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MG MGA - Dynamo running like motor

My fan belt flew off and I discovered the dynamo running by itself like a motor. Switching off ignition did not stop it. Have disconnected battery. Why is it doing this and how do I stop it please?
H L Davy

The cut-out relay in the control box has stuck closed, which feeds power back to the dynamo. This is generally caused by an internal short in the dynamo, which is probably also what caused the belt to fly off. The failure may have started either with the control box letting the dynamo overcharge, or the armature coming apart first. Good chance that both the dynamo and control box are toast, sorry.

FRM
FR Millmore

Goodness. Until I read what FRM had to say I thought you had invented perpetual motion!

Steve
Steve Gyles

Want to say, don't condemn things without testing. But, the belt throwing leads me to suspect the worst. When
I was much newer, we had some cars that would hang up the regulator without other damage. My father would stick a matchbook cover between the cut-out contacts to stop it, and remove the card after starting when he wanted it to charge. This sort of thing was standard depression/WW2 protocol!

FRM
FR Millmore

FRM-

I bet he had pennies in the fuse box at home as well ;-)

Chuck
Chuck Mosher

Yep the control box is not working as it should and it is a good observation about the reasons or it.

Had the fan belt not broken then after you had parked the car the battery would have gone flat? Or do you use an isolator switch?

When I have seen an overcharging system (twice) on both ocassions the dynamo has got so hot that the comutator segments have become unsoldered and the dynamo has failed, I have not seen this cause the fan belt to break?

Had you had some symptoms previously H.L.?
Robert (Bob) Midget Turbo

I reconnected the battery this morning and the dynamo no longer runs by itself. The points are open so if closed they have reset themselves. This control box is 'new' and looks clean and shiny bit I have run paper between points and sprayed the whole thing with switch cleaner. Will leave to dry overnight try again and update the story. Afraid may have given you a red herring about the belt. It's a bit ancient and the water pump pulley bent which I have not yet got round to fixing.
H L Davy

This thread was discussed between 07/01/2012 and 08/01/2012

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