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MG Car Repair/1971 MG Midget-generator to alternator


I just bought a 1971 MG Midget. After a few weeks of driving, the battery died. We replaced the battery, and within another 3 weeks, it was dead.  We replaced the voltage regulator and the generator.  We have not been able to polarize the generator for some reason. We are now thinking of changing to an alternator system, as I really want to be able to drive my care this spring.  Can you give me any advice on changing it over, or polarizing the generator?


Hi Mary,

I will tell you both.

First to polarize a generator first loosen the belt and remove it off of the generator. Now make up a jumper wire out of 12 gauge wire long enough to reach the battery. (heavy wire like the brown wire) At one end put either a large spade connector like the harness had or if you can't locate one just put a alagator clip on the one end and attach a small pig-tail wire to the same end with either a small spade connector or another aligator clip.

Now you have a heavy gauge wire with clips on one end with a small wire also attached.

Now remove both the large wire and the small wire from the back of the generator and put your two clipped wire on the back two connections on the generator. (the two connections on the back of the generator are now connected together on the heavy wire)

Now, touch the other end of your test wire to the power post (+) of the battery (your car is negative grounded).

The generator should run slowly like an electric motor and in the direction that the engine turns (clock wise as you face it from the front) Let it run for about 20 seconds and remove the end from the battery. Then repeat that several more times. Then remove your test lead and reconnect the harness wires and it is polarized. Nothing needs to be done with the regulator.

To test it just put a volt meter across the battery posts and read the voltage. It should read 12+ volts. Now start the engine and set the RPM at about 1500 RPM (a fast idle) and read the battery voltage again and it should read 13.8 volts to 14.5 volts if the system is working correctly.

If you decide to switch to an alternator you should get a two wire alternator with an internal regulator. There are some three wire alternators that can be used but they too must be a internal regulator type.

This requires some wiring changes. Remove the regulator and discard it, Connect the two large brown wires together along with another large 12 ga wire that you will run to the large post on the alternator. Take the brown w/yellow tracer wire off of the regulator and connect a wire to the end of it and run it out to the small post on the alternator. Tape up the other wires that were on the regulator as they must not get in contact with anything.

Again run the voltage test at 1500 RPM and you should get the 13.8 v to 14.5 v reading.

Let me know.


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Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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