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.