94 Chevrolet C3500 5.7 TBI

Vehicle has poor idle quality, would not stay running when first troubleshooting, rich condition present. Cleaned grounds to eliminate injectors flooding engine, poor idle quality still present. Service engine light does not prove out and unable to communicate with PCM. I’m sure I have more than one issue but I need to establish communication first. I’ve verified power and ground to PCM and need to know what direction to go next. Don’t want to just throw a module at it. Wondering also if I have a bad O2 sensor as well. Customer has shotgunned this thing with no results. How should I proceed with scope testing? 

Does the CEL illuminate during the bulb check, KOEO?

How should I proceed with scope testing?

 

Pin M is Serial Data. You should see an active 0v to 5v square wave. If you do not see a square wave, re-check the all of the powers and grounds to the ECM and verify the integrity of the Serial Data line that runs from Pin M of the DLC to the PCM, Pin F9.
 

Also, shorting pins A and B together with a paper clip should enter a diagnostic mode that will flash codes via the CEL.

Turn Key OFF

1. Use a paperclip or jumper wire to short pin A (ground) to pin B (diagnostic enable)
2. Turn Key ON (engine off)
3. The "Check Engine" light will begin to flash:
   • Code 12 = 1 flash, pause, 2 flashes — repeated three times. This indicates the ECM is working.
   • After three repetitions of Code 12, any stored DTCs will flash (also repeated 3x each).
   • When finished, Code 12 will flash again.