Most diesel tachs work off the AC output of the alternator. If the alternator is not producing any output for whatever reason there will be no signal for the tach. At the dock one of my engine tachs is erratic when the battery charger is charging the battery since that alternator regulator has cut way back.
Could be a bad regulator, is the battery being charged, what is the voltage output of the alternator?
There should be a connection on the back of the alternnator for the tach. Look up the alternator on the web and it should show the connections.
Bob
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