Hi my name is Hal.
I will work as quickly as I can to help you. I will need more information about your concern to help identify you root cause to this concern.
Is this a recent concern, or has this been progressively getting worse?
Can you here a hissing noise near the A/C control while the engine is running?
If yes, does it change at all when you try to move the selector, to different modes?
You will need to remove the heater control assembly. Disconnect the vacuum hose connection and inspect for oil in the line. If found, then check the vacuum mode door actuators for oil to. This was a common problem on these year models. If oil is found in any vacuum line to the control head, the control head and actuator s will need to be replaced.
The oil comes from a failed tranfercase vacuum switch, pulling transfercase oil up and into the vacuum hoses. there was a preliminary service bulletin on this concern, PIT 3153-A.
Here is part of that information. if you need more help just let me know.
HVAC mode switch is inoperative or is stuck in vent mode. After some diagnosis, oil in HVAC control head and/or vacuum lines may be discovered.
Replace the transfer case vacuum switch and all contaminated parts in HVAC system, inspect the following parts for oil contamination: Vacuum Tank/Canister, HVAC control head, all Vacuum lines, and Vacuum actuators, Vacuum check valve.
The transfer case vacuum switch is located on the transfer case.
The following should also be inspected:
If any of the above conditions exist, please repair as necessary.
Any of the three conditions listed above could cause the transfer case vacuum switch seal to fail. Any of these three conditions may cause pressure to build in the transmission or transfer case. When the transmission or transfer case heat up, the oil expands and finds its way into the vacuum switch. This oil expulsion causes the vacuum switch seal to fail and allows transfer case oil to be drawn into the HVAC vacuum lines. The Transfer Case Vacuum Switch has been revised. As of 08/04/04, this new switch has been used on production vehicles built at the Linden plant. The revised switch part number is XXXXX below
Switch - Transfer Case Vacuum
Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
(2001-2003 Chevrolet Blazer, S10) and (2001-2003 GMC Sonoma) and (2001 GMC Envoy, Jimmy) and (2001-2003 GMC Jimmy - Export Canada Only) (with 4WD and Two-Speed Electric Shift Transfer Case (RPO NP1))
Did you inspect the vac line to the mode vac switch behind the dash? Also inspect the mode and defrost door actuators? Is there vac to the switch?
If the mode vac switch and actuators check good, then it is possible for the defrost door or mode door to be broken or stuck from something getting down the defroster duct.
This will involve disassembling the and removing the HVAC case. The doors are plastic and do break. You may be able to inspect with a video bore scope if you have acess to one.
Here is a picture of the HVAC case. look on the left side and you can see the actuators for the mode and defrost doors.
The second picture, shows the mode and defrost doors with the upper case removed.
If the door is broken you will experience no defrost. This system does not use voltage to switch the mode doors. It only uses vacuum to switch the doors . Temp is controlled by an electric actuator. ( located on top of the rt side of case)
If you need more information please let me know.
You are welcome. Let me know if I can Help in the future.