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Hello. My name is Tan.
The tach works off the signal coming from the negative side of ignition coil gray wire. The hour meter is run by the voltage from the ignition key purple wire.
The only thing you can have is a broken wire unlikely or bad tach.
The only way to test the signal coming from the coil is with a peak reading volt meter or an known good tach.
The tach is the most likely failure and is not uncommon.
Does this all make sense?