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Old 06-02-2013
malyea malyea is offline
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Yanmar tach reads low rpm

My 1982 Hunter 30 has a Yanmar 2GM and what looks like the original tach has this issue...the onboard boat tach shows exactly half of the actual engine rpm.

With the engine running at 2000 rpm (as measured by a handheld digital tach) the onboard boat tach shows only 1000 rpm. It's the same for every throttle setting - regardless whether I'm in neutral or in gear. At full throttle I get 3600 (as measured by the handheld tach but only 1800 on the boat tach).

The boat tach uses a flywheel pickup mechanism to read rpm and does not get a signal from the alternator. What's the problem?

Looking at the back of the tach body, there is no selector switch to select the number of teeth on the flywheel and there is no calibration screw. (Both items were mentioned in some literature I found - but my tach has neither).
What's the fix?

This is not a new problem. I've owned the boat 3 years and it's always been this way...just finally getting around to addressing it.

Thanks!
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