How to diagnose tachometer?

[belizesailor]

Originally Posted by dkenny64

does the engine have a TACH connection or is it from the alternator?
if its from the alternator...is it slipping under load..
I've seen ours do exactly what you're describing..
tighten the alternator belt..tach works fine..
of charge the batteries by other means before running the engine.


-dkenny64

Along these lines:

It looks like that engine could be equipped with either a feed off the tach or a magnetic sensor screwed into the flywheel housing. Most likely its a tach feed, but you should take a look to confirm.

And yes, check the tension of your alternator belt.

[BigAl.NZ]

Originally Posted by belizesailor

The tach itself failing is far less common than an alternator/regulator or wiring problem.

Do you have an internal or external regulator?

+1 - our Tach was playing up and it was the external regulator on the alternator.

We just replaced the whole thing (regulator+alternator).

[Tetepare]

I spent years on and off troubleshooting a tach that exhibited the same problem. In frustration I replaced the tach. Problem solved for $60.

[Cat36Mahalo]

The only way to test the alternator tach signal is with a oscilloscope or frequency meter. The reason for asking about external regulators is when the battery is full the smart ones turn off power to the field, so no output, no tach signal, Balmar 614 have a setup option to always provide a minimal field so the tach keeps working. If your Tach sometimes goes to zero and sometimes works perfect, try turning on a bunch of high amperage 12V (or 120V if using an inverter) and see if the tach comes to life again.

If you change your alternator to something different the number of windings may be different requiring you to recalibrate your tach.