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An Accurate Voltmeter
The Motometer voltmeter fitted to BMW 'RS' and 'RT' machines (and available as an optional extra on all models) has a poor reputation for accuracy, to the extent that, when a question is asked concerning strange behavior in the charging circuit, the first advice has to be "First check that the Voltmeter is working right".
To be fair, most of the meters on the road are up to 25 years old and they do not exist in the most benign environment so it is hardly surprising that the reading is not entirely reliable. This doesn't alter the fact that, to be of any use, the meter must be both accurate and reliable otherwise it isn't worth having.
When, a few weeks ago, my voltmeter started to be not just inaccurate but inconsistent, I dismantled it and found that the problem was not repairable. This left me with a quandary. Should I:-
Put the meter back together, stick it back on the bike and regard it as nothing more than a plug for the hole in the dash.
Buy a replacement... Not cheap and no guarantee it would be any more accurate than the original.
Buy an aftermarket voltmeter. Cheaper, but not quite 'BMW' and still no guarantee of accuracy.
Build an accurate 'solid state' voltmeter. Still not 'BMW' but acceptable (to me anyway) and above all accurate.
Needless to say, I decided to go with option 4 and these pages are a chronicle of how I went about it.