The tach gets its sense from the negative terminal of the ignition coil. I would check the sense connection on the back of the tach. The most common fault is a bad tach which has a limited life time warranty from Faria so you can send it to them for repair.
A voltmeter can be attached to the gray sense wire, if it has a reading between zero and 12 V or is pulsing the tach is bad.
1997 2400 SCR
5.7 Vortec / Bravo 2