Originally Posted by svHyLyte
A "bouncing tach" on a Perkins 4-108 is often caused by the alternator cycling on-off when one's batteries are fully charged. Try leaving your charger off for a day or so with a small load applied to to your batteries--lights, a refrigerator or whatever-- and then see if you still experience the same issue. We found this to be the case on our boat and now overcome it by simply applying some loads to the batteries when it happens.
How could that work - that would require a feed back circuit and none exists.
The voltage regulator is what does or doesn't send a charge to the batteries based on voltage/state of charge (depending on how sophisticated it is).
The tach is simply reading and counting RPM's either from the alt (which is still turning exactly the same whether or not it's making amps) or from the flywheel (ditto, still making amps).
I'll admit I've had a similar problem with RPM's bouncing around once I get past 2400 RPM, but that's a matter of calibrating the gauge, not the alternator (flywheel in my Yanmar's case).