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MGs and many other British sports cars thru 1967 were wired at the factory with the POSITIVE battery terminal connected to the car chassis (GROUND). Thus the cars were labeled "Positive Ground."
Effective with the 1968 models, British auto manufacturers switched to wiring their cars "Negative Ground," as most of the auto industry had done many years previously. In "Negative Ground" cars, the NEGATIVE battery terminal is connected to the car chassis (GROUND).
Our cars can function equally well with either system, so many of us have no reason to convert to negative ground. But those of us who want to equip our cars for modern electronic devices, like a stereo or car phone, will benefit from the conversion. It is very easy to do. Most car radios and electronic items made in the last 30 years or so are wired for negative ground only. There are three ways to install one in a positive ground car:
Use a voltage inverter.
This device sells for about $30.00 and converts positive ground power to negative ground. The inverter sometimes makes a humming sound and they do not last forever.
Isolate the radio from the rest of the car.
This requires careful mounting of the radio and antenna so that they do not touch any metal part of the car. The radio is then wired so it's metal case is "hot." Any contact between the case and the car's body, dashboard, etc., will produce a blown fuse, burned wires or worse. Anyone working on the car who doesn't know that the radio is wired "backward" may trigger an expensive mistake.
Convert to Negative Ground.
This is the method we recommend. Here's how.
First, disconnect one of the battery cables; it doesn't matter which one. Then reverse the wires at the ammeter; otherwise it will read backward after the conversion.
Second, reverse the two small wires at the coil. The car will run with these wires connected either way, but the spark plugs will fire more efficiently if the coil is connected properly. This is a good time to check if your coil is wired properly.
Third, reverse any other solid-state, non-stock devices your car may have, such as an aftermarket fuel pump. They usually have a red wire and a black wire, or other clearly marked terminals. Make sure the red wire is connected to the POSITIVE side of your car's electrical system, and the black wire to the NEGATIVE side.
Fourth, reverse the battery connections. For MG T-Series cars this usually just requires turning the battery around.
Fifth, polarize the generator. To do this, make sure you have done the above items first and that the key is turned off. Then remove the small yellow with green wire from the "F" terminal of the voltage regulator (control box) and touch it momentarily to the "A" or "A1" terminal on the voltage regulator. Do it twice; you should see a small spark. Then reconnect the wire to it's proper "F" terminal.
That's it! You can now install a modern radio, accessory socket, etc. Just follow the device's directions just like you were installing it in a modern car.
