I'm helping a friend fix up an '87 and we've encountered a problem with the tach: at times it will work normally and other times it will read around 3000 RPM when the engine is at idle.  Reving the engine a bit and then letting off the throttle will cause it to shoot up to between 3 and 4 thousand RPM, then drop back.

The dash wiring is a mess, but I think we now have all the grounds connected.  We have verified that the power and ground connections to the instrument cluster are Ok, and cleaned the connector that connects the instrument cluster to the wiring harness.

We haven't yet looked at the signal from the coil with an oscilloscope.  I did notice I get a shock if I touch the signal lead from the coil, which could just be normal inductive kick from the coil, but I would expect there to be a diode clamp in the ECU.  Anyone know if this is normal?

Any ideas?

Bill