well i got a 88 fc and the rpm guage after i washed the motor it work for a second but after that it was history. i was wondering if there was a cable or something that could have gotten disconected so if you got an idea please ill need the help.

 

If the other gauges besides the speedometer aren't working either then try replacing the Meter fuse. If only the tach gauge is not working then try disconnecting the plug to the trailing coil which has four wires and make sure the Yellow/Blue wire is clean and dry. Also, there is a Black wire dangling from both the leading and trailing coils which is short lengthed and connects to nothing. You could jumper both of these Black wires and the leading coil will then provide the signal for the tach to work thus providing a way to prove that the tach itself does work. If it does work with the jumper wire in place then the problem lies within the trailing coil or the wiring from this coil to the tach.