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MG Midget and Sprite Technical - More questions on starter motors

I have a curious problem with my 1500 midget starter motor.

In really cold weather it turns over sluggishly as if the battery was flat. I've substituted the good battery from my other car and it does the same.

After I've turned it over once and left it a few minutes for the heat from the current to warm things up, it turns over at normal speed and starts the engine instantly. Now that the weather here is milder, it behaves ok.

I've put a two channel oscilloscope on the battery terminal and on the starter motor terminal at the same time and that tells me that there is minimal voltage drop across the leads and solenoid.

I've stripped the motor a couple of times and cleaned it and can see nothing wrong. I can't think of anything in a starter motor that would be temperature dependent. I thought maybe dampness was being driven off by heat but surely any dampness would be a much higher resistance than the motor coils so unlikely to affect them.

Any thoughts/previous experience?
Allan ('76 Midget 1500)

Maybe the magnets have lost strength, replace the starter.
Alex Sturgeon

I was told once that the grease in the starter motor will get cold and make it turn slower in cold weather but you said you cleaned it up so I don't know.
J Van Dyke

I am impressed, a twin channel oscilloscope?? what ever next?

Allan if you are capable os using such a device or indeed having access to such a thing, then you ought to be telling us what the problem is?

I personally would have suspected a poor battery, but you say that has been proved not to be the case by substitution..

Whilst I am not in the magnetism camp (they are electro magnets) I think it must be an issue with the motor. They are cheap enough and whilst it is against my better judgement I think I would change the motor.
Robert (Bob) Midget Turbo

This thread was discussed on 01/03/2009

MG Midget and Sprite Technical index

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