PDLJMPR Web Magazine, April 1, 1996
Written
by Norman Nock, British Car
Specialists, Stockton, California
Reprinted with Permission of Author
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While strict originality is important to the concours entrant, even most fanatics won't notice that an old British positive-ground car has been converted to negative. The only tell-tale is the battery cables; where they lead to and their size.
If you're using your car as a driver and wish to fit a modern stereo or other accessory that is available only as negative ground, your options are twofold: isolate the negative-ground accessory through a complex system of rubber washers and special fastening arrangements, or simply convert the whole car to negative ground. Conversion is really quite simple.
In general, generator-equipped models were positive ground, and if you're going further and converting to a modern alternator, you're already one step there.
The first thing to do is to disconnect and remove the battery or batteries from the car. If you have any special positive-ground electronic equipment in the car - which is doubtful - remove it now to have its polarity reversed.
Also, if you've replaced your original SU fuel pump within the last few years, it may be one of the newer designs with a diode across the points in place of a condenser. If so, reverse the diode, or it will expire when power is run through it backwards.
Now, reconnect your battery (or batteries) in reverse of the way they were originally connected. The ground cable that originally went to the (larger) positive battery terminal will now be too large to fit securely on the smaller negative terminal, and vice versa. In this case, fit new cable ends. If you have the old-style Lucas "helmet" connectors, new ones are also available to fit the cables as needed.
The residual magnetism of the field cores in your generator must be reversed, so disconnect the 'D' and 'F' terminal wires fron the generator, and temporarily connect a length of wire to the positive terminal of the battery and touch the other end of the wire several times to the small 'F' terminal on the generator. This will repolarize the field windings. Replace the 'D' and 'F' wires to the generator as they were hooked up before.
No polarizing is required for the regulator, but the electricity will now, of course, flow through the points in the opposite direction. If the points are pitted from many years of use they may stick. Either clean them, or replace the regulator.
Most ignition coils will work either way, but you can easily switch the SW (switch) and CB (contact breaker) wires to be sure. High tension coils marked +VE or -VE should have the -VE lead going to the distributor points.
For the ammeter, if fitted, reverse the connections to show the proper readings. Likewise, any permanent magnet motors (radiator fans, wipers, screen washers, etc.) will work properly with just a reversing of the power and ground cables. Otherwise they will run backwards.
Now you're ready to install your new stereo, CB, car phone, alarm, or whatever.
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