1988 TPI with a Firebird dash and harness. All of my gauges work fine, except the tach. It works properly most of the time and it reads pretty accurately, but lately when I really put the car under load it goes to zero. The engine seems a little down on power, but no jerking or hesitation. I've also noticed that sometimes the dash lighting will go out, but the other gauges stay working. I had a problem awhile back with the ecu ground on the driver side head at the rear, but I fixed it and the cylinder select error went away. The coil and dizz are used, not sure how many miles. New ac delco plugs and accel wires. New MSD cap and rotor.. Small cap dist.

 

My 1991 Formula is doing the same thing, if you figure out what is causing it I'd like to know!

 

Make that me too. Under WOT it'll drop to 0. I have a MSD 6AL and Vortech Boost/Retard box. I suspect it's the 6AL box but never got around to much testing.