Intermittent Tachometer – Yanmar

[ryancheff]

I have just recently purchased a 1983 Pearson 303 with a 13hp Yanmar. Among the other issues I am dealing with is an Intermittent Tach. It works for about 30 mins, then begins to stick. After shutting down the engine it will fall to 0 then restarting it sticks at about 1500? It will begin working correctly after it fully cools down (6-8 hours).

Anyone have similar issues, tell me where to start? Where is the tach pick-up located on the engine?

Thanks!

[gc1111]

I don't know the Yanmar, but there are two choices for tach input on a diesel. Either a mechanical takeoff (kind of rare) or a connection to the alternator (much more common). The alternator connection is to a special terminal on the alternator that has the AC the alternator creates before it is rectified. The tach works by measuring the frequency of the AC which depends on the rotation speed of the alternator. These require calibration and usually have adjustments available on the back of the tach.

Unless the Yanmar has a mechanical takeoff, I can think of nothing that would cause the behaviour you describe. With a mechanical takeoff and a rotating flexible shaft connection to the meter (similar to what is used on many automobiles) there are more possibilities.

[Faster]

I'm not sure what Yanmar uses.... but
Another type of sensor for rpm is an inductive pickup on the flywheel - the pickup sits in the bell housing/gear adapter and detects a ferrite slug embedded in the flywheel/ring gear. RPM is based on the number of times the slug goes by the pickup. Sometimes if the distance is too far the pickup loses signal. This is unlikely to result in your symptoms, however.

Your problem could be simply with the gauge itself... is the needle hitting the lens of the tach and sticking? or something similar inside...

[ollie820]

As GC stated the signal probably comes from the AC tap on the alternator. The Tach may be fine and the problem lies at the source. Check the output voltage to ensure that it is putting out enough DC as the Alternator may be on it's way out.

I installed a new alternator as the DC had dropped to below acceptable levels. I put in a new alternator and the tach worked fine and the charging was back to normal.

[camaraderie]

Ditto the alternator comments from ollie and GC.
Just wanted to add that IF the issue turns out to be the alternator and you replace it...make sure that the belt pulley spindle on the new alternator is the same size as the old one...OR...recalibrate your tach usinging a laser counter on the flywheel or shaft.
Failure to do this could resut in vastly incorrect readings from the new alternator. Don't ask me how I know!! (G)

[SVCarolena]

Ryancheff - Did you ever figure out an answer? I have a P303 that I bought this summer. The tack was working fine until I took it out this past weekend, at which point it was sticking and would only move if I tapped on the glass. I'm thinking it may be due to the cold (was in the high 40s) and am wondering if there is something in the tach that can be lubricated.

[billangiep]

The Yanmar Tach. (head or gauge) is pretty well sealed. I wouldn't open it up without inspecting first if there is a problem else where. 99% of the time the problem is the spade connectors on the sending unit itself located on top of the bell housing. As faster mentioned Yanmar uses the inductive type units.
Bill,

[Lion35]

Just to cover all the basics I would also check for a voltage drop on your ground to the Tach. Fire up the engine and measure the voltage between the ground at the back of the Tach and the center of the negative battery terminal. You want to measure voltage drop under load, not resistance on a "dead" circuit. Ground loops can play havoc with Tachs.

Just my two cents

[HoffaLives]

a basic test is to tap on the tach itself. Mine is twitchy as well, and a few taps reminds it of it's business. Something internal is bad - probably a cold solder joint. i would expect a problem with the tach before the sensor just because the pickup is really a passive device while the tach does all the interpreting and is a lot more complicated. when sensors go they generally provide no output rather than the wrong output. as has been mentioned connections are very important as well and prone to failure

[Omatako]

Hmmmm.. . .

I was wondering whether this had anything to do with output. I replaced the standard 35amp Yanmar alt with an upgraded 85amp (physically identical) unit. All the connections were the same, no left-over wires without a home but since then my tach doesn't work at all.

I don't think it has to do with the change in output but I've sort of run out things to check. Haven't checked my bellhousing but must say the coincidental non-operation is too obvious to think mine is anything but alternator driven.

Andre