The air valves that control the flow of air through the A/C module are operated by vacuum actuators. Solenoid valves located in the vacuum/electric solenoid apply vacuum or vent the actuators. The solenoids are controlled by solid state circuits in the heater-A/C control. The MODE button in the heater-A/C control selects the operating mode manually. The AUTO mode button allows the heater-A/C to automatically select the appropriate mode. Depending on the mode, the heater-A/C control determines which air valve should be operated.

Two air temperature sensors are used by the heater-A/C control. Their resistance decreases as temperature increases. The inside air temperature sensor measures the inside air temperature and the ambient outside air temperature sensor measures the outside air temperature. The difference between the two sensor measurements is used by the heater-A/C control to determine whether the incoming air should be cooled or warmed and to what degree in order to bring the inside air temperature to the selected temperature as quickly as possible. If a malfunction should occur in either the inside air or ambient air temperature sensor circuits, the heater-A/C control will set a Diagnostic Trouble Code (DTC). It will also substitute a temperature value of 24°C (75°F) if the inside air temperature sensor circuit is at fault, or 9°C (49°F) if the ambient outside air temperature sensor circuit is at fault. The substitute temperature values allow the A/C system to continue operation.

The sun load temperature sensor is used to control the blower speed and BI-LEVEL modes when the heater-A/C control is in AUTO mode.

The air inlet valve is closed in most of the operating modes permitting outside (fresh) air to enter the A/C module. When selecting the Bi-Lev or RECIRC mode at a temperature setting of 18°C (65°F) or higher, the heater-A/C control energizes the recirc solenoid. This action causes the air inlet valve to open, allowing air from inside the vehicle to recirculate for maximum cooling.

The air temperature valves are positioned by the left and right electric actuators. When the temperature door is in the cold position, air is prevented from passing through the heater core. The door can be placed in any position between cold and warm by the actuator to provide any degree of warming to the air after it has passed through the A/C evaporator core.

The defroster valve and the heater outlet valve are connected together by a mechanical linkage allowing two valves to move together. The defroster valve and the heater outlet valve can be moved to one of three positions. The heater/defroster mode valve vacuum actuator consists of two chambers operated by two vacuum actuators.

When the A/C system is operated in the DEFOG mode, both vacuum chambers are vented and both doors are in position B. Airflow is then directed to both the defroster outlets and heater outlets. When selecting the LOWER mode, the heater-A/C control energizes the heater solenoid, causing the heater/defroster mode valve vacuum actuator to move the defroster valve and the heater outlet valve to position A. In position A, airflow is sent to the heater outlets.

When selecting the FRONT DEFROST mode, the heater-A/C control energizes the defrost solenoid, causing the heater/defroster mode valve vacuum actuator to move the defroster valve and the heater outlet valve to position C. In position C, airflow is directed to the defrost outlets.

When selecting the LOWER, DEFOG or FRONT DEFROST modes, the heater-A/C control energizes the A/C solenoid. This action causes the vent valves to move to position A, and airflow is not sent through the I/P outlets.

When selecting the UPPER mode, the heater-A/C control energizes the bi-level solenoid. This action causes the vent valves to move to position C, and airflow is sent through the I/P outlets.

When selecting the BI-LEVEL mode, the heater-A/C control de-energizes both the bi-level and A/C solenoids. This action causes the vent valve to move to position B (neutral position), and airflow is sent through both the I/P and heater outlets.

The delay mode is used to prevent the vehicle occupant's breath from fogging the windshield when the outside temperature is low and the engine is cold. The heater-A/C control will set the doors so that airflow is directed to the windshield with low blower operation. The system will stay in the delay mode until engine coolant temperature reaches 49°C (120°F) or after four minutes whenever comes first. The heater-A/C control then operate normally in the selected mode.