Tried diagnosing why none of my scanners will not communicate with the ECM on my 1992 Cavalier. Recently my ABS light came on after a very hard stop. Initially the light would go out the longer you drove. For about a week, you could hear the ABS pump cycle under the hood and the light would go out right after starting the car. Now it doesn't cycle at all, and the ABS light stays on constant.
I first tried using my Snap On MT2500 scanner on the car and it would not communicate except for one time. I have not had the scanner very long as it was an Ebay purchase, and I didn't know if it had a problem.
With the bad weather and time issues, I didn't look at the car again until yesterday. I tried two other scanners that I have that have communicated with it in the past with no problems. Both of these would not communicate either. I have the factory service manual and found what lines of the DLC do what. It only has four lines going to it. Pin A is ground to the ECM, B is the diagnostic test line to the ECM, M is the serial data line. The fourth pin is for TCC. The voltage for the diagnostic test line is supposed to be 5 volts which I had 4.96. The voltage for pin M, the serial data line is supposed to be 4.7 volts and I had .006 volts which I assume is my problem. Pin A, the ground, did show 0 volts. These lines apparently go through the instrument cluster connector so this might also be a problem area. The ECM is mounted above the glove compartment behind a plastic panel, so a decent amount of stuff has to be removed to get to it.
The weather will be bad again in the coming days so any more trouble shooting will have to be on hold since I have to work on it outside. I'm going to access the instrument cluster connector before trying to get to the ECM and test there.
All of this work is mainly to see if I can diagnose this communication problem more so than trying to fix the ABS problem as I don't really care if the ABS works and getting parts to fix whatever is wrong is probably impossible. I don't think it's an ECM problem because when I short pins A & B together at the DLC, I do get code 12 to flash continually. It's the only code that shows up. Service information does say though that it could still be an ECM problem if I don't find any problems with the serial data line such as an open or short to ground.
I first tried using my Snap On MT2500 scanner on the car and it would not communicate except for one time. I have not had the scanner very long as it was an Ebay purchase, and I didn't know if it had a problem.
With the bad weather and time issues, I didn't look at the car again until yesterday. I tried two other scanners that I have that have communicated with it in the past with no problems. Both of these would not communicate either. I have the factory service manual and found what lines of the DLC do what. It only has four lines going to it. Pin A is ground to the ECM, B is the diagnostic test line to the ECM, M is the serial data line. The fourth pin is for TCC. The voltage for the diagnostic test line is supposed to be 5 volts which I had 4.96. The voltage for pin M, the serial data line is supposed to be 4.7 volts and I had .006 volts which I assume is my problem. Pin A, the ground, did show 0 volts. These lines apparently go through the instrument cluster connector so this might also be a problem area. The ECM is mounted above the glove compartment behind a plastic panel, so a decent amount of stuff has to be removed to get to it.
The weather will be bad again in the coming days so any more trouble shooting will have to be on hold since I have to work on it outside. I'm going to access the instrument cluster connector before trying to get to the ECM and test there.
All of this work is mainly to see if I can diagnose this communication problem more so than trying to fix the ABS problem as I don't really care if the ABS works and getting parts to fix whatever is wrong is probably impossible. I don't think it's an ECM problem because when I short pins A & B together at the DLC, I do get code 12 to flash continually. It's the only code that shows up. Service information does say though that it could still be an ECM problem if I don't find any problems with the serial data line such as an open or short to ground.
