New Tachometer isn’t working, possible causes?

[Jasindude1]

I bought a new tach on Amazon, set it to the correct setting, wiring is good, ground good the tach does turn on and display the other items, (water and outside temp) so the gauge does work but no RPM signal, my purple/grey wire is connected all new wiring and connectors. I have a 88 Merc 60hp outboard with a pre-94 force wiring, so purple is the tach. How can I verify the grey wire is sending a signal? I have my voltmeter with me at the lake, what reading should I get from the grey wire to the tach? Also was reading could be my rectifier? The engine is charging my starting battery when running, I am able to see the voltage while the engine is running, but what reading should I get on the rectifier? Or at the grey wire on the rectifier? Any help would be appreciated im at the lake for another few days so would like to get this ironed out if possible.

 

[Chris1956]

The tach runs off the pulsing signal from the stator wires from under the flywheel. You should test the yellow stator wire disconnected from the rectifier, and see if you get any voltage. I would recommend testing high AC voltage range and reducing the scale. Any reading is likely fine. as the count of the AC signal frequency drives the tach.

This would be a good time to test the rectifier. They are usually a full wave bridge rectifier, with the -DC post grounded to the block. the internet will describe how to test it with an ohmmeter.