I have a 91 camaro rs with a 3.1 engine and i have a issue with my speedometer and over drive. About a year ago i was having an issue with my speedometer, it would work only after it warmed up as i drove the car, then it got progressively worse by taking longer to warm up, after that it stopped working all together. So i thought it was the VSS or speed sensor, so i replaced that and still nothing. along with no speedometer my overdrive isnt working and hasnt worked since the problem has started. some guy said something about a white box under the drive shaft, which i cant find, SO im thinking my ecm is bad, is there any way to tell? because the VSS goes directly to the ECM which distributes the code to, among other things the trans to tell it when to shift, correct? if someone could help that would be great because this is irritating me

 

You may be mistaking OD for the TCC lockup. The TCC is controlled by the ECM, overdrive is not. Without a working VSS the TCC won't lock up correctly.

As you mention the VSS goes directly to the ECM, where it is buffered and sent to the cluster (speedo), and cruise control unit. With losing TCC it seems that the ECM is not receiving or buffering the proper VSS signal.

If the VSS itself is known to be good, that leaves the wiring to the ECM and the ECM. It may also be the VSS driving gear on the transmission output shaft. On the up side there is no longer a working speed limiter in the ECM

RBob.

 

You can't believe how many times I've had to tell people WHY they need a working VSS (TCC won't lock being one of the issues). Maybe I'll send them to this post in the future.

Anyway, the TCC won't lock up until the engine attains a certain temperature. It sounds like you should find out why it is taking so long for your car to warm up. I really cannot think of a reason your Speedo only works when the engine is warmed up. Could be a wiring issue for that problem.