Since the system is manual, there is very little scan information that would apply. A/C request and A/C pressure sensor data is it. There are a couple bulletins, one of which matches the concern of goes full hot when one click off cold, I will include that in the following. It references replacing the control unit with a specific part number, was that the number that you used? It also states that the control head defaults when feedback voltage is out of range, although no specification on what the range is. Looking at the schematic and reading the description, I would suspect outside being the extreme of either 0 or 5 volts.
#PIT4704A: HVAC Temperature Goes Full Hot When One Click Off Full Cold - keywords A/C AC actuator air compressor door heat heater mode - (Feb 18, 2010)
HVAC Temperature Goes Full Hot When One Click Off Full Cold
2006-2008 Hummer H3
This PI was superseded to update the Recommendation/Instructions. Please discard PIT4704.
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Some customers may complain that the temperature will go to full hot when they turn the temperature control one click off of full cold. The control head is defaulting to Emergency Mode (this provides HOT air for safety reasons which would be available to clear the windshield in Defrost mode). This will happen if the feedback voltage for the air temperature actuators are out of range.
Replace the HVAC control head with part number 25920831.
Wednesday, November 30th, 2011 AT 5:28 AM