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Positive Ground, Why?

MG Merlin
08-19-2003, 01:31 PM
I'm just wondering about the significance and integrity of running on a positive ground system. Why did they even do it that way?

Also, how does that affect things like, say, me putting in a stereo? How do a go about working around that. Thanks.


08-19-2003, 08:44 PM
I think its one of those great mysteries that will never be solved.
Simply a case of the British being British.

As for fitting a stereo, you could either convert the car to a negative earth, possible, and with a new wiring loom not all that hard to do, but you might simply have to hard wire it with fuses straight to the battery, and make sure all the wiring is insulated from the cars body.

It must be a very old MG if its still got a positive earth on it though?

MG Merlin
08-21-2003, 09:42 AM
It's a 1967 b roadster. Are there any websites of how to convert it over to negative earth? Thanks.

08-21-2003, 10:58 PM
I dont have much on the MG off hand, Im more of a Triumph fan myself, but will be a good place to start, if you can find one site with a large links page that should you take you anywhere, it might just take a little time, as the MGB has to have one of the most devoted followings of any car ever made, and there are literaly thousands of websites dedicated to it.

Finding you local branch of the MG owners club would also be a good place to start.

Good luck, if I come across anything anywhere Ill be sure to let you know.

MG Merlin
08-22-2003, 08:40 AM
Thanks very much. I'd appreciate that.

01-20-2004, 12:52 PM
All cars were positive ground at one time including american cars. I haven't converted a car but I did an old tractor. The only thing I had to change was the charging circuit. Remove the generator and replace it with an alternator and, of course reverse the battery. You can hook up a stereo in a pos gnd car but you have to be extremely careful not to groung anything on the stereo. The chassis would have to be connected to the hot wire (neg) from the battery and the 12v connection from the stereo would have to be connected to the car chassis. Also be careful not to ground any of the speakers.

01-30-2004, 09:23 PM
This may be of interest. Also the BBS at this site is quite good, as well

09-06-2010, 12:08 AM
TO MG Merlin:
Converting my 1962 MGA from Positive to Negative ground was insanely easy. The only part of the car that cared about polarity was the original 1962 radio, which no longer worked (dead tubes for which I could not find replacements). Like all automobile radios, it had a metal case the bolted to the metal behind the dashboard and was expecting to be bolted to a positive ground. As it no longer worked anyhow, I removed it. Then I simply reversed the connections on the car battery. This left me with a red cable attached to the black, negative pole of the battery, and a black cable attached to the red, positive pole. Not a problem as such, the wires are just wires, but liable to badly confuse anyone else working on the car in the future, so I later replaced both cables with correctly colored, black negative, red positive cables. I also added a sticker saying "NEGATIVE GROUND! (just like all modern American cars)"
All of the worries about other electrical componants proved to be false.
Fuel pump, lights, fan, wipers and so on did not care and ran just fine. The generator also worked fine. Reversing the polarity of the battery reversed the direction of current through the field coils of the generator, so the electricity it generated also reversed polarity, that is, came out with negative ground just like the battery. All the arguments we had had about the generator needing to be replaced because it would be double-reversed were just a matter of thinking too hard.
And finally I bought an ordinary 1991 12-volt negative ground AM/FM radio and cassette player, that had the right-sized face plate to fit into the empty space in the dash. Had I wanted greater purity of restoration, I could have mounted the dead original radio and never turned it on, or installed the dash plate that covered the space if you bought the MGA without the radio option back in 1962. But I was willing to compromise a little on the absolute originality of the restoration, to get a reliable radio..
Car and new radio are still running fine.

By the way, if you have an MGB with a positive ground ALTERNATOR not generator (and I am not sure they ever built MGBs with alternators) you may find that it will continue to produce rectified AC with positive ground after you reverse the battery connections. That would be a big problem, and require replacing the alternator.

09-06-2010, 01:22 AM
After my previous answer, I browsed through the internet and found this site,
which seems to cover everything that might need adjusting. I do not remember doing everything it describes when I rewired my 62 MGA, but that was 19 years ago, and I may have jusrt been lucky about the ignition coil wiring. Check the above site for some well-worded instructions about converting to negative ground.

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