94 G20 Tach is malfunctioning/fluctuating

[gniknave]

Just installed a used transmission in a 94 G20, has about 107k miles on it. For some reason the tach goes crazy. It's fluctuating from 500-2000 rpm at idle, and badly fluctuating from 1000-3000rpm while driving. However the motor is not revving with it. There are no drivability problems whatsoever.

Tried to swap crank sensor from the old transmission but this didn't fix the problem or make it any better.

Any ideas?

[maxhopper]

This is a pretty common occurence with the G20. The least costly method that I've heard of is to buy an entire gauge cluster out of a junk yard and swap out the spedo/odometer (so it doesn't look like you rolled back the mileage).