Ask Sail: Tachometer Gone Wild

My sailboat is equipped with a Yanmar diesel. Often while motoring, the tachometer suddenly pegs out beyond the highest value. At these times there is no change in the engine itself; it is not racing.
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Chris Bosworth, Newport News, VA

A: My sailboat is equipped with a Yanmar diesel. Often while motoring, the tachometer suddenly pegs out beyond the highest value. At these times there is no change in the engine itself; it is not racing. The only solution seems to be to kill the engine and turn the key off. (Simply killing the engine does nothing for the tachometer until the ignition key is turned off.) Might this be an intermittent bad connection? If so, should I look at the engine or behind the instrument panel?

Nigel Calder Replies
A: Yanmar has two types of tachometer: one of them (the GM series, which I suspect is what you have) employs a sensor screwed into the flywheel housing; the other (which is found on most other models) takes information from the alternator. The alternator type senses the frequency of the three-phase output of the alternator before it is rectified to DC current to charge your batteries, and when it fails it will likely default to zero, which is why I think you have the other type. The flywheel type could potentially go to full rpm if shorted. (Full disclosure: I am not entirely certain of this.) According to Yanmar, the most likely problem with this kind of tachometer is the connection plug on the wiring harness, though this doesn’t seem to fit your having to turn off the ignition to reset things. Still, I recommend you check this plug, as it is easy to do. If this doesn’t fix things, find the sensor on the flywheel, unscrew it, clean it and check all the connections and wiring, including those at the gauge end. The following website has some useful information on tachometers and more: yanmarhelp.com/s_tach.htm.

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