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I have a similar problem.

A few weeks ago while motoring out of the marina a guest slipped and kicked the throttle lever. The rpms jumped way up b for a second or two until I was able to throttle back. Immediately the tachometer, which had always worked perfectly, dropped down to around 1500 when it should have read 2000. It eventually dropped to about 500 and then would wander back and forth from 500 to 1500. Sometimes it rests on zero. All the time with no change to the actual engine rpms and also no response if I do move the throttle.

The engine is a yanmar 3g. I have looked for a loose wire on the tach and alternator but haven't found anything. The yanmar manual says there is a sensor on the engine, but I can't find it. It seems like it ought to be somewhere near the crankshaft. I think it would look like a temperature or oil pressure sending unit.

Any suggestions? Thanks.
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