I inherited a 1992 F-150 5.0L V8 w/ Trailer Tow Pkg. The tach quit working the other day... When started, it immediately goes to 2400 rpm. It seems to rise above that with engine speed, and shift back down to 2400 rpm. When it does rise above 2400, the engine is not running at the registered RPM. For example, it says 3400 rpm when cruising at 70 w/ overdrive engaged.

The truck has 133k on it, and has an E40D tranny and 3.55 rear end.

 

Mine did the same thing about two years ago. I have 190,000 miles on it and the tach messed up about 30,000 miles ago. Did you ever figure out what was the cause? A friend of mine said that more than likely it was a cracked ground connection on the tachometer gauge circuitry. It will let a capacitor charge, but will not discharge fast enough. I have not tried to rip out the gauge cluster to see if that was the cause. I hope someone has some better advise than this though.

 

I will pull the guage cluster apart this week and check out the ground problem. It was also mentioned that it could be a speed sensor, but he speedo works...

 

The tach doesn't use any speed sensor - it's driven directly off the coil/ignition module.

The fault is definitely within the tach module, which is easy to swap out of the cluster, but difficult to find by itself. Click my signature link & look in the Low Fuel Light album for some pics of a disassembled cluster.