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 thirty, 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:

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.

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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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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.

Once this work has been completed the re-wiring can begin. Connect wiring to the fuel pump in the normal way ensuring a good earth. If the original Positive earth coil is retained the two low tension wires should be reversed as follows;

The coil will have to be re-located on the right hand inner panel in the same position as on later models.

Connect the wiring to the alternator with the heavy-duty brown/yellow wire to either of the large spade connectors and the brown/green wire to the small spade connector. If the dynamo connectors are not suitable for the alternator or are in poor condition then they should be replaced. A suitable alternator plug kit is available from specialists.

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Next the voltage regulator should be wired as follows;
a)  The brown/yellow wire from the alternator should be connected to the 'D' terminal.
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.

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.

'Kingpin'