they have had a lot of problems with the vacuum switches on the transfer case itself, i will give something from gm, read this it will help.please click on the diagrams thanks
Service Information 2003 Chevrolet Blazer - 4WD
| Blazer, S10 Pickup, Jimmy, Sonoma (VIN S/T) Service Manual
| Bulletins By Category
| Document ID:(NNN) NNN-NNNN
#03-01-38-009: HVAC Mode Switch Inoperative or Stuck in Vent Mode, Oil in HVAC Control Head and/or Vacuum Lines (Replace Transfer Case Vacuum Switch and Contaminated HVAC Components) - (Jun 25, 2003)
|Subject:||HVAC Mode Switch Inoperative or Stuck in Vent Mode, Oil in HVAC Control Head and/or Vacuum Lines (Replace Transfer Case Vacuum Switch and Contaminated HVAC Components)|
|Models:||2001-2003 Chevrolet Blazer, S10|
|2001-2003 GMC Jimmy (Export Canada Only)|
|with 4WD and Two-Speed Electric Shift Transfer Case (RPO NP1)|
Some customers may comment that the HVAC mode switch may be inoperative or stuck in the vent mode. After some diagnosis, oil in the HVAC control head and/or vacuum lines may be found.
When the transmission or transfer case heat up, the oil may expand and find its way into the transfer case vacuum switch. This oil expulsion may cause the vacuum switch seal to fail and allow transfer case oil to be drawn into the HVAC vacuum lines.
Replace the transfer case vacuum switch. Refer to the Transfer Case Vacuum Switch Replacement procedure in the Transfer Case sub-section of the Service Manual. Also, replace all contaminated parts in the HVAC system.
Switch -Transfer Case Vacuum
Parts are currently available from GMSPO.
For vehicles repaired under warranty, use:
Switch, Vacuum Control Transfer Case - Replace
With Skid Plate
Any additional time for replacement of HVAC components must be submitted as Other Labor Hours and requires appropriate authorization.
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.
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© 2008 General Motors Corporation. All rights reserved. this is also a heating issue fix. this switch cause's 2 problems, also check for proper vacum at the switch