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Dynamo with a mind of its own !

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OWD724

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Hello to all from UK. Yesterday a very strange thing happened to my Healey 100 that I do not understand. I was driving along quite slowly - about 45mph, when suddenly I noticed that the water temperature had gone up from its usual 160F to 220F. I immediately stopped and switched off. I opened the hood ( we call it bonnet ) and found the fan belt had come off the three wheels but had not broken but looked destroyed as the fabric was torn in placed down to the rubber. BUT what was very odd was that the Dynamo was spinning around and would not stop until I disconnected the battery !!!! Where I reconnected the battery some minutes later the Dynamo did not start up again. How could this happen? I have ordered another Belt from AH Spares here in UK, but do not want to refit until I understand what is going on with the Dynamo. The fan/water pump turns freely by hand and so do the Dynamo. Some advice please ? Thanks and regards John
 
Perfectly normal and not a problem. The points in the reg take a second or so to open and if the fan belt comes off then the genny goes straight from charging to discharging (spinning the generator as a motor) before the points can open. Andy.
 
Yes its called motoring the generator and is actually a test that can be done to test if a generators windings are good or not.
 
healeyblue said:
Yes its called motoring the generator and is actually a test that can be done to test if a generators windings are good or not.
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Can you elaborate on this test? You know, I've been around cars a long time, but first I've heard of it.

Oh sure, been an electrical and controls specialist since the early 70s, and know about DC motors/generators, but still never encountered a generator that thought it was a motor__I'd have been laughing until I cried, if it happened to me!
 
For a negative ground system (like the Porsches I generally work on) you ground the field terminal and apply 12 volts to the D+ (the terminal that would normally be the output to the battery for charging) and this allows the generator (if the windings are in good condition and has no shorts) to run as a motor, but should only be done briefly for testing. This is also the way you would "polarize" a generator, only then you only briefly touch the terminals and get a spark and the generator does not have the chance to motor up.
 
healeyblue said:
For a negative ground system (like the Porsches I generally work on) you ground the field terminal and apply 12 volts to the D+ (the terminal that would normally be the output to the battery for charging) and this allows the generator (if the windings are in good condition and has no shorts) to run as a motor, but should only be done briefly for testing. This is also the way you would "polarize" a generator, only then you only briefly touch the terminals and get a spark and the generator does not have the chance to motor up.
Click to expand...
Okay, simple enough. In the past, I'd polarized several MGB generators (1967 & earlier). Of course in those instances, the drive belt was tight on all the pulleys, so not much chance of those generators powering up and spinning.

Thanks for the explanation, now the next time I turn up a generator in the garage, I know what I'll spend a few minutes doing ;)
 
Yes a simple test for us Porsche guys is to pull the fan belt (easy to do) and do the motor test on the generator if having charging problems. If it passes the motor test then most likely the generator is not the culprit.
 
A good thing to do whenever starting an unfamiliar engine that hasn't run for some time is to connect the battery and press down on the cutout regulator in the reg box as this will ensure the genny is polarised the right way round. The points don't open themselves as the OP found out, you have to prise them apart again. Usually can see the fan belt twitch a little as well.

Old motorcycles used a device mounted on the crankshaft that functions as a generator when running and a rather inefficient motor to get started. We had a gokart at high school powered by a Yamaha RD200 engine which is a twin cylinder 2 stroke and it had one. I'm pretty sure the start button just shorted across the cutout points.

Andy.
 
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