Will Replacing The Wires Fix The Tach Readings

[Joe Mullee]

I have a Yanmar 3GMF on board my 1983 Hunter 34 and my tach is off. I've messed with it for a few years but have never gotten it to work right. I've cleaned the wire connections at both ends but it's still not reading correctly. A local Yanmar mechanic checked the RPM's with a tach gun(?) while in neutral with the throttle wide open and it read around 3600 while the guage reads between 1800-2200 depending on the day. Under power my SOG was 6.4 with a dirty bottom (this past weekend).

Could I possibly fix this problem if I completely replace the wires between the guage and the tach sensors on the engine with continuous wires taking out the connections that are in the harness?

I recently had the starter button problem and after trying to clean all the connections between the button and the selenoid and still having the problem I just replaced the entire wire with one continuous 10AWG wire and haven't had that problem again. Would replacing the tach wires be as simple a solution as the starter button problem? If so what guage wire should I use?

Thanks for any advice,
Joe Mullee

 

[kloudie1]

Joe, I have the same problem to a lesser extent.. I show 2600 on a true tach at 2000 indicated on the Yanmar. I believe it is a problem with the spring/bearing for the needle in the gauge itself.. I haven't taken mine apart yet but that is on the list .. This particular system is basically a milli-ammeter.. The counter counts the flywheel teeth passing it over time and converts that to a current which is measured by the gauge .. The flywheel has 97 teeth and the tach should have a "97" in the bottom center of the gauge. Unlike a starter circuit, there is very little current in this system.. if cleaning the connections didn't help, changing the wire probably won't either.. but it is easy to get to and worth a shot..
At the sensor on the flywheel, a real 1000 rpm should generate a 1616.7 Hz signal at about 2 volts peak to peak.. can be measured with a good oscilloscope.. or frequency counter. The stated resistance for the flywheel sensor is 1600 K ohms ..

 

[Dan Johnson]

An O-scope?

You still have an O-scope? And know how to use it? Amazing! :dance:

Joe, Claude's right. I used a laser tach on my 3GM last year and found about a 200 RPM difference (tach lower than the laser reading) at different throttle settings in and out of gear. I haven't attempted to find a solution to correct the difference.

Boat show weekend I went down to Georgetown and back under power (wind on the nose or no wind). Only the second time out of the slip all season. Ran around 2,800 RPM indicated and was making an average of about 6.3 Kn SOG. Sounds like you ran about the same speed, which should equate to approximately the same RPM give or take 10% or so.