MGB models built before November 1967, when the Mark II was introduced, have a Positive earth electrical system. There are a number of advantages to be gained by converting these cars to the later Negative earth system. The early cars were fitted with a dynamo, which was barely adequate for the job. The alternator has a higher output and is therefore better able to cope with demand and also keeps the batteries in better condition. This is especially significant where a car covers low annual mileage. In addition it can often be difficult to buy accessories such as radio/cassettes and CD players for Positive earth cars. This conversion should be well within the capabilities of the owner who carries out his own servicing and maintenance.

It is possible to convert an MGB to Negative earth using the original Positive earth wiring loom. Before undertaking a conversion the wiring loom must be in good condition and free from any cracked insulation or badly made repairs. If in doubt the entire loom should be replaced. In any case, a car, which is forty, plus years old will most probably need to be rewired as a matter of course. There are three items that should be replaced and these are:

  • the fuel pump
  • the tachometer
  • and the dynamo

The coil can be retained along with the voltage regulator although both will need modification, which will be explained later. The tachometer should be replaced by a negative earth example from a 1967 - 72 car and the dynamo should be exchanged for an alternator. If you choose to re-new the coil then a Negative earth type should be purchased. It may also be useful to replace the main battery lead from battery to solenoid, especially if it is old and the insulation is worn or has hardened. The Positive terminal on the battery is larger that the Negative terminal. During the conversion the batteries will be reversed and the Negative terminal will be in the Positive terminal position and vice versa.

The work involved in the conversion is as follows:

First remove the battery terminals to isolate the power source. Turn both batteries through 180 degrees so that the Positive terminal for the right battery is in the rear right corner and the Negative terminal for the left battery is in the rear right corner. The battery interconnecting lead should also be reversed to enable the terminals to fit. The earth strap on the left battery should be replaced by a lead with a smaller Negative earth connector. Do not connect the Positive terminal to the right battery at this stage.

Replace the fuel pump with a Negative earth example. This is a simple exchange except that the Negative earth pump has a breather pipe that terminates inside the boot. This should be fitted to allow the pump to breathe during operation. It is also a good opportunity to ensure that the fuel line is in good condition. If replacement is necessary the alternative copper system is advisable. The fuel line should be checked during annual MOTs and corrosion or damage could result in failure.

Remove the tachometer and insert the Negative earth replacement using existing connections. The replacement should be from a 1967 to 1972 car which using a wire loop similar to the earlier car. The wiring connections into the loom are the same as for  the '67 - '72 car. If in doubt refer to the appropriate wiring diagram in the workshop manual.

  Hit the picture for a larger view
MGB Alternator BracketReplace the dynamo with an alternator. If you wish you can use the original dynamo but it will need to be re-polarized before use to convert it to negative earth. New alternators are now being manufactured and are generally supplied with a new pulley and fan. It is worth checking that the water pump pulley and crankshaft pulley are suitable for the new alternator fan belt. The Lucas 16ACR variant is the most suitable but higher output alternators are available. If a higher output alternator is used heavier wiring from alternator to solenoid is advisable to cope with the higher current. Fitting the alternator requires a modified rear mounting which is available from specialists. However, I found that the bracket supplied to me, when fitted, did not allow the alternator to align properly. Consequently I made a bracket that allowed proper alignment using the new one as a pattern. Proper Alignment is vital to allow the fan belt to run smoothly and to avoid undue wear on the fan belt and alternator bearings.

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MGB Alternator Bracket
When fitting the alternator you may find that the adjustment on the bracket is insufficient to take up the slack in the MGB fan drive belt. In this case you have two alternatives. You can get a shorter drive belt, a belt for a 1986 > Ford Granada 2.0L should fit or alternatively make a longer bracket. The bracket on the right in the picture was made from a piece of 30mm x 4mm bar. It is 135mm long with a 50mm slot for adjustment.  The MGB fan drive belt will now fit which saves sourcing problems in the future when a new belt is required during servicing.  The original MGB bracket is shown on the left of the picture.

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MGB Regulator Modification
The voltage regulator can be retained as a junction box in its original position on the right side of the engine bay. However, it must be modified to allow it to be used as a junction box. The only modification required is to solder a bridging wire from the 'D' terminal to the 'B' terminal on the back of the box as shown in the photograph. Whilst viewing the back of the voltage regulator you will note that the 'B' terminal is a double outlet. The copper terminals are exposed on the back of the box and soldering should be straightforward. Ensure that new wire is well insulated and protected to prevent the possibility of a short-circuit on the back-plate. The voltage regulator can then be re-fitted.

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MGB Voltage Regulator
 Next the voltage regulator should be wired as follows;
a) The brown/yellow wire from the new alternator should be connected to the 'D' terminal. If necessary this wire should be up-graded 40 amp to cope with the additional output from the alternator.
b)  The brown heavy-duty wire from the solenoid should be re-connected to one of the 'B' terminals.
c)  The double brown wire, which goes to the ignition switch and fuse box, should be connected to the second 'B' terminal.
d)  The brown/green wire from the alternator and the brown/yellow wire to the ignition light do not get re-connected to the voltage regulator. Instead, replace the terminals with bullet connectors and connect them together using a snap connector.
e)  The earth wire from the voltage regulator should not be re-connected to terminal 'E' on the box but attached to a convenient earth point instead. This will ensure that the voltage regulator bracket is not earthed.
f)  Carefully re-connect the Positive terminal to the right battery and test the circuits.

If the coil is to be retained switch the two LT leads over. One side will be market '+' or SW (switch) and the other marked '-' or CB (contact breaker). If the leads are not reversed the car may run but not as well.

It is worth fixing a 'Negative Earth' warning decal onto the bonnet locking platform as a reminder. These are available from specialists. Finally, road-test the car to check that the ignition warning light goes out when the engine is running above tick-over and that all is in order. The conversion works well with the minimum of modification and can relatively easily be brought back to original specification.


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