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MG MGA - Alternator conversion issue

I completed a conversion from a generator to an alternator using the Moss Motors kit. Almost everything went according to the instructions except for one issue. When I turn off the ignition switch the ignition warning light comes on. I still have 12 volts (battery voltage) at the light. If I turn the ignition switch on again the warning light goes off, the opposite of what I would expect. But when I again turn the switch off the light comes back on. Turning the switch on (light goes out)and powering anything like the fan or lights the power stops and the ignition light comes on. Then I can turn the ignition switch off and the warning light goes off and no power to the light.
The regulator only has a brown/Blue at A1, A has the yellow from the alternator and 2 brown wires to the battery, and fuse box, and D has the yellow/green wire from the alternator to the yellow wire to the warning light, F is empty. E has 2 black ground wires.
The car runs well and I have 13.87 volts at the battery when running.
Hopefully someone can direct me to a fix.
Terry Marcus

Terry, do you still have the generator control box connected? The alternator has a built-in regulator so the old control box is redundant.
Lindsay Sampford

Quite a common occurrence if you are using the regulator box as convenient terminal posts for the new set-up. There are bits about this in the archives.

Here is one comment by Barney: "Specifically, the old box may have the cut-out relay contacts stick closed so the ignition light remains on after shut-down. You can isolate the contacts with a bit of insulating material in between the contacts. Or you can cut the conductors inside the control box. Or you can remove the control box."

Steve Gyles

Terry, I agree with Lindsay and Steve, your old voltage regulator is still partially operating and the cut out contacts are sticking down.
This is ok whilst the engine is running but if the ignition light stays on when the ignition is switched off and the engine has stopped, the battery may be discharging through your new alternator and could damage it.
So you need to isolate the battery once you have turned off the ignition.

When I had this same problem I fitted thin plastic strips between the contact points inside the old regulator and this prevented the problem from re occurring, exactly as Steve suggested.

Later on I did actually rewire the alternator connections avoiding using the old regulators connectors.
I had experienced total failures of two brand new and expensive electronic distributors in quick succession.

This was traced to poor connections of the alternators wiring where they used the old voltage regulators screw connectors.


c firth

Thanks to Lindsey and Colyn. I insulated the relay and everything is fine. I wanted to keep the regulator box for appearances.
Terry Marcus

This thread was discussed between 15/12/2014 and 21/12/2014

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