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weird alternator behaviour


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Hi all. What might cause an alternator to perform well while the engine is idling but drops down to about 12.5 volt output when the engine is running? The battery light comes on on the dash at the same time.

incidentally, this is on my girlfriend's old merc van, not a Landy, but I assume it's an alternator thing not a merc thing.....

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Sticking brushes - can't follow the commutator when turning at speed?

If it drops after a few seconds rather than straight away, I'd suspect a bad regulator - working OK at low currents, but failing under stress at high outputs.

When you say it performs well when idling, what do you mean? Good voltage or just no warning light?

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Sticking brushes - can't follow the commutator when turning at speed?

If it drops after a few seconds rather than straight away, I'd suspect a bad regulator - working OK at low currents, but failing under stress at high outputs.

When you say it performs well when idling, what do you mean? Good voltage or just no warning light?

as in putting out about 14ish volt and no warning light on the dash. so i assume, working a-ok.

a mate of mine saved his alternator by tapping out all the fillings that had built up in the brushes, his had a nice easy to remove cap and the brushes were behind that. hope the mercs the same....

i'll see how quickly the volts droptomorrow. other than that, how do you test a regulator? or recognise it?!?

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as in putting out about 14ish volt and no warning light on the dash. so i assume, working a-ok.

a mate of mine saved his alternator by tapping out all the fillings that had built up in the brushes, his had a nice easy to remove cap and the brushes were behind that. hope the mercs the same....

i'll see how quickly the volts droptomorrow. other than that, how do you test a regulator? or recognise it?!?

Good chance it's a Bosch type unit with integral brush and regulator pack. They aren't usually expensive so I'd probably change it on the off chance if knocking out the brushes doesn't fix it. The only real test is pretty much what you have already done.

Look up your alternator on the Wood Auto site and you'll probably find a pic of the regulator and/or brush pack.

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