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.

charging schematics
fault diagnosis
converting dynamo to alternator
converting 16AC alternator with separate regulator to later alternator with integral regulator
or just read on.

Ignition Warning or 'Idiot' Light & Charging

Why 'Idiot' light? I don't know, but it seems to be an Americanism. 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.

Charging Theory

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.

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. This current from the ignition, through the bulb and through the field windings so lighting the bulb, also gives the initial excitation current to the field windings, which mean the output windings start to generate a voltage, which is fed back to the fields 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 stops glowing. From this it can be seen that the ignition warning light is necessary to give the alternator its initial excitation. However, 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. Note that once it has started charging, it will charge normally i.e. down to idle speed. 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.

On a dynamo the warning light is connected to the output terminal (and hence has a low-resistance path to ground to light the warning lamp), but because the dynamo control box has a cut-out relay that only connects its output to the battery once it has started charging, the effect is just the same. However the initial excitation for the dynamo field always comes from residual magnetism which is why you may have to 'flash' the field terminal to battery when you install a new dynamo, and must do if you are converting a dynamo-equipped car from positive ground to negative. I repeat, NEVER flash an alternator's terminals to battery.

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. 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. 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.

Some early Lucas alternators, possibly up to 1971, seem to have had two 12v connections - one labelled + and the other labelled B+ or BATT+. The first is the output connection and the second seems to be a 'sense' connection which either ensures correct voltage regulation, or possibly protects the alternator against the engine being run with it unplugged. Originally both these had brown wires (heavy gauge for the output and 'standard' gauge for the sense) and were run down to the solenoid, but they can be linked at the alternator instead. Later alternators used an internal connection anyway. These still seem to have two large output spades which are connected together internally. For standard loads you only need use one of them, but if you are getting a high power alternator and intend to add some heavy loads to the cars electrics then using one heavy gauge brown wire from each terminal to the solenoid will reduce the voltage drop.

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 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.

There were several different connection arrangements to the alternator over the years ranging from 4-pin of the 16AC with remote regulator (best avoided for a conversion) through 5-pin using two connectors, and 3-pin and 2-pin using a single connector for the other Lucas variants. 3rd-party alternators will be different again. In the 3-pin at least there were two further variants - one with a single large spade and two standard-sized spades either side of it, and one with two large spades and a single normal-sized spade (click thumbnail). Even with this last 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 (e.g. my 73 roadster). On the other (probably more common) both large spades are outputs and either (or both together for more current carrying capacity in the wiring) can be used and the normal-sized spade is the IND terminal (e.g. my 75 V8, pictured). So some care needs to be taken to determine just which you have and which wires need to be connected to which terminals, I can't help more.

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 damage it. Confirm the polarity is correct before continuing.

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 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 control box 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 control box, 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 (see below) of the new alternator and remains on the solenoid.

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

That should leave the brown going from the control box 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 control box and which to the fusebox. The one going to the fusebox must remain connected, but the one going to the old control box 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 control box 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.

Lucas 16ACR, 17ACR and 18ACR alternators are pretty easy to connect. Most seem to have one or two large spades and the remaining one or two standard sized. Where there are two large both of these are usually outputs, and either one can be used, and indeed you can run another heavy gauge brown from the second large spade to the battery cable stud on the solenoid for extra current carrying capacity and less volt-drop. The brown/yellow goes to the remaining standard sized spade if there are two large output spades. But if you have one large and two standard sized, or a non-Lucas alternator, you will have to experiment.

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.

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.

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.

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 amp-hours at the 10-hour rate or 58 amp-hours at the 20-hour rate. These days automotive batteries are quoted in 'Cold Cranking Amps' (CCA) or 'Cranking Amps' (CA) as starting an alternator equipped car is its main use not a continuous discharge over a period. 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. The current recommendation for 6v batteries for the MGB is 66 amp-hour capacity and 360 CCA. If you regularly start the engine in temps below freezing you 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 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.

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.

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 by the time you have got the box in the hole, a battery small enough to fit in the box, holes cut in the box for the cables and ventilation (battery fumes are explosive and will come up into the cockpit otherwise, and the high concentration above the battery will accelerate corrosion), insulation on the underside of the lid to stop the batteries jumping up and shorting out or more holes in the box for the clamp rods and cutting the clamp plates to fit, it is an awful lot of bother when the top of the battery stays pretty clean anyway. And even if you do decide to fit the clamps I have been told that the boxes do not sit on the cradle but are supported by the lip, which seems to me like quite a lot of load on a bit of plastic, even worse if you are considering using the boxes because your cradle has rotted away.

And finally... The batteries, particularly the single 12v in a rubber bumper, are a tight squeeze through the hole. 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.

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.

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

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 you can reach it quickly in an emergency. The +ve lead passes right by here so that is the most convenient place to put it. 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 you are merely tightening any of the other posts, i.e. you are going to remove the it/them you are going to be removing the earth connector anyway so having a switch in the earth lead is no advantage. A disadvantage is that to have the switch on the heel-board in the earth lead you would need to replace the earth lead with a longer one, and have more cable snaking around the batteries in an area which already quite cramped.

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.

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

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.

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.

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.

The tank unit base-plate should have 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. Note that later tank units with the integral fuel outlet may not have the ground tag.

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)?

Replacing the tank unit: The tank unit 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. The tank unit is usually 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 gettat the sender, can be done at any time i.e. with a full tabnk, 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've added a section on gauge identification, click on the final thumbnail. Also I debunked my long-held opinion that the thermal stabiliser was needed with the thermal gauge to eliminate fluctuations caused by ambient temperature variation, click on the fourth thumbnail. However the thermal stabilisr does result in a faster initial movement of the gauge from rest that 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.

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.

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 is 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.

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.

Adding a Relay: