Tach Needle Sticks at 4,000 RPMS

i had the same problem with my 90 tach , the problem is in the circut board on the back of the tach, may be the 4 nuts holding it to the tach came loose, or a bad connection in one of the 2 adjustable resistors on the circut board.

This seems to be a common problem on the mustangs, before buying a new one try taking the tach out of the cluster and check to make sure the nuts are tight and with a small screw driver turn the resisters pots a little back and forth make sure to set it back to the same spot you started with and try it. or take it to someone that can repair circut boards and have them check it. this is how i fixed mine and it would go to 3000 and stay there also