1998 gmc c7500 no communication

THE OBD2 CONNECTOR IN THIS TRUCK HAS PINS AT 1,4,5,6,12 AND 16. PINS 4 AND 5 HAVE GROUND AND PIN 16 POWER. PIN 6 WITH KEY ON IS A STEADY 5V. PIN 1 IS A 3.5 TO 0 VOLT VARYING SQUARE WAVE THAT LOOKS LIKE A DATA LINE. PIN 12 IS A 12 TO 0 VOLT SQUARE WAVE FORM. I WAS THINKING GM USED PIN 2 AS THE DATA LIINE AND WAS A VPM. BUT THIS HAS NO PIN THERE. I HAVE TRIED 5 SCANNERS AND NOTHING WILL COMMUNICATE. TRUCK RUNS LIKE IT IS MISSING PCM INPUTS AS IT IS OVER FUELING. 7.0 CHEVY MOTOR. I CAN WATCH SOMETHING TELLING THE INJECTORS TO GO WIDE OPEN. I JUST WANT TO MAKE SURE BEFORE GOING DOWN A RABBIT HOLE THAT THIS DLC CONNECTOR IS PINNED OUT RIGHT. IM THINKING I HAVE A PCM ISSUE BUT WE WORK ON VERY FEW OF THESE TRUCKS.

SHREP, I've attached the wiring diagrams that I could find for this truck. Couldn't find much, as I'm sure you've found out.

 

 

Normally I'd agree that pin 2 should be the Class 2 network on this year of GM. But not this one! You can disregard DLC pin #12, as that's only for EBCM communcation. Pin #1 definitely looks like it should be communicating data.

Pin #6 is... an enable circuit? Not really sure what you're supposed to see on that one. Does pin #6 change from 5V if you have the scan tool connected? I would think the scan tool would have some impact on that circuit.

How are the DLC pins themselves? If any of the pins are loose or spread, it won't matter how many scan tools you try.

How are the DLC pins themselves? If any of the pins are loose or spread, it won't matter how many scan tools you try.

That's a very good point. I went down a rabbit hole on an Astro van a few years back that only would not communicate with inspection station machines but would communicate with my scanner. That was my first go around with spread pins in the DLC.