Negative/Positive Ground
By Norman Nock
Originally published in the Austin-Healey Magazine
An electrical circuit needs a feed wire and a return. The return can be a wire itself, creating an insulted return system, or it can be a common grounding element—in this case the chassis of the car. Cars built after the late '60s used the negative pole of the battery as ground to the chassis. Negative ground was also used on all British cars up to about 1936.
At that time, it was found that there were certain advantages to grounding the electrical system via the positive pole, and the practice was applied almost universally to British cars for over 30 years. There were two main advantages: grounding the positive side of the supply makes the polarity of the central electrode of the spark plug negative, resulting in improved plug performance and longer service life for the plug and the high-tension cables. Positive ground also reduced the formation of electrolytic sulphation at the positive battery terminal lug, and also reduced electrical leakage from battery cells to ground as a result of acidulated moisture. Positive ground also reduced corrosion at points, connectors and soldered joints, especially under conditions of high humidity.
Positive ground had its limitations, however, not the least of which was that if by some chance your car got into the hands of a mechanic not familiar with the fine features of early British cars, he might easily assume the car was negative ground, causing inadvertent damage to the electrical system. Also, if you wish to install an FM radio, an AM radio of any quality, a tape deck or a burglar alarm, you would either have to totally isolate the device from the frame, use a converter with all its limitations, or convert the Healey to negative ground.
Conversion is not that difficult. First remove the battery(s) and any positive-ground equipment (like an original radio) you have in the car.
If you have replaced your original SU fuel pump in the last few years—one with a diode across the points, rather than a condenser—reverse the diode, or it will blow out when power runs through it backwards. You will also have to convert an electronic tach, and I suggest you remove it and have an instrument shop do the work.
Next re-install the battery (s) in the reverse way you took them out—that is with the grounding cable attached to the negative terminal. Since the two battery tugs are of different sizes, you may have to fit new cable connectors.
Once the battery(s) is installed, you’ll need to reverse the residual magnetism in the field coils of the generator. Disconnect the “D” wire (the thick wire) and the ‘F” wire (the thin one) from the generator. Connect a temporary line to the positive pole of the battery and stroke the “F” terminal on the generator (the thinner one). A brief spark will indicate that the field terminals have been repolarized. No changes are required for the regulator.
Reverse the coil so the wire from the coil to the points is attached to the “-“ terminal of the coil. Reverse the ammeter connections if you’ve installed one.
You’re all done. It shouldn’t take more than 30 minutes.
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