Electrical Issues

[k4ben]

I have a 2006 GMC Sierra 2WD 4.3L with electrical issues. A scan tool will not communicate with the ECM. I have a single line oscilloscope with which I see all the modules except ABS talking to the ECM. 
I have discovered that terminals 4 & 5 of the DLC have resistance. I have gone to the trouble of removing the dash in order to check the ground termination points. I cleaned them and re-attached the grounds. There was no difference with the readings, both had the original values, (4 ohms and 29 ohms).
I don't see any damage to the wiring harness but I have noticed a water leak that seems to be near the pillar/windshield on the driver side. It does not seem that the water penetrated the harness.
How should I proceed? 

[Jsdirt]

Do a resistance check between pins 6 & 14 (CAN high and low) - should be 60 ohms. If 120 ohms, there's probably an open in the comm lines somewhere.

 

If that checks out, the first thing I'd be doing is assuming that water DID penetrate anything in that area, and focus my attention there. If it is indeed good there, do a visual of all wiring under the hood and along the frame, looking for anything obvious. Keep the scan tool connected and wiggle sections of the harness. If it suddenly starts working you know where to focus your attention. 

 

If that is all ok, then I'd check ALL powers and grounds to the ECM. Test them with a halogen bulb to be sure they can each carry that load individually. Then check all the pins to make sure they're clean, not GREEN, and that none are bent, since you'll have it unplugged anyway to do the test.

 

If that checks out, check ALL grounding points. Start with the battery cable and move out from there. 

[k4ben]

Thanks- Spent some time examining the harness for water intrusion, nothing so far and no obvious damage.  It's older, only pins 2,4,5 & 16 to look at. I'll look at ECM power & grounds next.