MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - revers polarity

I have 59 mg where the polarity is negative ground. I have re-wired the whole thing using mga wiring diagram. Too late I realised that the diagram shows positive ground. Does this mean that guel gauge shown going from fuse box(-) to fuel gauge to tank unit to ground(+) shall be reversed such as fuse box(+) to tank unit to guel gauge to ground(-) or fuse box(+) to fuel gauge to tank unit to ground(-). On viper motor the switch location in positive or negative line maters? Can someone put me straight?
martin

There's complete instructions in the Moss MGB parts catalogue on changing a + ground B to - ground. I don't know why they don't put the same instructions in the A catalogue as the procedure is the same.
You have basically three things to worry about in the A conversion:
1. The generator must be re-polarised for - earth. Very easy: disconnect the field wire from the generator; use a test lead or small jumper wire to temporarily connect (for about 1 second) the F terminal of the gen to a +12 source. You should see a small flash. Connect F wire back and you're done.
2. Reverse the connections to the ignition coil. 'CB' should now go to ignition switch and 'SW' to distributor.
3. Check that the heater fan motor is rotating the right way 'round: if not, reverse the connections.
All the rest of the gauges should work ok on - ground, unless you have an electronic tachometer and/or an ammeter.
Hope that helps. Fraser
Fraser Cooper

The car is already -ground. where I am confused is if resistor (tank unit) in negative line have the same result as in positive line. Same apply to regulator will it work the same with wiring polarity reversed?
martin

Hi, Martin - no problem on either of these issues, as they depend on thermal (gauges) or isolated magnetic (regulator) properties, which are not polarity sensitive.

The deposition of metal on the horn contacts will start to reverse itself, which *might* cause a problem in a few hundred years, though <grin>.

Best of Luck - - Alec
Alec Darnall

Sorry, but I don't understand how you wire a car differently for negative vs. positive ground, other than where it all meets the battery. What am I missing?
Joe Cook

Joe,
You are correct, up to a point. There are several polarity sensitive components on a MGA. As listed above, they are the generator, coil, and heater fan. The rest of the electrical system doesn't care which way the smoke is traveling inside the smoke tubes!
Ed Bell

Martin - you said the car had been running reverse polarity already, so maybe this does not apply, but the later electric fuel pumps were polarity sensitive, so that's another thing to watch out for - Cheers - - Alec
Alec Darnall

Martin

Look to Barney Gaylord's site, he has an article on reversing polarity from Pos to Neg earth. You can just make sure that you have followed his steps to make sure you have wired correctly and made any necessary changes.
dominic clancy

The radio (if fitted) is likely polarity sensitive as well, I believe. Particularly for the original Radiomobile unit.
Tom Heath

This thread was discussed between 31/03/2005 and 01/04/2005

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.