you have a strange year ford tach. 84-85 neds a positive pulse not a negaive like every other car in the world. I'm assuming your'e no longer running stock tfi. if so you can buy a tach converter for $60 or make your own. what i did was took apart an old pos radio(20-40 dollar variety about 10 years back) and stole a little transformer or get one from radio shack a 1:1 audio variety works(im thinking this is the same thing in a ground loop isolator). hook the 2 primary wires to tach output(negative on coil for duraspark) and switched positive. then hook the secondaries to tach and ground) this did the trick in 85 capri.