Heating and Air Conditioning: Description and Operation

HVAC

DESCRIPTION

NOTE: LHD model shown in illustration. RHD model similar.

A manual temperature control (MTC) single zone type heating-A/C system, MTC front and rear heating-A/C system or a fully adjustable automatic temperature control (ATC) dual zone type heating-A/C system is available on this vehicle.

To maintain the performance level of the heating, ventilation and air conditioning (HVAC) system, the engine cooling system must be properly maintained. The use of a bug screen is not recommended. Any obstructions in front of the radiator or A/C condenser will reduce the performance of the A/C and engine cooling systems.

The engine cooling system includes the radiator, thermostat, radiator hoses and the engine coolant pump. Refer to Cooling system before opening or attempting any service to the engine cooling system.

FRONT HEATING AND A/C SYSTEM




All vehicles are equipped with a common heater, ventilation and air conditioning (HVAC) housing (1). The heating-A/C system combines A/C, heating, and ventilating capabilities in a single HVAC housing mounted within the passenger compartment beneath the instrument panel. The HVAC housing includes:
- Recirculation-air door and actuator (2)
- Blower motor (3)
- Blower motor resistor (MTC) or blower motor power module (ATC) (4)
- Blend-air door(s) and actuator(s) (5)
- Heater core (6)
- Mode-air doors and actuator (7)
- Evaporator temperature sensor (8)
- A/C evaporator (9)

Based upon the system and selected mode, conditioned air can exit the HVAC housing through one or a combination of the three main housing outlets: defrost, panel or floor. The defrost and panel outlets are located on the top of the HVAC housing and the floor outlets are located at the rear of the HVAC housing. Once the conditioned air exits the HVAC housing, it is further directed through molded plastic ducts to the outlets within the vehicle interior. These outlets and their locations are as follows:
- Defroster Outlets - Two large defroster outlets are located near the center of the instrument panel top cover, near the base of the windshield.
- Side Window Demister Outlets - There are two side window demister outlets, one is located at each outboard end of the instrument panel top cover, near the belt line at the A-pillars.
- Panel Outlets - There are four panel outlets in the instrument panel, one located near each outboard end of the instrument panel facing the rear of the vehicle and one located on each side of the instrument panel center bezel.
- Floor Outlets - There is one floor outlet located above each side of the floor panel center tunnel near the dash panel. There is also one outlet located under each front seat.
- Console Outlets - There are two console outlets located at the back of the center floor console facing the rear of the vehicle.

REAR HEATING AND A/C SYSTEM




The manual temperature control (MTC) front and rear heating-A/C system is equipped with a rear heater and A/C housing (1) that combines A/C and heating capabilities into a single unit mounted within the passenger compartment behind the right quarter interior trim panel. The rear HVAC housing includes:
- Blower motor resistor (2)
- Blower motor (3)
- Mode door and actuator (4)
- Wire harness (5)
- Heater core (6)
- Blend door and actuator (7)
- A/C evaporator (8)
- A/C expansion valve (9)

Based upon the rear temperature selected, conditioned air can exit the rear HVAC housing through one or a combination of the two outlets at the top of the rear HVAC housing. Once the conditioned air exits the rear HVAC housing, it is further directed through molded plastic ducts to the air outlets located in the rear of the passenger compartment. These air outlets and their locations are as follows:
- Upper Air Outlets - There are two air outlets located in each interior quarter trim panel.
- Lower (floor) Air Outlet - There is one lower outlet located in the right quarter interior trim panel located behind the right rear door area.

OPERATION
The standard front and the optional rear heating-A/C systems 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 front 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 allows an almost immediate control of the output air temperature of the systems. Both the front and rear A/C systems are designed for the use of non-CFC, R-134a refrigerant and use an A/C evaporator to cool and dehumidify the incoming air prior to blending it with the heated air.

FRONT SYSTEM




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 serviceable from 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 not 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 connected to the rear of the HVAC air distribution housing. Two plastic rear distribution ducts and one center console duct attach to the rear of the air distribution housing 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.

This A/C system uses an A/C expansion valve (TXV) to meter the flow of refrigerant to the A/C evaporator. To maintain minimum evaporator temperature 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.

REAR SYSTEM
The manual rear heating-A/C system pulls interior air through the opening in the rear quarter interior trim panel and into the rear blower motor in the rear HVAC housing, then through the rear A/C evaporator. Air flow can be directed either through or around the rear heater core. This is done by adjusting the blend-air door with either the front or the rear temperature control (depending on the front A/C-heater control setting). The air flow is then automatically directed to the floor or upper air outlets depending on the conditioned air temperature selected by the rear mode-air door. Air flow velocity can be adjusted with the blower speed control located on either the front A/C-heater control or the rear A/C-heater control (depending on the front A/C-heater control setting).

The four upper air outlets receive airflow from the rear HVAC housing through molded plastic ducts, which connect to the air outlet located in each interior quarter trim panel. Airflow direction from the upper air outlets can be adjusted by moveable grilles which can be serviced separately.

The rear floor outlet receives airflow from the rear HVAC housing through the molded plastic floor distribution duct, which connects to the rear floor outlet located in the right interior quarter trim panel. The airflow from the rear floor outlet is directed by fixed vanes in the outlet and air flow direction cannot be adjusted. The rear floor outlet is integral to the right interior quarter trim panel.

This rear A/C system uses an "H" valve-type thermal expansion valve (TXV) to meter the flow of refrigerant to the rear A/C evaporator.