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Old 06-02-2013
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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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Old 06-02-2013
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Re: Yanmar tach reads low rpm

You could try adjusting the pickup ever so slightly closer to the flywheel.. could be a proximity issue, but be sure to avoid contact

Alternatively get a label maker and put "x2" on the tach readout
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Re: Yanmar tach reads low rpm

could be as simple as a corroded wire/conection. Otherwise, bad pickup or tach
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Re: Yanmar tach reads low rpm

Your pickup is for a 1GM. If not for a 1GM, it's the wrong one.
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Last edited by sony2000; 06-02-2013 at 01:42 PM.
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Re: Yanmar tach reads low rpm

This may be enlightening; its from the Island Packet forum:
http://www.iphomeport.com/forum/attachment.php?id=1260
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Old 06-03-2013
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Re: Yanmar tach reads low rpm

Ugh, the IP forum requires me to have an account to read that webpage. Would it be a lot of work for you to cut and paste the useful post?
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Re: Yanmar tach reads low rpm

Hope this photo comes through.
Attached Thumbnails
Yanmar tach reads low rpm-tachometer-wiring-diagram.jpg  
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Last edited by Seaduction; 06-03-2013 at 09:29 AM. Reason: Added photo.
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Re: Yanmar tach reads low rpm

Thanks for the tach schematic...I wonder if some tach disassembly is required to get at the 'range selector switch' and the 'calibration' screw.
Looking at the back of the tach as it's installed in the engine control panel, it looks as if maybe the 'body' could be removed as it may be covering/protecting the adjustments that may be the culprit.....
If the 'range selector switch' is set to the wrong tooth count range - seems like it would cause a large error in reading accurate rpm.......
Anyone disassembled a tach before?
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Old 06-04-2013
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Re: Yanmar tach reads low rpm

The schematic of course stops before getting into the tachs inner workings...anyone have any insight if I do try to disassemble the tach what success I may have in adjusting the rpm readout and then reassembling in working order?
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Re: Yanmar tach reads low rpm

I think that schematic is for a Teleflex tachometer, not necessarily the stock Yanmar one. Look at who produced it.

My Yanmar tach has a fixed setting for number of teeth. It is written on the face of the tach (something like P=127).

Thanks for copying the image over!
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