Switching to alternator but staying positive ground

My ‘62 Mark2 has a non original generator that has given out. I’m looking to change to an alternator but unsure if I should keep the car positive ground or switch to negative ground when I change the charging system. All else is original and I don’t plan to change the radio, fuel pump, etc. if I don’t need to (which a polarity switch would entail).
Looking for advice on how best to fix the system. I could just buy an original generator, but they are expensive and I don’t have a core to exchange. I see that modern positive ground alternators (which look like generators) can be purchased. But not sure if I should just switch the system while I’m updating the system. Are there other pro/con reasons to change to neg ground if all else stays original in the car? Thanks for any advice.

If it were me I would simply keep the car as is and find a rebuildable used original generator and get a competent shop to rebuild it. If it is the large C48 version you will have a small ball bearing in the front and back of the unit and brushes. If you renew all these and clean the commutator you should be good to go assuming the armature and field coils are good. If you car has a power steering pump at the back of the generator that complicates changing to something other than the original type.

Hi Lucas,

When you say it has given out, do you know what has gone wrong? It may be simple enough to get working - new brushes, etc.

Changing to an alternator will mean some rewriring as the voltage regulator box is no longer needed. Provided you have not installed a polarity sensitive fuel pump, or have accessories such as radio, the only thing that ought to be done is to reverse the connectons to the ammeter.


Thanks for the reply and input. I can’t say exactly what’s the problem. The generator has worked for the last 30 years but it’s not the original to the car. I tried swapping the voltage regulator with a new one, but still no charge at the battery and the red light is on inside the car along with no charge indicated at the amp meter. I did bench test the generator and it’s putting out proper juice, so I’m at a loss. I thought maybe it was time to update the system instead of chasing old ghosts, but if keeping a generator is the better route I’ll need to do more sleuthing.

At least with Lucas generators you can test them by disconnecting the wires at the generator, connecting a jumper wire from the field terminal (the small one) to the output terminal (the large one) and running a volt meter from that wire to ground. Start the car and SLOWLY increase the engine speed and watch the voltage, it should slowly increase from about 5 volts to about 17 as the speed increases. If it does this the generator is likely OK and the issue is with the regulator or the wiring to the regulator. Do not go above 18 volts.

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You say you bench tested it and it’s putting out proper juice, what is proper juice?
If the voltage output is ok look for the problem elsewhere, regulator or polarity.
If you changed the battery recently the gen may need to be repolarized. If that’s not the issue look to replacing the regulator.

I pulled out the generator and had a shop check check it out; they said it performed within spec.
I then installed a new regulator and it didn’t change the charging performance. I’ve also attempted to ensure polarity is correct by a process that has been addressed on this forum and online.

I must now assume there is a wire issue in the system, which led me to consider changing the generator to an alternator (and switch polarity to negative ground). As I’d have to fiddle with some rewiring I’d hope to resolve the issue.
Also expected some benefit to the updates but wasn’t sure if it merited the cost.

I now see they sell positive ground alternators and I’m considering going that route. It would negate the regulator and need some rewiring but I’d still have use of the original radio and wouldn’t need to change the positive ground SU pump.

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I would still persue the regulator problem. Does your ammeter show discharge when other circuits are on? If it does I wouldn’t think its a wiring problem. Did you polarize at the regulator? I battled with a new regulator on my 3.8 E-type, it took a few polarizing attempts before it burst into life.

I was under the impression that only the generator had to be polarized when changing from + earth to - earth. I remember doing a few cars that way long ago and they are still working fine. Am I mistaken, or does polarizing the generator also transfer to the regulator?

I would try polarizing the generator. Depending on regulator you touch A terminal to F with a jumper wire, or if you have two B terminals you touch B to F just for a second or two.

Yes I’ve read that its best to polarize the gen at the regulator, which presumably polarizes the regulator as well.

Colour me confused. I always thought polarizing was the domain of rotating magnetic fields and nothing to do with regulators. Just saying.

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I’m no expert, but there are coils, resisters & magnets in the regulator that are sensitive to/by polarity. Is there an electrical engineer out there whi can comment?

If converting to alternator you’ll have to convert to negative ground.

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The regulator is not polarity sensitive. One thing to check is the earth connection (a simple wire from the reg to earth). I had that connection go bad once and the charging stops.

Check this, applies equally to 12 volt regulators.

The main component in a VR is a copper wound coil on a ferrous core. Direct current(DC) goes in and creates a magnetic force which pulls a metal arm which is attached to contacts much like a relay.This off / on function controls the field. Oh, forgot to mention, magnetic force will be produced regardless of direction of DC voltage, thus not polarity sensitive as stated by others above.

The WEB is FULL of misinformation.

G’day Lucas

the other thing that is always forgotten in these change of polarity issues is the clock. Yours probably doesn’t work anyway but if it does you need to electrically isolate from the body if you switch to negative earth.



Many have upgraded their non working clocks to electronic winding. Mike Eck can do this per this website. Positive or negative earth is possible.

Thanks for the advice. I will do some further assessment and try to resolve the generator issue first. If there’s no luck I’ll change the system over to negative ground and install an alternator. Although I’ve read that an alternator must be neg ground, I see suppliers sell positive ground alternators.