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:
- Verify that transmission and transfer case vents are working properly and are not clogged.
- Visually inspect for transmission to transfer case seal damage.
- Adaptor plate that mounts the transfer case to the transmission mounted incorrectly.
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
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))