Fluxuating Tachometer

[Saa Bear]

40 yr old Yanmar 2QM15, Tachometer used to be rock steady but this season now not. Engine seems to run fine but tach needle can jump up and down, sometimes as I hit waves and sometimes seemingly on its own. Any thoughts would be welcome.

[StuM]

Poor electrical connection.

[tkeithlu]

Ditto on Stu's "poor electrical connection."

[Wotname]

Agreed it is 99% likely to be electrical connection (loose or corroded) and the first spot to look is the spade terminal on the tach sender mounted on top of the bell housing (above the flywheel).

Unless you have an after market tach which may be sensed off the alternator, if so look for a loose connection on the alternator - sometimes marked 'W', sometimes a fly-lead.

[Compass790]

Agree with all of the above. We had problems with spade connection and ( from admittedly dodgy memory ) a plug that the sensor lead goes into. Our ysm8 ( Basically 1/2 of a 2qm15) vibrates so much that it shakes loose. Solution was to cable tie the plug together. Hope that helps

[BillKny]

If the tachometer is run off the alternator as I seem to remember that it is on this engine, it could also be a slipping alternator belt. If fact that’s pretty likely…

[Wotname]

FWIW, Yanmar offered a factory tach option for the 2QM15 which was sensed off the flywheel teeth.

Other (non Yanmar) aftermarket options would more likely to be alternator sensed.

[GordMay]

Agree, check electrical connections, or perhaps the proximity sensor gap [should be between 0.032 ➛ 0.062" gap, and clean].

[Saa Bear]

Thanks for the various ideas. I'll be going to my boat later this week and trying these options out!