I have an 85 GT with 120 speedo and matching tach.. Everything worked fine till a few days ago then the Tach stopped functioning properly. The tach will show almost 1k more then the engine is running, so Idle at 1k shows almost 2k on the tack. Revving the engine shoots the needle passed redline.. I took the filter out of line and didn't change anything. Before I go blindly digging into wiring and such, I'm hoping someone here may have had the same issue with a fix or can offer some places too look first. All other gauges work, speedo has a little bounce but that's for another time..
If there was a topic discussing this issue, I apologize for the dupe post.. I just didn't see it...
Google "GM tach reading too high" there is a helpful thread that tells how to replace the resistor in the tach with a variable one so you can calibrate the tach. The resistor loses it resistance over time and causes the tach to read too high.
I have similar problem with my 1986 Fiero. It has always read about 30% to high and now reads approx. 40% high due to deterioration of the electric components over time.
