Yanmar Tach Calibration...

[Dan Johnson]

This task finally came to the top of my "Wanna Do" list. My H34 has a Yanmar 3GM30F with the "standard" engine panel. After a tach problem two seasons ago that was cleared up by cleaning the wiring connections, I've wondered just how accurate these tachs really were, especially after some recent posts regarding RPM's, boat speeds, and prop pitches.

I acquired a Neiko laser photo tach (#20713A) for a very reasonable price from Amazon. It was a simple matter of first applying a reflective square (from provided material) on the crank pulley at the engine front. Then I fired it up and setting the throttle to a set value on the tach then shooting the laser at the revolving reflector spot on the pulley. The results were somewhat surprising.

The Yanmar panel tach indicated about 8 to 10 percent low compared to the laser tach's reading. Here are the comparisons:

Yanmar Tach vs Laser Tach Indicated
1000 RPM 1094 RPM
2000 RPM 2167 RPM
2500 RPM 2756 RPM
3000 RPM 3367 RPM

With the engine in gear (12x15 RH 2-blade prop):
1000 RPM 1035 RPM
1500 RPM 1620 RPM
Any higher revs while still tied to the floating dock would have put too much lateral stress on the dock section hinge pivots and dockline cleat attachments.

Consequences are that I will adjust my indicated cruising RPM to keep the revs at the recommended 80% of max level.

Has anyone else done any calibration comparisons?

 

[Don12364]

Mine reads 5.6% low on 3JH2E. i.e. the engine's tachometer reads 3400rpm when the engine is turning at 3600rpm as measured with conventional engineers mechanical type of tachometer.

 

[kloudie1]

Yanmar Tach

It is on my To Do list, Dan but I haven't gotten to it yet (Forgot optical tach at home last weekend).. I plan on doing it and posting the results here on your thread.

 

[jsalley]

Re: Yanmar Tach

I used the same laser tach on my Yanmar 3YM20 after replacing the alternator with a Balmar high output unit, and found the panel tach read exactly (at least within 25 RPM) the reading on the laser. Just lucky, I guess.

 

[alexco38]

On the back of the tech there is a seting rotary switch to match the number of poles on the alternator rotor. Most alternators are either 6 poles or 8 poles. They must be matched or the tech will give wrong info. The tech uses the AC pre-rectified output to drive the needle.

 

[SamLust]

The Yanmar Tach is nothing more than an analog display figital pulse counter. There is a sensor aimed at the edge of the flywheel that counts ring gear teeth. It converts that info to a needle position on the face of the tach. I've never noticed any sort of calibration onn the back. Many after market tachs read an output from the alternator. Their accuracy is affected by the state of wear of the belt. Be aware that Mack Boring sold complete accessory instrument panels and kits for the Yanmars. They were from VDO and looked as good as original. No one currently at Mack Boring that I've spoken with, with the possible exception of Larry Berlin knows about them.

 

[sailaway78]

As far as I know

On the Yanmars that use the flywheel for the sensor, there is no adjustment or calibration that I am aware of. The ones that drive off the alternator, have an adjustment for the number of poles in the alternator, but not sure that there is a calibration feature on these either.