Have you tried adjusting the pot on the back of the tach ( small hole straight slot screwdriver). If that can't bring it down you probably need a new one. To really check you need either a o-scope or frequency meter.
I had the same problem or at least tack going crazy all over the place and some times calms down and works normal but not always and most times is crazy and new Yanmar tack made by VDO $$$$$$$$$
Found one VDO for $99.00 but it still goes crazy.
Nick
on my project list is to epoxy some rare earth magnets to the flywheel - and use a hall effect sensor to pick up the pulses - should be a cleaner signal than the alternator.
The older Yanmars did that with a prox probe counting flywheel teeth which works fine until the old tacho starts loosing its mind.. Don't know what engine you have.. ?
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