I have a 93 5.0 and my tach has been acting weird since i got the car in 05, when i start it and let it idle is sits around 600rpms but when i drive it or rev it up it has a mind of its own. If anyone has any info that can help me trouble shoot this it would be a big help, Ive gotta the Ford Fuel Injection & Electronic Engine Control book and on the TFI on the dist it shows one wire running off of the module.

 

so the rpms actually feel fine... but the needle bounces a lot ??

fyi the stock tach is VERY innacurate

at minimum you want a shift lite, but aftermarket tach is much better

as far fixing the problem... not sure, the tach should pickup off the coil.. so find the wires and see if they are loose ???