Vader, After some searching I read some threads where you talked about a buffer for the vss and that it was probly the only thing that was bad if the speedo was working properly. I have a 87 iroc with the tpi and 165 ecm. Im pretty shure all my problems are something to do with some part of my vss system. Mine has the sensor on the speedo. The problem Im having is my idel wants to stay at 1200 or 1500 when I pull to a stop and wont go to normal till the speedo reaches 0. Even if im going like 5 or 10 mph it will stay up till I reach 0 and stay stopped for a second or 2. Also my curise dosent work and I have replaced the switch and tested it like is shows on here and it is good also, my tcc is locking up as soon as it hits 3rd gear (although i also read where you had that problem and got a stock replacment eprom and fixed it; how much was that replacment eprom?). There are no vss dtc's and when I unhook the cable the tcc dosent lock and I then get the code so I know that the ecm is getting somekind of signal but, Im thinking and this is just a guess by the way its acting that as soon as I energize the key switch the ecm thinks im moving at some speed because iv hooked a test light to the ecm to see exactly when the tcc locks and the light stayed on and only went off when I hit the brake or when the gas pedal was released. it would even stay on when I parked and turned the ignition off. I couold get it to go off by unhooking it and then hooking it right back and it would be off till I started the engine back. Also when The car sits a while in cold weather and I dont drive it everyday the speedo will make a loud scratching noise and read way to high for about 10 or so miles and then when the noise quits it reads right agin like something in the speedo is sticking, that may be something unrelated but I thought I would add it in also. What would you think I need to look at for the cause of this and what parts should I replace? Im stumped and dont think the vss is bad just the signal its sending is wrong and dont know how to fix that. Any ideas or any test procedures welcome!!!!

 

Yet another of the often ignored differences between Camaros and Firebirds is the differences in the VSS systems. Camaros of that vintage had the VSS incorporated onto the rear of the speedometer, while Firebirds had them on the transmission since 1985. Camaros caught up in about 1989 and moved them to the transmission as well, eliminating the cable drive to the speedometer.

The signal from the speedometer-mounted optical VSS on a Camaro should be a square wave DC output varying in frequency. At 30 MPH the output should be in the range of 16-17 Hz. Camaros used an optical trigger array which produced a pulsed (square wave) DC signal which was split at the VSS buffer module for use by the ECM and cruise control module.

Another difference in Camaros is that they use a SINGLE output from the VSS buffer to feed both the cruise control module and ECM. If the cruise module has a circuit problem, it may be shunting the signal to the ECM, potentially causing digital flake-out. Try disconnecting the cruise module electrical connector just to check that possibility.

A plain analog or digital multimeter will not be very useful in diagnosing the system, unless your meter has a frequency scale (most don't unless they are higher-end units). About the best you can do with a plain meter is to test the speedo and VSS Buffer DC outputs by connecting a DIGITAL voltmeter to the output and ground, then turning the VSS sensor/speedometer cable very slowly (one pulse at a time) to cause the VSS Buffer output state to pulse off/on.

Start at the buffer module since it is easier to access and splits the system in half. If you detect the signal state changes at the VSS buffer outputs, you might presume the buffer module is doing its job correctly If the signal at the buffer is good, so you can work downstream toward the ECM. If the signal at the buffer is bad, the optical pulse generator on the back of the speedo or the buffer may be bad and you can work upstream to the speedo head. You can also test the square wave output of either the optical array or the buffer module with an oscilloscope.

Hopefully, you find only a corroded electrical connector. That should keep you busy for a few minutes.

 

I unplugged my cruise and no change. I have my speedo out, but I dont know if I need to put 12v to it anywhere, or just hook the neg to the buffer center black prong and then test each side for the on off pulse. If so, without supplying 12v anywhere, its not doing anything at the buffer, on either side the red side or green side. But Im not shure if Im testing right. Or do I need to run some jumpers out from where the buffer hooks to the long plug on the dash and put it all back together and slowly spin the cable from the tranny and check for pulse then while in drive and key on?

 

The optical array on the speedo gets its power from the VSS buffer module. The devices must be connected and powered to test. You can check the signal between the brown wire and ground while turning the speedo.