HVAC - Service Information: Operation

WARNING: This page is about a different car, the 2006 Dodge Magnum, 2006 Dodge Charger, and 2006 Chrysler 300. However, it is still accessible from the selected car via links, so may be relevant.

Both the manual temperature control (MTC) and the automatic temperature control (ATC ) heating-A/C system used in this vehicle are blend-air type systems. In a blend-air system, a blend-air door controls the amount of conditioned air that is allowed to flow through, or around, the heater core. In the available dual zone system, two blend-air doors are used to provide completely independent side-to-side temperature control of the discharge air. The temperature control(s) determines the discharge air temperature(s) by operating the blend door actuator(s), which move the blend-air door(s). This design allows almost immediate control of output air temperature(s).

Fig 1: Typical Blend-Air HVAC System

Courtesy of DAIMLERCHRYSLER CORP.

The heating-A/C system pulls outside (ambient) air through the cowl opening at the base of the windshield, then into the air inlet housing and through the A/C evaporator (3). Air flow can be directed either through or around the heater core (1). This is done by adjusting the blend-air door(s) (2) with the temperature control(s) located on the A/C-heater control in the instrument panel. The air flow can then be directed from the panel, floor and defrost outlets in various combinations using the mode control located on the A/C-heater control. Air flow velocity can be adjusted with the blower speed control located on the A/C-heater control.

The outside (fresh) air intake can be shut off by selecting the Recirculation Mode with the mode control. This will operate an electrically actuated recirculation-air door (4) that closes off the fresh air intake and recirculates the air that is already inside the vehicle.

The A/C compressor can be engaged in any mode by pressing the snowflake, A/C on/off button. It can also be engaged by placing the mode control in the mix to defrost positions. This will remove heat and humidity from the air before it is directed through or around the heater core. The mode control on the A/C-heater control is used to also direct the conditioned air to the selected system outlets. The mode control uses an electric actuator to control the mode-air doors (5 and 6).

The two slot-type defroster outlets receive airflow from the HVAC housing through the molded plastic defroster ducts, which connect to the HVAC housing defroster outlets. The airflow from the defroster outlets is directed by fixed vanes in the defroster outlet grilles and cannot be adjusted. The defroster outlet grilles are integral to the instrument panel top cover.

The side window demister outlets receive airflow from the HVAC housing through the molded plastic demister ducts. The demisters direct air from the HVAC housing through the outlets located on the top corners of the instrument panel. The airflow from the side window demister outlets is directed by fixed vanes in the demister outlet grilles and cannot be adjusted. The side window demister outlet grilles are serviceable from the instrument panel. The demisters operate when the controls are set in Heat, Bi-level, Mix and Defrost modes.

The four instrument panel outlets receive airflow from the HVAC housing through two molded plastic main panel ducts. One duct directs air flow out of the right side instrument panel outlets, while the other duct delivers air flow to the left side outlets. Each of these outlets can be individually adjusted to direct the flow of air.

The floor outlets receive airflow from the HVAC housing through the floor distribution ducts which are integral to the rear cover of the HVAC air distribution housing. Two plastic rear distribution ducts and one center console duct attach to the rear cover and provide conditioned air to the rear seating positions. The two console outlets can be individually adjusted to direct the flow of air, but the floor outlets cannot be adjusted.

NOTE: It is important to keep the HVAC air intake opening clear of debris. Leaf particles and other debris that is small enough to pass through the cowl opening screen can accumulate within the HVAC housing. The closed, warm, damp and dark environment created within the housing is ideal for the growth of certain molds, mildews and other fungi. Any accumulation of decaying plant matter provides an additional food source for fungal spores, which enter the housing with the fresh intake-air. Excess debris, as well as objectionable odors created by decaying plant matter and growing fungi can be discharged into the passenger compartment during heater-A/C operation if the air intake opening is not kept clear of debris.

The A/C system is designed for the use of non-CFC, R-134a refrigerant and uses an A/C expansion valve to meter the flow of refrigerant to the A/C evaporator. The A/C evaporator cools and dehumidifies the incoming air prior to blending it with the heated air. To maintain minimum evaporator temperatures and prevent evaporator freezing, an evaporator temperature sensor is used. The sensor is located downstream of the evaporator and supplies an evaporator temperature signal to the A/C-heater control. For the ATC system, the A/C-heater control broadcasts the A/C request on the controller area network (CAN) B bus, where it is read and processed by the front control module (FCM), which in turn broadcasts it on the CAN C bus, where it is read and processed by the powertrain control module (PCM). For the MTC system, the A/C-heater control sends the request for A/C to the CCN via a dedicated mux circuit. The CCN then broadcasts the A/C request on the CAN B bus, where it is read and processed by the FCM, which in turn broadcasts it on the CAN C bus, where it is read and processed by the PCM.