Does it blow cool air, or is it always warm? A couple of possibilities are low refrigerant, or an actuator fault in the rear. On a system that is only partially low, the rear unit will be the one most affected, as it is farthest from the pump(compressor). If it is stuck in a position, like full hot, it may be a default action in response to an actuator fault. You could have it checked for a fault code to confirm this. There is a bulletin I have included, it may apply.
#06-01-39-015A: Unable to Control Rear HVAC Temperature or Modes, Intermittent Ticking Noise from Rear HVAC Actuators or Rear HVAC DTCs B0434 or B3764 (Replace Auxiliary HVAC Control Module) - (Jan 12, 2007)
Subject: Unable To Control Rear HVAC Temperature Or Modes, Intermittent Ticking Noise From Rear HVAC Actuators Or Rear HVAC DTCs B0434 Or B3764 (Replace Auxiliary HVAC Control Module)
Models: 2003-2006 Cadillac Escalade Models
2003-2006 Chevrolet Suburban, Tahoe
2003-2006 GMC Yukon Models
with Automatic Control HVAC System and Rear A/C (RPOs CJ2 and C69)
This bulletin is being revised to correct one of the blower motor control module part numbers. Please discard Corporate Bulletin Number 06-01-39-015 (Section 01 -- HVAC).
Some customers may comment on one or more of the following conditions:
An intermittent ticking/clicking noise from the rear Heating, Ventilation and Air Conditioning (HVAC) system.
Unable to control rear HVAC system temperature.
Unable to change rear HVAC system modes.
Technicians may find that Diagnostic Trouble Codes (DTCs) B0434 and/or B3764 have been set.
These conditions may be caused by HVAC actuators that hunt for the correct position. This cycling may cause a clicking or ticking noise.
An overtravel of the HVAC system control doors may cause one or more of the above system conditions. If an overtravel occurs, a DTC will be set, and the door will go to a preset default position.
When a system door defaults, that door will stay at the default position until the DTC is cleared. Clearing the DTC will allow the door to operate properly until the overtravel condition reoccurs. The following table lists the DTCs that may have been set and the door that is associated with that code.
System Door Affected
Auxiliary (rear) Air Temperature Actuator
Auxiliary (rear) Mode Actuator
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Thursday, June 30th, 2011 AT 5:02 AM