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On a '98 Holiday Rambler Diesel pusher, The dash Voltage gauge slightly pulses, like you would expect in response to your turn signal blinking on and off. The tach gauge swings in unison with the pulses of the volt gauge. I've found that if I turn on some load, such headlights, the tach needle levels out and stays steady. With the diesel, there's little to no elecrical load on the alternator at idle. What's failing?
There may be nothing failing. You may have a poor ground in the Alt field cicuit, Tach Circuit(The tach May run off the Alternator tach terminal) A bad ground, poor conection in the alternator or a poor tach head will all show up like this. The tach reads the AC ripple from the alternator, and with an alternator with a problem or the bad conection I said earlyer the tach will do this. I have seen the tach do this for 250,000 miles on comercial units and never get fixed even changing out alternators etc. In my fleet we just remind the drivers they are to have their head lights on all the time and the symptoms go away. In short nothing to worry about unless there is other symptoms. Good Luck.