I assume you think the transmission is working properly and only the tach is the problem.
While idling in neutral, rev the engine quickly to around 4000 rpm and back to idle. (If you have a Chrysler scanner, you can run the gauges up and down without running the engine). Look for free movement of the tach needle. If it appears to stick or jump, take the cluster apart a look closely at the plastic laminate that's glued under the gauge needles. Sometimes the glue allows it to slide sideways a little, and the base of the needle rubs on it. Rather than trying to reposition the laminate, it's easier to use a small hobby knife to trim the hole a little so the needle doesn't touch it.
There may also be a circuit board plugged into the back side of the instrument cluster. Unclip it and pull it out, then lightly sand the silver-colored contacts. Just removing and reinstalling it can scratch the contacts enough that the tach might work fine for a few months.
Naturally, if you hear the engine speed up when the tach jumps, the tach is just verifying what you are hearing so it's working properly.
Thursday, March 19th, 2009 AT 5:30 PM