I can't seem to get my speedometer to work in a mix-matched car I threw together. It is: 1991 Z28 body, 1991 305 TBI motor, wireing harness and ECM, 1992 z28 700r4 trans. I have tried numerous speed sensors, tested the wiring from the sensor to the ecm and then to the cluster, taken apart the cluster to make sure nothing was stuck. I don't know if this may have anything to do with it, but the tach. shows the rpm about doubled what the motor is actually running at. Please help

I'm having a similar problem though not in a car with mixed components. I have a 1992 RS with 80k miles and the speedometer/odometer quit working. I've tried swapping out the VSS assembly and swapped out a cluster from another 1992 RS. Neither solved the problem so I am thinking it must be the ECM. The reason this thread caught my attention as to being a similar problem is I am getting the same readings from the Tach as you describe. It shows normal 1500rpm at idle but it definitely reads way off as the RPMs increase. I can redline 6.5-7k rpm before it ever shifts and I guarantee the motor isn't turning the rpms showing. I'm taking it to a mechanic tommorrow who is the one that thinks it's the ECM and hopefully he'll have it resolved in a few days and I'll post a response.

Re-reading your post it seems like yours too may be ECM related. My mechanic mentioned to me to make sure that I had the right year VSS and Gauge cluster when I attempted to fix it myself because he couldn't remember for certian, but something with the VSS changed either from 90 to 91 or 90 to 92. If it's the latter 91 to 92 you may just have a communication issue with your VSS to your ECM since your tranny and thus VSS is from a 1992.

Thanks! I would appreciate if you would let me know what you find out with your camaro. I have also heard that the TBI motors needed a pulse 2 VSS and the TPI's used a Pulse 4.?. Don't know if this is true or not.

Well I'm going to pick up the car now and he was unable to fix or locate the problem. He checked the signal from the VSS and it was fine, the single out of the Buffer box to the ECM was fine and the signal to the gauge cluster itself was fine. So I'm stumped. I may go pick up a Buffer box from a junk yard since I imagine it would be a cheap troubleshooting step before replacing the ECM. Ok so it took me a 3rd time to comprehend what mix/match parts you have. TBI cars require a "Buffer Box" to convert the signal from the VSS so the ECM can understand it. I would imagine since your car is basically a Z28 with a TBI motor your problem lies somewhere in the fact that your ECM has 0 clue what the VSS is telling it since your car is most likely lacking this buffer box.