Heating and Air Conditioning: Description and Operation
HEATER AND AIR CONDITIONING SYSTEMBoth the heater and the heater/air conditioning systems share many of the same components. This information will deal with both systems together when component function is common, and separately when they are not.
For proper operation of the instrument panel controls, refer to the Owner's Manual provided with the vehicle.
All vehicles are equipped with a common A/C-heater unit housing assembly. When the vehicle has only a heater system, the evaporator and recirculating air door are omitted.
An optional zone control HVAC unit is available. This unit has dual blend-air doors that can be regulated independently of each other. The temperature setting can be different from driver's side to passenger side. There is also a rear (aux.) heating and A/C system available when the vehicle is equipped with zone control.
The air conditioning system is designed to remove heat and humidity from the air entering the passenger compartment. The evaporator, located in the heater A/C unit, is cooled to temperatures near the freezing point. As warm damp air passes over the fins in the evaporator, moisture in the air condenses to water, dehumidifying the air. Condensation on the evaporator fins reduces the evaporators ability to absorb heat. During periods of high heat and humidity, an air conditioning system will be less effective. With the instrument control set to RECIRC, only air from the passenger compartment passes through the evaporator. As the passenger compartment air dehumidifies. A/C performance levels rise.
A/C PRESSURE TRANSDUCER
A/C Pressure Switch:
The A/C Pressure Transducer monitors the refrigerant gas pressure on the high side of the system. The transducer is located on the liquid line.
The Powertrain Control Module (PCM) monitors the A/C compressor discharge (high side) pressure through the air conditioning pressure transducer. The transducer supplies an input to the PCM. The PCM engages the A/C compressor clutch if pressure is sufficient for A/C system operation.
A/C SERVICE PORTS
Valve Service Ports:
The A/C service port valve cores are located on the A/C refrigerant lines.
The service ports are used to charge, recover/recycle, evacuate, and test the air conditioning refrigerant system. Unique service port coupler sizes are used on the R-134a system, to ensure that the system is not accidentally contaminated by the use of the wrong refrigerant (R-12), or refrigerant service equipment.
The High Side (Discharge) valve service port is located on the liquid line near the right frame rail. The Low Side (Suction) valve service port is located on the suction line near the compressor.
The High Side service port is a two piece port and is serviceable. The Low Side service port is not serviceable, the suction line would have to be replaced. After servicing the refrigerant system, always re-install the service port caps.
COMPRESSOR HIGH-PRESSURE RELIEF VALVE
The High Pressure Relief Valve prevents damage to the air conditioning system if excessive pressure develops. Excessive pressure can be caused by condenser air flow blockage, refrigerant overcharge, or air and moisture in the system.
The high pressure relief valve vents only a small amount of refrigerant necessary to reduce system pressure and then reseats itself. The majority of the refrigerant is conserved in the system. The valve is calibrated to vent at a pressure of 3450 to 4140 kPa (500 to 600 psi). If a valve has vented a small amount of refrigerant, it does not necessarily mean the valve is defective.
The High Pressure Relief Valve is located on the compressor manifold at the discharge passage.
NOTE: Special effort must be used to keep all R-134a system components moisture-free. Moisture in the oil is very difficult to remove and will cause a reliability problem with the compressor.
CONDENSATE DRAIN
Condensation from the evaporator housing is drained by a rubber tube through the cowl panel and on to the ground. This tube must be kept open to prevent water from collecting in the bottom of the housing.
The tapered end of the drain tube is designed to keep contaminants from entering the Heater A/C unit housing. If the tube is pinched or blocked, condensate cannot drain, causing water to back up and spill into the passenger compartment. It is normal to see condensate drainage below the vehicle. If the tube is damaged, it should be replaced.
ENGINE COOLING SYSTEM REQUIREMENTS
To maintain ample temperature levels from the heating-A/C system, the cooling system must be in proper working order.
The use of a bug screen is not recommended. Any obstructions forward of the condenser can reduce the effectiveness of the air conditioning system.
EVAPORATOR PROBE
The evaporator probe is located in the HVAC unit housing and inserted into the evaporator fins. The evaporator probe uses a thermistor probe in a capillary tube. The probe prevents evaporator freeze-up.
As the temperature of the evaporator increases, the resistance of the sensor decreases. As the resistance of the sensor changes, the voltage monitored by the Control Module also changes. The monitored voltage indicates to the Control Module the temperature of the evaporator.
HANDLING TUBING AND FITTINGS
Kinks in the refrigerant tubing or sharp bends in the refrigerant hose lines will greatly reduce the capacity of the entire system.
CAUTION: The system must be completely empty before opening any fitting or connection in the refrigeration system. Open fittings with caution even after the system has been emptied. If any pressure is noticed as a fitting is loosened, retighten fitting and evacuate the system again.
A good rule for the flexible hose lines is to keep the radius of all bends at least 10 times the diameter of the hose. Sharper bends will reduce the flow of refrigerant. The flexible hose lines should be routed. For they are at least 3 inches (80 mm) from the exhaust manifold. Inspect all flexible hose lines to make sure they are in good condition and properly routed.
The use of correct wrenches when making connections is very important. Improper wrenches or improper use of wrenches can damage the fittings.
The internal parts of the A/C system will remain stable as long as moisture-free refrigerant and refrigerant oil is used. Abnormal amounts of dirt, moisture or air can upset the chemical stability. This may cause operational troubles or even serious damage if present in more than very small quantities.
When opening a refrigeration system, have everything you will need to repair the system ready. This will minimize the amount of time the system must be opened. Cap or plug all lines and fittings as soon as they are opened. This will help prevent the entrance of dirt and moisture. All new lines and components should be capped or sealed until they are ready to be used.
All tools, including the refrigerant dispensing manifold, the manifold gauge set, and test hoses should be kept clean and dry.
High pressures are produced in the system when it is operating. Extreme care must be exercised to make sure that all connections are pressure tight. Dirt and moisture can enter the system when it is opened for repair or replacement of lines or components. The refrigerant oil will absorb moisture readily out of the air. This moisture will convert into acids within a closed system.
HVAC CONTROL MODULE
HVAC Control Module:
The HVAC control module regulates the operation of the various actuator motors. The actuator motors are used to move the mode, blend- air, and RECIRC. doors.
The control module is included in the A/C control head located on the instrument panel. The control head includes the blower speed switch, rear wiper and washer operation, front & rear window defogger,recirculation door operation, and A/C compressor operation if equipped. Refer to Instrument Panel Systems for Removal and Installation.
NOTE: The RECIRC. function on the HVAC control module automatically defaults to the OFF position after a ignition key cycle. To reactivate the RECIRC. function, the RECIRC. button must be repressed.
The rear blower speed switch is serviced separately from the control head.
A/C RECIRCULATION PROGRAMMING
The recirculation control is programmed to cancel the recirculation mode when the ignition key is turned OFF and will reset to outside air mode when the ignition key is turned ON. The frequent use of outside air will help keep odors from building up within the air conditioner-heater housing. It is recommended that the recirculation mode be used as little as possible, especially in humid climates.
For hot and dry climates, or people who are allergic to pollen and find frequent use of the recirculation mode necessary, the recirculation control can be programmed to not automatically reset to the outside mode by using the following procedures:
- Start the engine and turn the front blower switch to any ON position
- Set the mode control to the panel-face mode.
- Turn the ignition key to the OFF position.
- Depress and hold in the recirculation button.
- Start the engine, and continue to hold in the recirculation button.
- The recirculation indicator light will flash repeatedly, when the light stops flashing, release the recirculation button and the programming is complete.
If the recirculation indicator light remains lit when the recirculation button is released, the recirculation mode will not reset when the engine is started. If the recirculation indicator light went out when the recirculation button was released, the recirculation mode will reset to the outside air mode when the engine is started. The programming procedures can be repeated to change back and forth between the automatic reset and the maintain selection preference.
BLEND/PASSENGER ACTUATOR
The door actuators are 12-volt electric servo motors which move the doors in two directions. The HVAC Control Module can reverse polarity to change the direction of the motors.
The Blend/Passenger Actuator can move the temperature door in two directions. When the voltage at Pin 12 of the control module is high, about 11.5 volts, and the voltage at Pin 17 is low, about 1.5 volts, the door will move towards the Heat position. When Pin 17 is High and Pin 12 is Low the door will move towards the Cold position. When both Pins are high or both Pins are low, the actuator will not move. The Blend/Passenger feedback signal is a voltage signal that is supplied by the actuator to the control. The signal will be about 4.0 volts in the Heat position and 1.0 volt in the Cold position. As the position of the Blend/Passenger actuator changes, so will the feedback signal. The feedback signal is necessary for the correct positioning of the temperature door.
DRIVER ACTUATOR
The door actuators are 12-volt electric servo motors which move the doors in two directions. The HVAC Control Module can reverse polarity to change the direction of the motors.
The Driver Actuator can move the temperature door in two directions. When the voltage at Pin 15 of the control module is high, about 11.5 volts, and the voltage at Pin 13 is low, about 1.5 volts the door will move towards the Cold position. When Pin 13 is High and Pin 15 is Low the door will move towards the Heat position. When both Pins are high or when both Pins are low, the actuator will not move. The Driver feedback signal is a voltage signal that is supplied by the actuator to the control. The signal will be about 4.0 volts in the Heat position and 1.0 volt in the Cold position. As the position of the Driver Actuator changes, so will the feedback signal. The feedback signal is necessary for the correct positioning of the temperature door.
MODE ACTUATOR
The door actuators are 12-volt electric servo motors which move the doors in two directions. The HVAC Control Module can reverse polarity to change the direction of the motors.
The Mode actuator can move the mode door in two directions. When the voltage at Pin 18 of the control module is high, about 11.5 volts, and the voltage at Pin 12 is low, about 1.5 volts the door will move towards the Panel position. When Pin 12 is High and Pin 18 is Low the door will move towards the Defrost position. When both Pin are high or when both Pins are low, the actuator will not move.
The Mode door feedback signal is a voltage signal that is supplied by the actuator to the control. The signal will be about 4.5 volts in the Panel position and 0.5 volts in the Defrost position. As the position of the Mode actuator changes, so will the feedback signal. The feedback signal is necessary for the correct positioning of the mode door.
SIDE DOOR HEATER A/C OUTLETS
The driver's and passenger side doors have supplemental air outlets and duct work.
Door Outlets:
Air is channeled from the instrument panel to the door duct and either to the lower floor or upper door outlets. The air can be adjusted to blow on the first rear passenger seat(s).
SIDE WINDOW DEMISTER
Demister Inlet:
The side window demisters direct air from the heater assembly. The outlets are located on the top forward corners of the front door panels. The demisters operate when the control mode selector is on FLOOR, MIX or DEFROST setting.
SYSTEM AIRFLOW
The system pulls outside (ambient) air through the Cool opening at the base of the windshield. Then it goes into the plenum chamber above the heater-A/C unit housing. On air conditioned vehicles, the air passes through the evaporator. At this point the air flow can be directed either through or around the heater core. This is done by adjusting the blend-air door with the Temp control on the control head. An optional zone control HVAC control module is available.
The HVAC unit has dual blend-air doors (if optionally equipped) that can be regulated independently of each other. The temperature setting can be different from driver's side to passenger side. After the air passes the blend-air door(s), the air flow can then be directed from the Panel, Floor, and Defrost outlets. Air flow velocity can be adjusted with the blower speed selector switch on the control head.
Ambient air intake can be shut off by closing the recirculating air door. This will recirculate the air that is already inside the vehicle. This is done by depressing the Recirc. button on the control head. On air conditioned vehicles, moving the control to Mix or Defrost depresses the A/C button and will engage the compressor. This will send refrigerant through the evaporator, and remove heat and humidity from the air before it goes through the heater core.
CAUTION: In cold weather, use of the Recirculation mode may lead to excessive window fogging. The Recirculation mode is automatically deactivated in Mix and Defrost modes to improve window clearing operation.
REAR BLOWER RESISTOR BLOCK
The blower motor resistor has multiple resistor wires, each of which will change the resistance in the blower motor ground path to change the blower motor speed. The blower motor switch directs the ground path through the correct resistor wire to obtain the selected blower motor speed.
The rear blower motor resistor is not serviceable separately. The resistor is integral to the blower motor.
If the resistor block is faulty, it will be necessary to replace the complete blower motor assembly.
The blower motor resistor cannot be repaired and, if faulty or damaged, it must be replaced.
With the blower motor switch in the lowest speed position the ground path for the motor is applied through all of the resistor wires. Each higher speed selected with the blower motor switch applies the blower motor ground path through fewer of the resistor wires, increasing the blower motor speed. When the blower motor switch is in the highest speed position, the blower motor resistor is bypassed and the blower motor receives a direct path to ground.
REAR BLOWER SPEED SWITCH
The rear blower speed switch controls the rear blower with the choice of low and high speeds.
When the switch is on it allows the blower speed switch located on the rear headliner to control rear blower speed. This switch will override the rear headliner blower switch. For operation instructions refer to the Owner's Manual. The rear blower speed switch is serviced separately from the A/C control module.
REAR HEATER AND A/C LINES
The rear heater-A/C unit plumbing is used only on models with the optional rear heater-A/C unit. The formed metal rear heater-A/C unit suction line, liquid line, and heater lines are available for separate service replacement. The molded and straight heater hoses used on the rear heater-A/C unit can be serviced in the vehicle.
Rear Heater and A/C Lines:
Front Lines Connected to Rear Lines:
Rear Heater Hose Connections:
Rear Heater Hose Quick Connects:
Rear A/C Block Connection:
The rear heater and A/C lines are all serviced as individual pieces. When disconnecting any line or block ensure that the area around it is clean of any contaminations that can get in to the system.
Any kinks or sharp bends in the rear heater-A/C unit plumbing will reduce the capacity of the entire heating and air conditioning system. Kinks and sharp bends reduce the system flow. High pressures are produced in the refrigerant system when the air conditioning compressor is operating. High temperature coolant is present in the heater plumbing when the engine is operating. Extreme care must be exercised to make sure that each of the plumbing connections is pressure-tight and leak free.
SYSTEM OPERATION
A/C Control Panel:
Rear Blower Switch:
The auxiliary rear heating-air/conditioning unit is located in the right rear quarter panel. The rear heater A/C control operates in conjunction with the front heater A/C control. A four position two speed blower (override) switch is located on the front Heater A/C control panel. The operator can use the rear heater A/C blower switch to operate the blower, regardless of the rear control setting. In the OFF position, the rear control will not function. In the ON (RR) position, the rear control will function normally providing three speeds. The mode setting is controlled by the front A/C control panel.
Air from inside the vehicle is drawn into the air intake grille in the right rear quarter trim panel. The air enters the blower and is pushed through the heater core and A/C evaporator coil. The air direction, floor or overhead, is determined by the position of the driver's temperature control lever on the front HVAC system control. If the temperature control lever is below 20% heat, the auxiliary HVAC system airflow will come from the overhead air outlets. In between 20% and 80%, the airflow will remain in the last position selected. In other words, if the driver started out in full heat and then adjusted their temperature lever to 30% heat as they became comfortable, the rear air would come out the floor outlets the whole time.