The air doors 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 to the actuators or vent the actuators. The solenoids are controlled by solid state circuits in the heater-A/C control. Push buttons in the heater-A/C control determine the A/C operating mode. Depending on the mode selected, the heater-A/C control determines which air doors should be operated.

Two air temperature sensors are used by the heater-A/C control. Their resistance decreases as the temperature increases. The inside air temperature sensor measures the inside air temperature. The ambient outside temperature sensor measures the outside air temperature. The difference between the two sensor measurements is used by the control circuits in order to determine whether the incoming air should be cooled or warmed, and to what degree. This is determined 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 outside temperature sensor circuits, the heater-A/C control will set a trouble code. It will also substitute a temperature value of 75°F (24°C) if the inside air temperature sensor circuit is at fault or 49°F (9°C) if the ambient outside temperature sensor circuit is at fault. The substitute temperature value(s) allow the system to continue operations.

The sun load temperature sensor determines the amount of sunlight produced by the sun. As the sunlight increases, the sensors current increases. This sensor is used in order to program the blower speed and the auto/bi-level modes.

The air inlet valve is closed in most of the operating modes, permitting outside air to enter the A/C module. It is opened only in the A/C modes and at a temperature setting of 65°F (18°C) or higher. When the air inlet valve is open, air from inside the vehicle is recirculated in order to get maximum cooling.

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

The defroster valve can be moved to 1 of 3 positions. The defroster valve actuator consists of 2 chambers operated by 2  vacuum actuators. When the A/C system is operated in the bi-level of the DEFOG mode, neither vacuum chamber is evacuated and the door is in position B. Air flow is then directed to both the defrost outlets and heater outlets. If the heater solenoid is activated, the actuator moves the valve to position C. Position C directs air flow to the heater outlets (lower mode). If the defrost solenoid is activated, the valve moves to the defrost outlets. This is the position the valve is in when operating the defrost mode.

The inner and outer mode valve is in position B when the air conditioning is activated, or when the bi-level or upper modes are selected. In this position, air flow is directed out of the A/C outlets. If the OFF, LOWER, DEFROST, or DEFOG mode is selected, the bi-level solenoid is de-energized. This will vent the vacuum from the inner and outer mode valve actuator, allowing the door to move to position A. The air flow will now be directed toward the heater and defrost outlets.

The outer mode valve performs the following functions:

The outer mode valve is in position B when the system is in the maximum A/C or normal A/C modes. The outer mode valve is in position A in all other modes.

The delay mode is used in order to prevent the vehicle occupants 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 air flow is directed to the windshield with low blower operation. The system will stay in the delay mode until the engine coolant temperature reaches 49°C (120°F) or after 4 minutes, whichever comes first. The system will then operate normally in the selected mode.