I had a nonworking tach on a used 88GT I bought. Tach filter was missing so I bought one of Rodney's. Now tach works but is very jumpy and unstable. I've never had a bad part from Rodney so I had another gauge cluster and put in the tach from the new old cluster and tach does the same thing. I tried a stock tach filter I picked up on ebay and same eratic jumpy tach. Could it be the output from the engine causing this? Two tachs, two filters and same jumpy needle. John

i have this same problem on my 88' GT, but ONLY when it rains and i happen to go through some water... huh? I was going to purchase Rodney's filter, but if that's not the fix, than i'd rather hold off and see what is?

chck the plugs on the module and the coil, corrosion can cause that. hard to clean em up, but often fixes that problem. I believe the tach line also goes thru the c500 connecotr, that's another possibility.
if the engines run fine, then it is most likely a connection, tho the module and pickup coil and even the spark coil can cause it, but there are usually other symptoms with either coil.

Unplug the connectors on your ignition module and coil, spray them with WD-40, and plug them back on. Can't hurt -- might help. Let me know the result if you try it.

I'll clean up the connectors and see what happens. Weather has taken a turn for the worse here is Tulsa, the car is outside so I'll wait till it warms up later this week. I'll give you an update then. Thanks for your help. John