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MG MGA - Dynamo (generator) testing

Last weekend I ran the MGA for the first time this year. It started right up and ran well, except the ignition lamp in the tachometer remained illuminated. A quick check with a voltmeter showed no charging. I removed wires D and F from the dynamo (generator), shorted the two dynamo tabs together with a test wire, started the engine, and measured zero voltage to ground. The dynamo was clearly not functioning. I installed a spare dynamo, and everything was fine.

How could a dynamo that has been working without any problem fail after sitting idle for the winter? I disassembled it and found no obvious fault. The brushes look good, and their springs are intact. All wires are connected without shorts. Nothing looks burned. The two field coils in series measure 6.0 ohms (measured from the small tab F to ground). I used crocus cloth to polish deposits off the commutator and reassembled the dynamo. I measured a small voltage when spinning the pulley, but how can I tell if it is now working properly?

When investigating the voltage regulator and cutout, I learned that the cutout contacts will remain closed if they are pushed together, even with the ignition off. Although possibly normal, this is clearly not a good thing. I also heard a clicking noise at the dynamo when doing this. A closer look showed that the dynamo was functioning as a motor and attempting to crank the engine.

Returning to the dynamo on the bench, I shorted the D and F tabs together and connected them to the negative terminal of a small, 12-volt battery. When I connected the positive battery terminal to the dynamo frame, it became a motor with the pulley spinning clockwise. Does this result show that the dynamo is now probably working and that the fault was a dirty commutator?

When the dynamo is stationary or operating at low speed, the voltage regulator shorts tabs D and F. When the output voltage rises at higher speed, the voltage regulator opens leaving F (field coil) connected to D (output) through a 67-ohm resistor. This limits the current to the field coil and reduces the output voltage.

-Brad
B. D. Schwartz

Hi Brad Re: I shorted the D and F tabs together and connected them to the negative terminal of a small, 12-volt battery. When I connected the positive battery terminal to the dynamo frame, it became a motor with the pulley spinning clockwise. Does this result show that the dynamo is now probably working and that the fault was a dirty commutator? YES. If the generator starts rotating on it's own when power is connected as described, then the generator is probably in good working order. Parts that sit unused over a long period of time often go "defective" In the case of generators, commutators can oxidize, and brushes can stick in their holders, rendering the generator inoperative. Usually a thorough cleaning of commutator and brushes gets a balky generator working again. Many other parts on our MGAs suffer from prolonged disuse, so it is a good idea to drive the car once in a while or at least start the engine occasionally, and run the car back and forth in the garage. This keeps moving parts free and lubricates engine transmission and rear axle components. Cheers, Glenn
Glenn

Generators are like everything else. IF YOU DON'T USE IT YOU LOSE IT.
Glenn is right.
Sandy
Sandy Sanders

Thanks for the comments.

When the dynamo is running, current flows from the armature brush to point D on the voltage regulator/cutout box then out through point A on the box to charge the battery. This should be a low-resistance path so there are no voltage drops between the dynamo and battery.

If the cutout is closed when the dynamo is not running, battery voltage at point A is connected directly to point D and the dynamo brush. I measured 0.2 ohms resistance between the brush connection and ground. If this is correct, the current draw would be 12/0.2 = 60 amperes. That would quickly drain the battery and possibly damage the dynamo armature and/or wiring.

After destroying a generator in a VW, a friend installed an in-line fuse that would pass normal charging current but blow out if the cutout stuck closed.

Always check that your MGA ignition lamp illuminates after turning the key but before starting the engine. The lamp will turn off when the dynamo is operating. After turning off the engine, turn the key on again to check that the ignition lamp lights. If the lamp does not light, the cutout contacts could be stuck closed.

-Brad
B. D. Schwartz

This thread was discussed between 25/04/2007 and 26/04/2007

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