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Unfortunately, the Trailblazer has a completely different set of HVAC controls than the old style S-trucks which used the vacuum as described above.

Trailblazers use all electric actuators for temperature and mode controls- and they are controlled by the control head through data lines and feedback controls- and require periodic calibrations done automatically.

Do you hear clicks coming from out under the dash? a sign of a broken actuator or actuator door. Does your dash temp display flash? If so, you have a HVAC code set and it needs to be read for help in diagnosis.

Here is a description of your mode door system.

Air Distribution
The HVAC control module controls the distribution of air by the use of a defrost actuator and a mode actuator. The
modes that may be selected are:

Defrost

Defog

Panel

BI-Level

Floor
The mode and defrost actuators are connected to the mode and defrost doors by a cam type linkage system. Depending on the position of the door, air is directed through the HVAC module and distributed through various
ducts leading to the outlets in the dash. If the HVAC control module detects a fault with the mode or defrost doors the HVAC control module will try to drive the actuator for a predetermined amount of time, to defrost, which is
the defaulted position for the mode and defrost door actuators. When the mode switch is placed in the defrost or defog positions the A/C is commanded on and the recirculation door is moved to the outside air position to help
reduce window fogging. A/C is available in all modes and recirculation is only available in the panel and bi-level modes