2006 year.
Is anyone familiar with the electrical routing from the transmission external barrel connector wiring to the ecm? I have an intermittent short to ground (tech2 shows it very random and intermittent) it’s on the 3-2 shift solenoid P0785 code. Goes into limp mode- clear it with scan tool and it drives fine, until it randomly shorts to ground (P0785) back to limp mode……solenoids have all been replaced in transmission (the replaced 3-2 shift solenoid tested fine with 9v battery outside transmission). ECM replaced. Still same issue… random P0785/limp mode- clear and fine until randomly occurs again. Suspecting the white wire (ground from ecm to 3-2 solenoid) is touching somewhere. I’m sure mechanic can follow the wire harness back, just asking if anyone has had this occur and if there is another connector between the ecm and where it plugs into transmission. Haven’t ruled out the transmission solenoid wiring under the pan…. Have to love these intermittent wiring issues

 

Assuming by "ECM" you are referring to the PCM?

Had a local H2 limo owner with a similar problem, replaced the entire transmission no-less (expensive) was about to hack the wiring, just in time he changed the PCM which fixed the problem.

Sounds like you've changed it, but worth the question.

 

PCM=ECM ? Its the Delphi module with the 2 large connectors attached on side vertically (held in center by 7mm bolt each) low in front of battery that "snaps in pace" about the size of a small book..... yes, just replaced and symptom identical. It has to be in the wiring between the solenoid - to this module.... I believe next step is for the harness to be handled/jiggled while attached to the tech 2 & see if the "short to ground - yes" appears. if the check engine sees it & the tech 2 sees it... its shorting to ground, somewhere. Bought a solenoid harness "in the transmission" and another solenoid since $24, will have the transmission inside wiring ruled out or discovered.. harness $75 and almost 20 years old so cant hurt...

 

Great news I believe, found where the loom had disintegrated. You could see individual wires where the loom had fallen off and I believe is where the wire that goes to the 3-2 solenoid must have been intermittently making contact with ground. For future reference if anyone starts getting transmission code ground errors, follow the harness towards the front left of the truck. You will see high up ,near where the transmission bell housing mates to the engine the 2 transmission cooler lines- right above that is where I believe it was making contact. To really get at the loom to "properly" retape/reloom the left cat/pipe really needs to be removed so you can get your hands up to re-insulate the loom in that area...

As this vehicles age I am guessing more random wiring harness shorts may occur. The plastic loom is so brittle from age & heat it just falls apart in your hand.
For us owners of these now 20 +/- year old Hummers... I strongly suggest buying a Tech 2. Amazing what info they can show for diagnostics...

So, crossing my fingers this was the fault. Have driven start/stopped etc for around 30 around town miles. Has been fine. Since this started, I couldn't drive a couple miles without the fault.

Another note- While reading about this OBD code- I read about several people who in fact replaced transmissions ....only to have the fault occur after the new transmission. I have never experienced a transmission "limp" mode before this issue and I could easily see how one would think the transmission is broken. Limp mode defaults you to basically 3rd gear & you stay in 3rd until you either clear the code or a certain amount of time without the fault passes (I am guessing).
Inside your transmission there are electrical solenoids (easily changeable & inexpensive to purchase). I believe its like 5 solenoids, wiring harness & manual valve position switch that are easily changeable and cost is approx $250 for all that (solenoids being OEM AC Delco and harness being Rostra). So, if you are doing a transmission filter & fluid change - since pan is dropped anyways, the fore mentioned parts are easily renewed. Also, if you do need to do the transmission service you can also buy a pan that has a drain plug in it - Makes it easy if you wish to drain and add new fluid at certain mile intervals for longevity.
see attached pics of the Tech 2 screen when running the transmission function-

 

Chrmer3 great job on the fix, great info, its always good to get feedback and to know what the solution was 👍👍👍

 

This is great information. You saved yourself a bunch of money and maybe some other folks from buying a new transmission.