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Posted: Mon May 16, 2011 1:32pm

Post Subject: Alternator/RPM

I have a BMC 1.8 with 70Amp Alternator fitted 2 years ago. The rev counter will sometimes change from 2000 rpm to 2400 rpm without any change in engine speed. The change can last for hours at a time and is always 400 rpm difference. Slowing the engine makes no difference - the range is always the same. It seems to show the higher figure when batteries are fully charged. Is it a faulty alternator or something that can be corrected?


Posted: Mon May 16, 2011 3:49pm

Post Subject: Alternator/RPM

If you had said that the revcounter was erratic and the variations had no pattern I would have said there was probably a bad connection in the alternator or wiring or faulty diode within the alternator. As this seems entirely consistent and almost predictable I am at a loss. I suspect it could be a problem with the revcounter electronics because all the alternator does is supply a series of pulses at a frequency related to speed and I can see no way that those pulses can increase with no increase in alternator/engine speed. As you say it seems related to fully charged batteries I wonder if it related to heat causing a problem in the chips inside the revcounter but its only a guess. I must say that my revcounter's hour meter produces gobbldy gook with well charge batteries and extra solar input so at a guess I would say revcounter but I doubt is is worth doing anything about it. You could well end up paying a lot of money and still have the problem. Tony Brooks

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