Vehicles with heating and air conditioning and vehicles with heat only provide front air delivery and temperature controls. The heater only option's heater control module operates similarly to the HVAC control module except that the A/C pushbutton and temperature dial MAX detent features are not provided for the control of the recirculation door. An auxiliary cooling fan is provided for vehicles with air conditioning and 7.4L gasoline engines. Refer to Engine Cooling Fan Description - Electric in Engine Cooling.

The following are powered by CKT 141 (BRN) when the ignition switch is in the RUN position.

Grounding for the door motors is passes through CKT 150 (BLK) to ground G202. CKT 141 (BRN) powers the HVAC control module when the ignition switch is in the RUN position. Grounding passes through CKT 150 (BLK) to ground G200 for the IP.

The HVAC control module ventilation controls for the front mode door and the recirculation door are disabled when the module front blower switch is in the OFF position. The ventilation controls are enabled when the front blower switch is in the HI, M2, M1, or LO positions. The front temperature door is not dependent on the front blower operations. The front temperature door can be controlled only when the switch is in the RUN position.

The mode doors and the temperature doors are positioned in proportion to the variable voltage signal received from the HVAC control module. The temperature dial at the HVAC control module adjusts a rheostat which provides a voltage signal proportional to the dial position. CKT 733 (LT BLU) carries the voltage to the front temperature door motor. The front temperature door motor responds to the voltage signal. The mode dial at the HVAC control module adjusts a rheostat. The mode dial provides a voltage signal proportional to the dial position. CKT 454 (WHT) carries voltage to the front mode door motor. The front mode door motor responds to the voltage signal.

The recirculation door is a 2-position door. The recirculation door is in the recirculation position only when input CKT 1614 (DK GRN) is grounded through the HVAC control module. The recirculation door is grounded when the auxiliary cooling fan A/C pressure switch operates the auxiliary cooling fan for air conditioned vehicles with 7.4L gasoline engines. Refer to Auxiliary Cooling Fan Circuit Description in Engine Cooling.

The recirculation door is normally in the outside air position but moves to the inside air position when the input wire is grounded. The LED on the HVAC control module is illuminated when the recirculation door is grounded through the HVAC control module.

The rear auxiliary air delivery and the temperature controls provided with the combined rear auxiliary heater and air conditioner option, the C36 + C69, work independently of the ventilation controls used for the front of the vehicle. The front system provides a recirculation door and the door controls. The rear auxiliary system does not have the recirculation door and the door controls. The rear auxiliary mode door and the rear auxiliary temperature door are exclusively controlled from either of the 2 auxiliary HVAC control modules. The front auxiliary HVAC control module blower switch is located above the IP. The front auxiliary HVAC control module blower has a permissive position called REAR. The REAR position enables control from the rear auxiliary HVAC control module. Relay contacts in the auxiliary HVAC control logic module change state in order to enable control input from the ventilation controls when in the REAR position with the ignition switch in the RUN position. The ventilation controls are located at the rear auxiliary HVAC control module. The logic module relay is de-energized when the front auxiliary HVAC control module blower switch is NOT in the REAR position. The logic module relay controls inputs from the front auxiliary HVAC control module that control the doors.

Each control module produces a variable voltage signal to the door motor. Each door motor is powered from CKT 341 (BRN) when the ignition switch is in the RUN position. Door grounding passes through CKT 150 (BLK) to ground G400. The variable voltage signal operates the door motor to a position consistent with the signals voltage. The voltage corresponds to the control dial selection on the selected control module.