H30 Yanmar 2GM RPM question

[Alan Gomes]

It's the tach !!!

I first checked the linkage (took about a minute) .... nope, linkage fine. Bought a handheld tach at Harbor Freight and bingo ..... the onboard boat tach reports/reads exactly half of the actual engine rpm.

With the engine running at 2000 rpm (as measured by the handheld 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 - the onboard boat tach shows exactly half of the actual engine rpm.
At full throttle I get 3500 (as measured by the handheld tach - 1750 on the boat tach).......
So.....now on to troubleshoot the onboard tach.......thoughts?

I have the workshop manual for the 1GM/2GM and it contains step-by-step instructions for troubleshooting both the sensor and the gauge. Private message me and I'll get it to you by e-mail attachment.

You'll also want to make sure that the tach you have is the correct one for your engine. It could be that a previous owner swapped out tachs and Yanmar tachs are not all interchangeable for different Yanmar engine models.

 

[Steve Dion]

Here is the deal. Get one of these to check our your basic problem.

http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html

 

[lesrrt]

Ground maybe. Ck sensor, open/ shorted?

 

[Jon Bastien]

(Bump!) I'm having the same issue...

Malyea, did you ever figure out why the tach is only showing half the actual RPM?

Thanks,
--Jon

 

[malyea]

Malyea, did you ever figure out why the tach is only showing half the actual RPM?

Thanks,
--Jon

No, haven't yet got around to troubleshooting the issue...other than now knowing (happily) that the engine does develop full rpm at full throttle - although the tach reads exactly half actual rpm...

 

[Nelsonhow]

2GM RPMs

I had the same issue withholds 2 GM. Teach was reading about 1,100 RPMs. Clean mixing elbow, clean fuel and filters, replaced injectors with no change. I finally bought a laser tachometer on eBay for less than $20. You attach a reflector to the drive Lilly on the front of the motor. Within 3 minutes I had my answer. The real RPM at full throttle was 3,550 RPM. Either the tack on the panel, the sensor prone of the wires is bad. For now I know that the tach reads 50% actual.