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MGB 74 MGB Chrome bumper Grounding (Polarity) Question

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rshaikh00

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74 MGB Chrome bumper Grounding (Polarity) Question.

Starting a new restoration project on my recently purchased 74 MGB.
Trying to connect battery with correct Polarity.
Is there a way to test if car is Positive Ground or Negative Ground, without cranking the engine.
Seller insisted at time of sale, it is Positive Ground, but I notice it has a Lucas 3 prong Alternator. There was no battery connected at time of sale.
So my assessment is it is a Negative Ground.
Please Help !!
 
1974 should have been factory built to negative ground spec. The positive ground cars were earlier ones that had generators. As far as I am aware, any MGB that came with an alternator from the factoy should already be negative ground.

The early positive ground cars could be switched to a negative ground by repolarizing the generator, but no similar process exists to convert an alternator-equipped negatve ground car to become positive ground (and I can't imagine why anyone would need to).
 
rshaikh00 said:
Yeah, except that there was NO battery in it when purchased. So chk'g if anyone knows how to chk without blowing up the starter.
Click to expand...
The starter itself is polarity insensitive. The battery terminals ought to be different sizes. Although tapered, one battery post has a larger diameter than the other. Our '80 MGB has a factory sticker in front of the radiator that states it's negative ground.
 
rshaikh00 said:
Yeah, except that there was NO battery in it when purchased. So chk'g if anyone knows how to chk without blowing up the starter.
Click to expand...

Another thing you could do would be to (if its still readable) look up the part number on your alternator and verify that it a negative ground unit. If its a 3 wire Lucas it is certain to be, but doing a crosscheck of the part number can't hurt if it sets your mind at ease.

My larger concern is that the seller thought it was positive gound. If he/she actually hooked up a battery with positive going to ground, they could have possibly damaged the alternator (or possibly the instrument voltage regulator and/or the tach). I don't think you'd blow up a starter on reversed polarity, it might just spin backwards and the Bendix mechanism wouldn't extend the drive gear.

There are not many electronics in these cars, but the diode/regulator pack in the alternator, the instrumentation and possibly the radio (if present) would not appreciate reversed polarity. By 1974 negative ground was certainly being used (even my 1970 was negative ground), but if the previous seller hooked it up backwards some damage could have been done (or possibly not, depends on a lot of variables).

I'd be very confident putting a battery in and connecting it negative ground - what I would do is connect the positive going to the starter first and then just quickly tap the negative to the battery post - if you see or hear any kind of spark you know something is passing current that shouldn't be (which could indicate damaged electronics/wiring or could be something as simple as the lights being left on - but you'll want to find out).
 
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