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Alternator charging


nelmo
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So my alternator is playing up, although I think it's an earth issue.

When cold, it seems to be fine, with 13.8v showing at 2k rpm. But as the car warms up, it drops down to 13.4v and 12.5v at idle ie. its doing nothing at idle. 

Does that suggest a dodgy earth possibly created by the heat? Something expanding and causing a bad connection - is that a thing? 

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One thing to understand is that most alternators do not give a constant voltage out.

At initial startup when cold they have a higher voltage to get the starting charge back into the battery.

Depending on pully sizes it might not be roataing fast enough to charge at tickover when hot.

Back in the days when cars were fitted with dynamos it was normal for no charge at tickover.

Dynamos have a longer shaft than alternators due to the presence of the comutator.

This means that for a given electrical output you can spin an alternator faster by fitting a smaller pully.

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I think richyb66 means that the alternator is earthed through the mounting bracket, so it has to be firmly mounted for the belt to drive it, hence should be a good earth otherwise it would fall off. This assumes though that the engine block is earthed. I think that the alternator should give pretty much a constant output voltage which is controlled by the regulator circuitry inside the alternator. The regulator varies the field coil current to keep the output voltage constant. When there is demand for current from a flat battery, or a battery just a bit low after starting, or from light bulb filaments, or cigar lighter socket etc this manifests itself in a drop in resistance at the ouput from the alternator and from school I remember V=IR, so to keep V constant with falling R the I has to get bigger, so the regulator increases the magnetic field coil current in the alternator to generate a bigger current output and the voltage constant. With a bigger magnetic field the alternator gets harder to turn and puts more load on the engine via the dtive belt. So possible drops in voltage could also be caused by a loose drive belt, although you normally hear this squeeling from cold. The regulator is the most likely source of the problem. What happens to the voltage if you turn on the headlights or any other current consuming device ?.

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Yes, it is bolted on well (and I removed the powder coating when I fitted it) and it's been fine up until recently. 

Maybe it is a loose belt - it has always squealed a little when cold and I have pulled it as tight as I could physically do it but maybe its a bit looser now? I'll try tighten that first... 

Thanks for all the advice - much appreciated 👍

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I've had a few alternators do this, I think it's heat related damage as it's close to my exhaust. Eventually the output drops completely until it wont take the load of headlights etc and I replace it. I've now moved it further away, haven't driven it enough to say it solved the problem though.

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