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MG MGB Technical - Positive ground?

I have a 67B GT parts car in my driveway, and the tach says positive earth.. I want to nab some of the electrical stuff for my negative earth roadster. I didn't think the B's ever came in a positive ground. Since I'm far too lazy to check the wiring set up, is it likely that the GT IS positive ground? or does it just have the wrong tach in it?
David Ryzuk

Yes, very likely - it will have a dynamo rather than an alternator if it is. You could always look at the battery connections.
Chris Betson

David. MGB's originally had positive earth on pre 1968 cars. See page 143
in the current Moss catalog for positive to negative ground/earth conversion.
If you don't have the catalog you can download the information at;
www.mossmotors.com
Clifton
Clifton Gordon

Chris, It has an alternator, not a dynamo, and I already looted the battery from it... the leads were made of old extensioin cords(of the same colour) and both disappeared into the rust below the car.

Clifton, Thanks, I'll check that out.
Dave

Dave - If your car has an alternator, it was most likely converted to negative ground at the time that the alternator was installed. Just to make sure, get ahold of a multimeter, set it to measure volts and the range set to 20V (or something above 12V). Put the positive lead to the positive terminal of the battery and the negative lead to a clean metal portion of the car. If the meter deflects upscale, then the car has been converted to negative ground and if the tach is working, it has been converted also.
Good luck - Dave
David DuBois

It sounds as though it has been converted. Alternators only run negative earth.

Most of the electrics are not polarity sensitive. The tacho is, but may have been converted. The fuel pump is - but you can reverse the polarity by swapping the condenser/ diode leads round inside the black cap.
The radio is - you may be able to see from the marking on the terminals.

Most other things, like starter, wipers, fans, lights, etc are not sensitive to polarity.
Chris Betson

SU pumps with a condenser/capacitor don't need swapping round, but pumps with a diode do. The wiper motor is polarity concious if it has the permamant magnet stator, but I doubt you will notice it running backwards except for possibly a slightly different park position. If the heater motor has one coloured wire and one black wire then it too is polarity concious, and will have to be reversed to maintain effectiveness - a relative term, I know. The very earliest Bs may have had a motor with a wound stator and rotor (instead of the later permanent magnet stator) and have two black wires, these are not polarity concious.
Paul Hunt

This thread was discussed between 16/12/2001 and 17/12/2001

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