I have a 84 with the factory instrument cluster with tachometer. The tach quit working a long time ago. Then a few months ago I had the tilt steering column rebuilt and when I put it back in, lo and behold the tach was running again. About a month later, it quit again. I don't know how the tach works so I'm asking how does it hook up ? Is it electrical ? Is there a wire or ground around the steering column that may have lost contact or connector that may have come unplugged ?

 

It is electrical. The wiring plug that goes into the rear of the cluster is where the input to the tach is. I am not sure how the harness is routed under the dash near the steering column, then goes to the firewall connection into the engine harness to the coil. As to grounds in the cab for the tach I am not sure where it would be

 

Check out the site at the link below.
It used pages directly from the FoMoCo EVTM (Electrical Vacuum Troubleshooting Manual).

http://www.garysgaragemahal.com/tachometers.html