There is a thermister in the fuel tank that has one circuit that is wired into the VFI module.
Then there is output circuit from the VFI to the light in the tach.
I would start by identifying and verifying the integrity of those two circuits.
If ok, you can check the resistance of the Thermister to verify it. They are available from S&S, but you have to remove the tank and fule pump assembly from tank.
