My 2008 Hummer H2 has a transmission issue that baffles competent, professional mechanics in my locale. A 2008 H2 has a GM 6L80-E transmission. My H2 had 108,000 miles when I bought it. Presently, it has 189,000 miles. With the exception of the current issue, the transmission has operated flawlessly. Within the past few weeks, the transmission began to infrequently "stick" in low gear. On those occasions, I shut down the engine, wait a minute or so, restart the engine and the transmission again works flawlessly. Codes recorded for the transmission are: PO700, PO752, PO842, PO872, and PO877. I took the H2 to a transmission shop recommended by my mechanic, a man of proven expertise and impeccable reputation. I've enjoyed a business relationship with him for forty-two years and trust him. The owner of the transmission shop is also a man of excellent reputation. He drove the Hummer H2 for a week during which the problem never presented. I asked about the codes and he explained that a code does not identify the specific cause of a transmission problem but points to an area requiring further diagnosis. He further explained that the problem may not be in the transmission itself but among other things, it may stem from external, related components that can affect the transmission's performance. He suggested that I continue to drive the vehicle and return it to him if and when the problem began to frequently present itself. Has anyone experienced this issue and if so, was the problem identified and corrected? If so, please tell me where the specific problem was found.

 

Are those the only codes present? It could be a wiring issue, but more likely the tehcm inside the transmission. Hard to diagnose being so intermittent. A common problem with these transmissions is the pressure switches and sometimes wiring within the tehcm.

First thing I would do is inspect the connector on the transmission for corrosion and dirt and clean it. Then I would inspect the harness as best you can for broken or frayed wires. Load test powers and grounds with a headlight bulb.

If you don't find anything, drop the pan and drain the fluid. If there's a lot of clutch material and/or metal, you need to just have the trans rebuilt. 100k+ miles is pretty much end of life for an early 6l80 in our heavy trucks. Normally the 456 clutch or torque converter blows out by then.

There's really no cheap fix for codes like this if your powers and grounds check out.
The computer/logic that sets these codes is inside the trans itself. The only thing the truck side wiring and connector on the trans contains is 12v power/ground, speed signal, park/neutral/reverse safety signal, and the 2 canbus signals.

Essentially what I'm saying is if you can verify power and ground integrity to the trans, your issue is inside it. You could try and put a reman tehcm in it, and I'll give you the link to the correct one for our trucks, but that is a 50/50 shot. We have no way to diagnose them. Trans rebuilder should be replacing the techm with rebuild anyway, so they might discount you since you already have a new one.

TEHCM - https://transend.us/products/transmi...tehcm/t104420b
Again. Only replace this if your powers and grounds are good, fluid is CLEAN, and the transmission shifts good. Verify your oem part number with the ones in the description.

You will also need this when dropping the valve body
GM:24236927