my tach starting bouncing then completely dying. The signal for the tach is on the blue wire from the ICM (Ignition Control Module) under distributer cap. I followed the teg tip on tightening the 4 terminals but they were pretty tight to begin with. I also used emery cloth on the 4 male terminals on the ICM. What I saw on the shaft of the distributor where the magnetic pick-up is (by the small transformer) was carbon tracking standing right on end as it was magnetized and it was completely bridging it. I’m assuming this was causing no output from ICM to tach. I used a thin feeler gauge to remove the carbon tracking, put everything back together and the tach has worked great since. Should also note that this AC signal (blue wire/gnd) also goes to the cruise control module and the automatic transmission control module. If your tach fails so does your cruise and your tranny will not go into 4th gear.
bad tach update
Update: Tach died again 2 days ago. This time I just removed the cap and blew out the inside of the distributor with compressed air. There was a lot of brunnish/reddish looking dust. I’m assuming it’s carbon wear from the cap and rotor button contact points. Anyways, it’s working fine again. Perhaps this will become a monthly thing I’ll need to do. I need my tach as I drive about 45 minutes each way to work.
Have you tried taking out your cluster and checking your tach cable behind it? I had that problem and the cable had broken off (mostly) from where the tach attaches to the back of the cluster. Replaced the cable… done.
good luck