Last year the tach on my '90 RS started to read higher in OD. I thought for the longest time it was the lock-up torque converter. I changed the fluid, etc. with no result. When I finally thought to check the RPM in 'drive', the result was the same!... A 20% higher tach reading for no reason!

This year I got the car out in the spring, and guess what... the tach has decided to add ANOTHER 20% to its reading! This is unacceptable to me and I will change the cluster if that is the problem, but the only explanation I can get from anyone is, "It's old... they just do that sometimes..."

I get the unfortunate feeling that I am the only one in the universe with this problem.

 

Nope, you are not alone, almost every one of these cars has a flakey tach or is going to have in a few years.

 

I had the same thing happen to me.

I found out thru a Speedometer repair place in Carrolton TX
that the Calibration resitor pack goes bad.

I had my tach repaired, My red line moved to 6000rpm, my
speedometer change to the 145mph and calibrated and all
lighting replaced for $145.00.

Works great, and I have white faced speedo cluster.