When the A/C compressor is not running, air flowing through the system can be heated above, but not dehumidified, or cooled below outside air temperature. When the A/C compressor is running, air flowing through the system is cooled, dehumidified, and then heated, as necessary, to bring the vehicle interior to the desired temperature.

System air is heated by passing it through the heater core. An air mix valve, located inside the heater and evaporator module assembly, determines how much air passes around the heater core and how much air is forced through the core. When all air is forced through the heater core, maximum heating is obtained. When all air is routed around the heater core, no heating is obtained.

The air mix valve is operated by the air mix valve actuator, which is mounted on the heater and evaporator module assembly. The IPM uses information from the control assembly and its network of sensors to determine the appropriate air mix valve position. The IPM then powers the actuator motor to run in the direction necessary to move the air mix valve to that position.

The heater and A/C temperature sensors are used to monitor discharge temperatures from the heater and A/C outlets. These sensors are located in the left and right heater and A/C ducts. The IPM monitors the discharge temperature sensors, and uses this information to help determine command signals.