Since you replaced your tach make sure the settings are correct on the back of the guage for your motor. If thats good the only item left is your voltage regulators. You should have 2 of them on the back of your motor. There will be 2 grey wires coming off the regulators, one connected and the other plugged off. Those are the tach signal wires. First try switching the grey wires. If your tach starts working correctly then you know its your VR. If it does not fix it then you may have 2 bad VR's. Not an uncommon problem. Also check for darkened connections where the VR's connect to the stator. If they are burnt then the VR's are bad. When replacing voltage regulators always replace both at the same time regardless. Changing VR's is simple and straight forward. The Sierra after markets are about half the price as the merc parts.