fluctuating voltages) to the positive post (not the clamp) of the battery and the negative probe of the meter to the heavy current lug on the starter motor, or if that is concealed on inertia starters to the heavy current lug on the solenoid. Note that if you use the starter lug the meter will show 12v immediately whereas if you use the solenoid it will show 0v. Disconnect a coil wire so the car cannot start, turn the key to the start position and note the meter reading.

Now do the same with the positive probe of the meter on the starter case and the negative probe on the ground post (not the clamp) of the battery.

In a perfect world you would see 0v while cranking both times. But even the heavy current cables and good connections will have some resistance, and hence be causing some volt drop, but ideally it should not exceed 0.5v in either path. If you get significantly more than 0.5v you have one or more bad connections, and by using the same technique of putting the probes at various points along the circuits you will be able to determine those that are causing the biggest volt-drops. These can typically be the cup-style battery connectors, the ground strap where it bolts to the battery box, and either end of the grounding strap. Incidentally make sure you do have a grounding strap either between block and chassis rail or gearbox and body or your starter current will be returning to ground via the heater and accelerator cables, heating them up and possibly damaging them in the process.

When checking voltages at components check the component terminal as well as the wire connector that is connected to it. With PO wiring or re-terminations also check the connection between the wire and the connector if you can.

Earth/Ground connections. In addition to his information on problems with number plate earths Felix Weschitz also sent me his thoughts on getting good ground connections. Additionally I always daub Waxoyl on all nuts and bolts, whether on the body or not, electrical connections or not, to delay the onset of the dreaded corrosion particularly where things are bolted to the body and where the paint is inevitably damaged.

Connectors. With normal care and use bullet and spade connectors from the factory will give good connections for many years. But with PO 'repairs' using dodgy components and techniques or if the car has been abandoned in a field for years corrosion can develop that doesn't occur in normal use and both will cause electrical problems.

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 of front lights, two-pin connectors for electric fans, and the ground point of parking/indicator light units on chrome bumper cars. This last is because they don't have a ground wire but rely on getting a good ground from their mechanical fixings to the wings. The back of these is in the full force of water and salt spray from the wheels and the resultant rust. Rear light clusters can also develop grounding 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 ('A' in the picture on the left) to make this job easier. I subsequently discovered there is a specialised tool for this, but at £20 I'll stick with my modification, thank you very much. The mod consists of cutting a notch at the bottom of each handle ('B' in the picture) that accepts the wire but will push on the back of the bullet ('C' in the picture). Squeezing the handles in the normal way pushes both bullets (or one bullet if it is the third wire in a 4-way connector) fully into the metal connector. Without adequate force it is easy to leave the back of the bullet hanging out of the connector where it is loose has a risk of shorting, and has less contact surface area. This means the joint is capable of carrying less current without overheating and corrosion will break the connection sooner. Click on the image on the left to see the modification and its use.

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) ground, some tach grounds, 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 ground 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.

Which Terminal is Which? Added July 2009:
Modern 12v batteries usually have the polarity symbols + and - moulded into the battery top by the respective posts, as well as being supplied with colour-coded caps (red for +ve and blue or black for -ve) over the post (discarded on fitting), and possibly coloured rings around the base of the posts (permanent). But some 6v batteries don't seem to have markings, even current supply. These seem to be the ones with the individual screw caps for the cells of which there are least two designs - the original tar-tops with black caps as well as a more modern smooth-topped battery with coloured caps. My present 6v batteries have a single rectangular cover (red) over all three cells and do have + and - markings. There is a possibility that some makes may have a distinct vertical groove in the negative post (no + or - markings), but this remains to be confirmed. Easy to use a meter to determine polarity - as long as you are sighted and the battery has some charge in it! Other than that all the batteries and cars in my experience have had the posts and connectors of different sizes - positive larger than negative. It's not much by sight or touch, only about 1/20" in diameter, but it makes a big difference if you try to put the negative connector on the positive post (it won't fit) or the positive connector on the negative post (it drops on and is loose). If you do need to test-fit the connectors, make sure you only do one at a time, and only one battery at a time, to avoid reverse connection and the risk of shorts from the loose end of the link cable. When changing polarity always change the connectors as well, junking the cup-shaped type (if you still have them) for bolt-up type, as whilst the bolt-up type can be made to fit the wrong posts it would be rather short-sighted. 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.

12v or twin-6v? Twin 6 volts in chrome bumper cars, single 12v in rubber bumper. Yes, twin 6v batteries are a bit more expensive to replace (typically 2*£58 vs 1*£70 from one source in July 2008, up from £40 some years ago for the 6v whereas the 12v is the same price) but I like to keep mine as original as I can. 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? Certainly mine last well enough - fairly early into my third set in 14 years, and that with very little use for several months over winter. 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).

Updated July 2008: 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 tend to be 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. It could be that the original 'tar top' type are using the old technology, but I'm pretty sure the others benefit from the modern technology.

Added January 2009: Battery Megastore (patience, it can be slow to load) have what looks like an MGB 6v (170l x 175w x 225h) and quotes 57Ah (virtually the same as the Workshop Manual 58Ah at the 20 hour rate) but only 250CCA for £70. Leacy are showing four different 6v batteries from £32 to £57. However they only supply the GBY3031 type now, suffix 'W' is 'Wet' for collection and suffix 'D' is 'Dry' for mail order, 56Ah and 270CCA. They also have 12v at £69 (no spec). MGOC are showing 6v (56Ah, 270CCA) for £49, filled for collection only, add £17 for acid packs (six needed for two batteries) for mail order. Moss Europe list 56Ah 270CCA at £64 and 63Ah 295CCA heavy duty at £73. Performance Batteries lists three batteries suitable for the rubber bumper MGB ranging from its 57024 of 70Ah and 600CCA at £63 via the 570 413 063 of 70Ah and 630CCA at £85 to the AX D26R of 75Ah (all at the 20 hour rate) and 750CCA at £130, up to 266mm x 175mm x 220mm. By comparison on the same date Halfords was listing its HB072 of 68Ah and 510amps starting power (i.e. probably CA and not CCA, i.e. about 400CCA) at £93 and its HCB072 of 68Ah and 550amps starting power (440CCA) at £97, both at 261mm x 175mm x 220mm. Both sources include VAT making Halfords at least 30% more expensive for less performance, but convenient as you can buy in store and don't have to pay shipping. Americans often quote 'group 26' as a suitable 12v battery for a chrome bumper MGB and this site quotes the dimensions of that as 208l x 173w x 197h. The Performance Batteries site lists two batteries that might squeeze in - the 544 402 44 of 44Ah and 440CCA at £50 and the 077 of 44Ah and 390CCA at £56. Both of these have quite a bit less Ah rating than the standard 6v, not surprising as they are half the physical size, and even though they have quite a bit more CCA it is still less than the 12v for the rubber bumpers and quite a bit less than the 475-575CCA I've seen quoted for group 26 American batteries. 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.

Twin 6v Link Cable The roadster came with a single 12v which needed replacing quite soon, 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 ground clamp in the other box, installed the batteries and clamps, checked the volt-drops, and away we went. Not too long afterwards I had occasion to remove the interconnecting cable, can't remember why, and saw with horror that it had been hanging down and rubbing on the propshaft! It had marked the insulation but fortunately not rubbed through. So I installed my own tube to support it up out of the way.

Added April 2009
When I fitted the battery cut-off switch to Bee last year I found a mass of corrosion around one of the terminals on the right-hand battery, where the link cable attached, and the fluid level in the cell closest to it was well down. I cleaned it all off (it made a terrible mess of the drive and is only now starting to fade 12 months on) and found it had eaten away quite a bit of the clamp, so bought a new one, this time of the correct armoured type. These are supported by a clip underneath the battery shelf, at the front between the two batteries. I found this clip is held by a bolt that goes into a nut in a box-section I didn't even know was there. Quite a bit of surface rust up there, and restricted space between the battery boxes and above the prop-shaft, but the bolt came out very easily. Almost certainly the first time it had done so since being fitted at Abingdon, so I treated it with due respect! The new cable went in quite easily with the clip and bolt, but when I went to attach it to the battery posts I found it was shorter than the old one, and would only just reach its post by lying tightly across the other clamp - not a good idea. I tried turning the battery round 180 degrees but then the main battery cable was nowhere near its post. Turned round just 90 degrees both clamps fitted, but one of them was very close to the clamping strip, which in any case was hard up against the cover over the refilling ports, again unsatisfactory. Fortunately because I had fitted the cut-off switch in the main battery cable I was able to remove the short length between the switch and the 12v post, remove the lug and connector, and transfer them over to a suitably longer length cut from the old link cable which would reach its post with the battery rotated 180 degrees.

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

Plastic battery bins - I can't see the point in these, in fact I think they are being mis-described and mis-sold. If you have converted a chrome bumper car from twin 6v batteries to a single 12v and use one in the empty space as semi-secure storage that is fine. But it seems to me that:

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 be very thankful for your foresight.

Battery Chargers Updated November 2007 I've just seen an episode in the new series of The Garage on Discovery last night about the 'English Mobile Mechanics' in Spain. They had a BMW with a flat battery so connected a boost charger direct to the battery in the boot (even though my son's BMW has jump-lead connections in the engine compartment). These boost chargers put a very high voltage to the battery and hence a high current, a lot of boiling and gassing. They got it started and the boss told one of the mechanics to disconnect the charger, which he did - by disconnecting one of the battery clips without turning off the charger first. Big spark from breaking the high current, ignited the gases around and inside the battery, and the battery exploded. The mechanic was very lucky in that he only got acid up his arm and not in his eyes, nor any injuries from plastic 'shrapnel'. Whilst I suspect that most of us have smaller chargers that would result in less gassing and a smaller spark, I wouldn't want to be the one to find out just how small a spark and an amount of gas would cause an explosion. This is also why the last connection to be made when using jump-leads should be made remote from the battery.

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 ground and -ve ground cars. The cigar lighter was an option from the beginning and standard from the 1973 model year, but was wired differently over the years, each needing a different approach as follows:

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

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

Another option is to use a different plug and socket with the socket in, say, the engine compartment connected to the purple fuse and ground 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. 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 the fitting and use of a cut-off switch is much less convenient.

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 Types Added December 2008: Battery technology and hence selection is getting ever-more complicated with technical advances.

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

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

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

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

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

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

References:

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 as far as cutting the power goes. The only 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. One slight disadvantage of having the switch in the earth cable but positioned on the (left-hand in this case) heelboard is that you may need a longer cable to snake round the battery to the switch, as well as an additional one back from the switch to the battery post. Not only from a space point of view but also from a length and cranking resistance point of view. It's marginal in either direction, but the earth cable does avoid cutting the 12v cable and fitting two lugs, and is probably the way I would do it in future.

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.

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.

Note: I have seen the dash harness for 77 and later UK models with wedge-type bulb holders

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

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

Tachometers: There have been two types of electronic tachometers - the earlier inductively-coupled type (which came in two versions - positive ground and negative ground) and the later directly connected type. The inductively coupled type uses the white wire that goes from the ignition switch to the coil, the wire is looped round the pick-up i.e. it passes through it twice, and the direction is critical. The directly connected type uses a black/white from the coil (the same terminal as the feed from the distributor i.e. the -ve on negative-ground cars) and terminates on a cylindrical connector at the tach rather than a flat spade. For a description of the circuitry, calibration and repair of the electronic tach see this article from Mark Olsen's Sunbeam Tiger pages. For a table of the tach (and speedo) types that are correct for each car (currently for the USA only) have a look at Ken Earnhardt's MGB Shop.

Voltage Stabiliser: 'Stabiliser' 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 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 input terminal (12v supply), and its moving end breaks and makes contact with the output terminal (to the gauges). A heating element is wound round the bi-metal strip and connected between the output terminal and ground. 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 ground. The heating element causes the bi-metal strip to bend, which opens the contact, which disconnects system voltage from the gauges. It also stops the flow of current through the heating element, so the bi-metal strip cools, closes the contact again, and the cycle is repeated. 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, which is how high system voltages result in shorter 'on' times than low system voltages.

The stabiliser has two spade terminals and its 'can' needs to be properly grounded (usually screwed to the firewall behind the dash on the RHS of RHD cars for example) for it to work correctly. The terminals should be labelled B (battery) and I (instruments), the green wire goes to B and the light-green/green to I. However the spades are male and female respectively so it should not be possible to get them the wrong way round. There is also a threaded calibration stud on the stabiliser, but remember that this will affect both gauges. If only one gauge is out, because of a replaced fuel tank sender for example, you should recalibrate the fuel gauge, not alter the voltage stabiliser.

Updated July 2010:
Incidentally the drawing in the Leyland Workhop Manual is misleading, as shown it couldn't possibly work! The contacts by the B terminal should be shown closed, as this is how they are when first turning on the ignition, to heat up the winding around the bi-metal strip. It is only when this has heated up and the moving contact bends away from the fixed contact that voltage is disconnected from the gauges as well as the heating coil, as shown in this amended drawing.

Fuel Gauge: 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.

The 62-64 cars used a direct-reading gauge using a completely different principle to the later cars, only the system for the later cars is described here. For the earlier gauge, which was the same as for the MGA, see Barney Gaylord's web site. Update January 2010: This system needs a local earth at the gauge as well as one at the sender to function correctly. If the gauge earth is missing it will likely swing from E to F and back again as the gauge lights are turned on and off. The schematic is not clear as to whether these gauges had a wired earth as the later cars definitely did, or if it relied on the mechanical mountings cutting through the paint on the back of the dash and where the dash bolted to the body.

Fault diagnosis
Tank Sender
Calibration
Gauge identification

Diagnosis: 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 a ground to the green/black wire on the back of the gauge. Does the gauge pointer move smartly towards to 'Full' (remove the ground as soon as you see the pointer moving)?

Tank Sender: Updated November 2006
The tank unit consists of a length of fine wire wrapped round a former across which a grounded 'wiper' contact moves as the fuel level rises and falls, making a variable resistance. One end of the wire is connected to the insulated terminal on the 'base plate' of the tank unit and the other end is open circuit. The wiper contact is connected to the body of the base plate via the float arm and its pivot 'bearing' in the tank unit structure. The wound former is tapered in shape and the distance between adjacent turns varies in an attempt to make the movement of the gauge needle bear some relationship to the graduations on the gauge, which themselves are non-linear. However it fails dismally. On my two cars I get infinite MPG for the first 40 miles 50-odd MPG for the first half of the tank, about 10mpg from half to a quarter, and something a bit closer to the actual mpg for the bottom quarter. The 'full' resistance on one of my tank units is about 35 ohms and the 'empty' about 300 ohms, although there seems to be quite a bit of variation between tank units, see the section on calibrating the gauge.

Up to September 76 the tank unit base-plate has two spade terminals - an insulated one to which the green/black wire goes and a smaller, uninsulated one which should have a ground wire on it which usually goes to the earthing point on the boot back panel together with things like reversing lights and fuel pump. It is possible for the fuel gauge to work without this ground 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 ground tag.

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 ground 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!)

Calibrating the gauge:
Whilst it is possible to bend the upper and lower stops on the sender to get a bit more travel (but run the risk of running off the end of the winding at either or both ends), or bend the float arm (which only moves the available travel up or down the range of the gauge to leave an even bigger 'dead area' at one end or the other), the real problem is that the resistance doesn't go low enough at F or high enough at E to get full travel of the gauge needle, so the only real solution is to alter the gauge to compensate for this. Getting at the gauge is also a lot easier than getting at the sender, can be done at any time i.e. with a full tank, and you won't get leaks afterwards! The back of the gauge should have two holes, one by each terminal post (they may be covered by cork plugs), each containing a slotted plate. Twisting the plate by the terminal at the 'F' end of the gauge will adjust the 'Full' reading, the other adjusts the 'Empty' reading. Do them in this order so that changing the 'Full' setting (easily done after filling up) does not upset the 'Empty' calibration as this latter is the more important one to have accurate. For the empty adjustment I ran the tank right out whilst carrying a spare gallon. I put the spare gallon in the tank and only then adjusted to E, to give me a gallon 'reserve'. I say 'slot' but the hole is more like an oval, probably for a specially shaped adjuster. Make sure you use an implement that is a good fit in the slotted plate, they can be stiff, the plate is only thin, and a poorly fitting screwdriver is likely to 'round out' the slot. You will see from the last two photos that the action is more of a 'twist and slide' as the slotted plates pivot about a point above the slots, and not about the slots themselves. There has been a suggestion that slackening the terminal post screws make the adjustment easier, you will see from the photos that this is not the case, the slotted plate is retained by rivets, the terminal post are mounted elsewhere on the insulated back-plate.

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

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

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

Electric Temperature Gauge: Added November 2008 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 also 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:

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:

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 here (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 as shown here. 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. 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. But 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.

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 The situation with the electric oil gauge is a little more complex. For the first year (67) of this gauge 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.

The first consideration is the batteries. Before doing anything else make sure the battery ground connection is the first thing you remove, and the last thing to reconnect at the end. All the batteries I have seen have different-sized posts for +ve and -ve so in theory you cannot connect them the wrong way round, therefore the connectors will have to be swapped over or replaced. The original 'helmet' type that completely cover the post and are secured with a small screw that goes into the post expand and get loose with age and repeated removal and replacement, giving poor connections, and some resort to using silver paper to get a tight fit. Seeing as you are changing the polarity originality is not an issue, so if you haven't already replace these with the bolt-up type which give a much better connection. The other thing with the helmet type is that they are usually moulded on, these have to be cut off and replaced with the clamp on type, which usually have two large screws to secure the cable. This results in shortening each cable by about an inch but that shouldn't be a problem. If it is, then you will have to replace the cable(s). If you already have clamp-up type connectors remove these from the hot and ground connections and swap them over. Unless you have already replaced the twin-6v with a single-12v you will also have to deal with the interconnecting cable in the same way, and unless it can be physically removed from the car and reversed you will have to cut off and replace moulded-on helmet-type connectors, or remove and swap over the clamp-up type.

If you are retaining the dynamo this has to be repolarised so that it generates the correct polarity voltage. Disconnect the wires from the F and D terminals of the dynamo and with the batteries connected take a jumper lead and connect it briefly between the brown at the fusebox and the F terminal of the dynamo so as you see a small spark. Just one flash is enough, then reconnect the dynamo.

Cars after 64 had the electronic tach and this has to be converted too. You have to get into the case, find the supply and ground wires from the case to the circuit board, and reverse the connections. But note that some cars (e.g. a 67 B belonging to John Schroeder) have the circuit board screwed to the case and pick up the ground connection this way. In this case you have to isolate the circuit board from the case, move the original 12v supply wire from the terminal on the case to the body of the case, and provide a new wire from the ground connection on the circuit board to the 12v supply terminal on the case. John intends to publish notes and pictures of this on the Chicagoland MG Club website. In all cases you have to reverse the direction of the current pulse through the pickup and this also varies. On early models the pickup is external and a continuous white wire comes out of the loom, through the pickup twice (i.e. one turn) then back into the loom. With these carefully note the route the wire takes now, remove it, and reverse the direction of the wire through the pickup, but keeping everything else the same e.g. the position of the loop. On later tachs the pickup is external with two flying leads with male and female bullet connectors to match up with their opposite numbers on wires from the loom. As you are doing wiring changes inside the tach it would be neater to reverse either the ignition wire as it passes through the pickup, or the output of the pickup to the circuit board, whichever is easiest. But an alternative is to reverse the bullet connectors where the loom joins the tach. As to which pair, that is up to you. Since it is the car that has changed polarity it might make sense to change the bullets on the loom wires. But as you have to change the polarity of the power supply at the tach, it might make sense to change the bullets on the tach wires as well. The only advantage of doing the other way is that if you have to replace the tach in the future the pickup part of the polarity change does not have to be done again.

Added December 2009: It's frequently stated that when changing the car's polarity you should also reverse the coil connections to keep the polarity of that and the HT spark the same. I usually mention it when the subject comes up, but you do end up with less HT voltage than before either way, replacing the coil with one intended for negative earth cars would be preferable, see Ignition Coil polarity.

If you have the heater fan motor with black and green/brown wires these may have to be reversed at the connectors by the motor. If in doubt try them both ways (you can't do any harm) and if one way blows more air than the other that is how to connect it.

Finally fuel pumps. Originally the pumps had capacitor protection (reduces the burning of the points) and are not polarity conscious. But if you have installed one of the later pumps with an 'X' in the part number e.g. AZX137 then these have diode protection (improved points protection) and need to be modified. Open the electrical end-cap and locate the black cube with two wires - one black and one red - coming out of the same end of the cube. These wires must be removed from the pump and connected the other way round. There is no immediate risk of damage powering-up without having done this change but it will largely disable the points protection so the points will burn faster than usual. Important If you have fitted one of the 'pointless' electronic pumps then you will probably have to replace the pump as converting the electronics in these is much more involved, and changing the battery polarity without doing anything about the pump will result in the pump not working at best or destroying the electronics and possibly burning the loom at worst.

That's it, unless you have any other electronic devices, which will be aftermarket and so up to you. The only possible other thing might be that the wipers now park in a slightly different place. If it bugs you then move the arms on the spindles. Start the car, check the tach is working, and measure the voltage on the brown at 3000rpm with minimal load. With a dynamo you should see in the order of 14.3v to 15.5v depending on ambient temperature (lower volts at higher temps), with an alternator you should see 14.3v to 14.7v.

Rubber-bumper cars up and including the 1976 model year did have a relay and powered the HRW from the purple circuit, but then from the 1977 model year it reverted to no relay and powered from the green circuit again, which can only have been a penny-pinching measure.

Updated June 2009: Originally an option, on Mk1 GTs the switch and warning light would have been on a little panel added somewhere. On Mk2 North American spec, 1972 model-year elsewhere, the switch moved to the centre console. There always 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.

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 turn signals 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

Adding a Relay:

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

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

Added October 2009: There often questions about the wiring and connections to the HRW on the tailgate itself. There were two types of HRW - the earlier embedded wire element type and the later surface-printed element type, I can only speak for the latter. On my V8 the wires exit from a hole in the rubber seal surrounding the glass very near the hinges, and terminate in bullets. The wires run down under the seal to the element connection points which are about mid-way down each side. The rubber seal is pretty hard to lift, and I don't want to damage the connections, so I've been unable to determine what lies beneath, but word is that it is a small spade connector. You should be able to test at these points with a voltmeter to see if non-functioning of the HRW is due to a break in a wire feeding the element, or a problem with the elements themselves.

Haynes indicates 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 ground from their physical mountings, these are known as '1-wire horns' (and saved about 3 feet of wire!). However the Bentley diagrams show 2-wire horns being used until the end of production, I haven't seen a late model Leyland Workshop Manual for a final decision.

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 ground to the column itself - it relies on its mechanical fixings for this and not a dedicated ground wire. This is only a problem with the earlier 2-wire horns. I used a relay to 'boost' the ground to operate the horns, but I have heard of others connecting a ground-strap between chassis and rack. Before going to the bother of adding a relay or ground strap check the other connections first.

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 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 column stalk until the end of production. (NB. Either arrangement is far preferable to that on the ZS, which has two little buttons to one side 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, hardly ideal when you need to give an urgent warning of approach!

Updated October 2009
The various arrangements for connecting to the wheel-centre horn contacts can be seen by clicking the thumbnails to the left. I'd like to dispel the myth that the sprung wire (69 and earlier) or 'pencil'/sprung rod (71 to 76) is a 'brush'. A brush is something that provides a rubbing contact between two independently moving electrical components, e.g. on the commutator of a dynamo or motor. 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 the back of the wheel (71 to 76). Some think that the sprung wire or 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) simply connects the column slip ring to the horn switch, the pencil (71 to 76) connects the back of the wheel slip-ring to the horn switch. They sit between both, 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 ground 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. Although 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).

Fault Diagnosis

Adding a Relay:

Note that this circuit will only get round a poor ground from the column and switch onto the purple/black. I opted to install the relay as the column on my V8 had a low-grade ground that would not even operate one 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 near where it emerges from the column multi-way connector and that way both ends may be long enough to reach the relay without having to extend one or both wires. Inside the cabin is also a better environment than under the bonnet/hood.

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 ground 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 ground, taken from a tag secured under the relay lug, out on the purple/black to the horns themselves.

Footnote: Some time later I decided to see just where the high resistance connection 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.