Having just fixed a broken tach there are only a couple of potential problems.
Bad sensor on the motor.
Should have 12V at the tach, good ground and the wire from the senor.
Bad connection between sensor and tach.
Bad ground.
In my case the tach itself was bad. New tach was $180 - $200 depending on where you buy it. Mine had a Faria with a limited lifetime warranty. I went to their website and filled out an RMA. Sent bad tach and $25 to them and they sent me a rebuild.