JeremyNYR

I have a 1995 LT1 swapped into my third gen T/A. I've tried two 3rd gen tachs now, with the same result. I can't say for certain whether or not this problem existed when the drivetrain was still in the original 4th gen car. I've used a 8000 RPM tach from a 350 TPI car that I know for certain worked perfectly with the original 350 TPI, and I've also used a 6000RPM tach from a 305 TBI car. For both, I will slowly accellerate and the tach will move up smoothly until 4200 RPM, then it drops off the table to zero and stays there until I drop below 4200 RPM again. The engine doesn't get choppy or anything when this happens, it appears to just be the tach that acts funny. Does anyone know what is causing this?

Red-LT1

I could only guess it would have something to do with the PCM. Did you swap that too? I would imagine you got one with the engine. A '95?

JeremyNYR

yeah, the pcm, lt1, t56 and the engine wiring harness was all from the same 95 Z28. I bought the car running but with body damaged. I don't remember seeing the tach act up when i test drove it, but i only drove it in that car once and may not have noticed it. I spliced into the 3rd gen dash wiring for the guages.

96' Formula WS6

Well the only thing i could guess is that when you go past that certain rpm there is too much voltage (or something) and your thirdgen tach can no longer keep up with it and get a good reading. Now im no electrician so dont flame me if im way off base.