Tach not working properly

[LakePirate]

I have a yanmar 2gm. Had no problems with the tach until today's sail. The motor starts and runs great. The problem is at an idle the tach is setting on 0, with the motor at what feels like about half power the tach is around 500. What could have caused this to start happening? Last time I took the boat out it worked fine. Things I have done since the last sail are had a bottom job, stuffing box gland replaced, cutless bearing replaced and installed two new batteries. I don't know how any of this could have an effect.

[Faster]

Many tachs are alternator driven. A failing tach can be an indication of bad brushes on your alternator if it's driven from there. Do your charging amps drop off too?

[Stu Jackson]

Are you on shorepower when you're not there? New batteries are most likely fully charged. Once charged, the alternator's regulator does not require output. Tach driven from alternator won't read.

[Hartley18]

Check the wires off the back of the alternator.

You'll likely find one of them will come off in your hand...

[Omatako]

Maybe it is an alternator driven tach but my Yanmar has one that has a sensor on flywheel housing that reads the ring gear teeth going by.

I really hope you don't have one of those because mine did what yours is doing then it died. The only solution was a new one from Yanmar which was then (2008) $350.

I now measure the engine RPM with my ears. If it's too loud, it's too fast.

Is it really true that a fully charged battery bank will cause the alt driven tach not to read? I was going to buy one of those but if this is so I guess I won't. With my new up-coming genset, my banks will hopefully be fully charged more often than not.

[Hartley18]

Originally Posted by Omatako

Is it really true that a fully charged battery bank will cause the alt driven tach not to read? I was going to buy one of those but if this is so I guess I won't. With my new up-coming genset, my banks will hopefully be fully charged more often than not.

No. An alt driven tach is wired to a different set of terminals on the back of the alternator and so will get its signal pulses regardless of the output voltage or the state of charge of the battery bank.

[mbsl98]

I have a 3gmD and just had a similar tach problem, after a new shaft/cutlas/engine mounts. It turned out to be a bad connector on the tach sending unit which is located on the top of the aft end of motor, just before the output to the transmission. It has two wires going from sensor into the main engine harness, and they are the only wires around that aft end of motor. I'd be looking at things that got disturbed while the work was being done, as I also found I had a slightly loose fuel line fitting at the engine mounted pump that was probably caused by motor being moved around.

[bwindrope]

Tiny Tach might be your solution. They run off the injector lines and are absolutely reliable. Far more accurate than an alternator tach. I've had one on my Gulf for a few years and love it. Cheap and solid.

I needed one because on the Gulf 32 the inside wheel has the engine gauges and there were none in the cockpit. To remedy this obvious problem, I used a winch pad and placed the Tiny Tach and a temp gauge right to my starboard near the wheel. Now I can glance every few minutes and see those vital things. And it is easy enough to actually do it as often as one should.

NO, I do not work for or know anyone who works for Tiny Tach! Just a great little product.

[capecodda]

For what its worth, my previous boat had a big battery bank, and would exhibit tack drop out when the batteries were fully charged from shore power. The tack would drop to zero for a second, then come back to reading properly, then repeat. After running for a while, with DC loads on, this would disappear and the tack would read correctly I was told this was the regulator dropping out because the batteries were fully charged. Once they got down a bit, the problem would go away. Never investigated further. Not sure this what is happening, or even if I had the right root cause for the old boat. If it is, you could try discharging the batteries a bit and see if the tack works.

[rikhall]

Originally Posted by bwindrope

Tiny Tach might be your solution. They run off the injector lines and are absolutely reliable. Far more accurate than an alternator tach.

1 - I had a TinyTach on one of my boats and agree with bwindrope if you have a dead tach - but
2 - I also had the intermittent tach thing on my current boat (2QM20H) and it was the wires - loose and finally one came off. New terminals and voila - works perfectly.

Rik