If your tach is not operating properly or at all- try this simple step before any of the more complicated paths: clean the electrical contacts on the top of the tach generator. And inspect the wires for breaks and missing insulation. I used an electrical contact spray cleaner of the connectors and tabs on the generator. My tach which has been jumpy and jittery for years now works perfectly!

your problem MAY be more complex but the downside to cleaning the leads first is zero.