Tach Problems

I sounds like a loose wire to me, you should take apart your dash and check it out. I don't know where the wires go, but if you know which wires are which (if you have a diagram or follow it from the tach) you should be able to find it. Another place to check is the negative side of the ignition coil (it'll say "tach" on the little plastic cover), check to see that the wire is completely on there. One more thing to check when following wires is if there's a spot where the wire may be grounded or exposed. Good luck

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'84 Mustang 5.0 T5, FMS aluminum radiator & 180* thermostat, 1 5/8 shortys/2.5" duals, '88 GT tail lights and wheels, Holley 4160 4 BBL, smog pump idler, more coming soon