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MG MGA - dynamo fail

help confused , my dynamo failed , so i replaced it with a recon unit , this failed after a short time so i replaced it ,& guess wot it has failed again am i missing something ?,please help, colin
colin daly

The dynamo is only one part of the charging system. A bad regulator and or a bad battery can kill the dynamo. You need to check out the complete charging system. This is a good step-by-step guide for checking out the charging system. www.vintagemg.com/ArticlePDFs/Tech103.pdf

Jeff Schultz

Hi, Colin - you don't say what type of failure you're experiencing - electrical (just quits charging), or mechanical (bearings, etc.)

Maybe you're running the fan belt way too tight. A dynamo requires only the barest minimum of tightness, or the bearing and bushing will both suffer.

And, of course, you could be getting replacement units from a remanufacturer with two left thumbs. In the last year I've pulled apart no less than a half-dozen where the rear bushing had been lubricated with grease! You know, whatever gets them out the door <sigh>. Then, for the electrical issues, several have had the brushes too large for the holders, or the holders have been bent so that one brush hangs up. No brushes - no charge, obviously.

It's such a simple mechanism, but many companies can find tons of ways to screw it up, sometimes catastrophically.

Anyway, best of luck - - Alec
Alec Darnall

Colin - Remove the wires from the dynamo and short the two terminals together. Attach a multimeter between the two terminals and ground (if your car is positive ground, the negative lead of the multimeter goes to the two dynamo terminals and the positive lead to ground. Reverse the connections for negative ground). Set the multimeter to read 20 volts (or whatever range it has that is greater than 12 volts) and then start the engine. Watch the multimeter as you SLOWLY increase the revs. If the dynamo is good, the voltage will increase with the revs rather quickly to 20 volts and above. As soon as the voltage hits 20 volts, drop the revs as the voltage will keep increasing when in this configuration until the dynamo becomes damaged. If you cannot get the voltage to increase to at least 20 volts, the dynamo is bad. If the Voltage increases to 20 volts and beyond, the dynamo is good and you need to be looking at the regulator, battery, wiring or all three. Good luck - Dave
David DuBois

Colin,
A simple first check... Remove the voltage regulator cover and check that either the voltage regulator cut out points and/or the regulator points have not fused together (perhaps as a result of the first generator failure.) If the voltage regulator is at fault, new replacement generators will be ruined. -M.S.
Martin Straka

As said the dynamo can very easily be damaged by the regulator. If the regulator allows too much voltage and does not cut out then the charging rate will be so high that the dynamo will overheat and melt the soldered commutator. I saw one like this that was actually allowing 19V thus the current was high enough to melt the internal connections inside the dynamo. To check this out simply put a voltmeter across the battery and start the engine. When the engine is revved check that the voltage does NOT exceed 14.5 volts. If it is above this figure then the regulator is malfuntioning and will thus KILL the dynamo.
Bob (robert)

This thread was discussed between 15/05/2005 and 16/05/2005

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