I bought another Z28 this past fall for my daily driver and am making my 95 Z28 the "race car". Anyways the car is a 1996 Z28, M6 with full bolt-ons. Whenever I turn the ignition to the on position the tach needle will jump to around 1500 rpms and then bounce up and down several times and settle at zero. Whereas, normaly the tach should gradually rise in a nice liquid smooth motion. The bouncing is very fast. When I am shifting the car like a champ (quality 2-3 shift) the tach is jerky in movement, kind of like it is retarded/slow. Anyways, I was driving home from work today and drove on a road that was rather bumpy and I noticed the tach needle would "bounce" a bit whenever I would hit a decent bump in the road. I find this activity extemely odd. Where should I start looking? BTW, the previous owner wired up a remote start (on a M6 car is a little scary) could he have wired into the tachometer somehow and that is causing its weird behaviour? Any help on where to start looking would be awesome! Thanks!

Jeff

 

Is your speedo quirky too, or just the tach?

 

Just the tach. Speedo is perfect.

 

First check for any codes. The SES light will not be on for some codes. If there are no codes, you probably have a wiring/ connection issue between the IP and the PCM. If you do have codes, the problem is else where. More than likely the opti wiring. But let us know.

 

Well I had this problem once upon a time. Replaced Opti, WP, etc. Still did not fix it, until I was haveing my AC serviced because of a hole in my line. The line and stap had been rubbing, the ground strap was just about in two.

 

Pigtail connection on opti can cause that if its corroded. Also your Tach pin out off the ECM can do the same thing if they had a shift light or something attached to it, and then removed it.