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Fig.1 Common Blend-Air Heater-Air:
HEATING & AIR CONDITIONING
Heater And Air Conditioner
All vehicles are equipped with a common HVAC housing assembly. The system combines air conditioning, heating, and ventilating capabilities in a single unit housing mounted under the instrument panel. On heater-only systems, the evaporator coil and recirculation door are omitted from the housing.
The heater and optional air conditioner are blend-air type systems. In a blend-air system, a blend door controls the amount of unconditioned air (or cooled air from the evaporator on models with air conditioning) that is allowed to flow through, or around, the heater core. A temperature control knob on the A/C Heater control panel determines the discharge air temperature by controlling an electric actuator, which moves the blend door. This allows an almost immediate control of the output air temperature of the system.
The mode control knob on the heater-only or A/C Heater control panel is used to direct the conditioned air to the selected system outlets. Both mode control switches use engine vacuum to control the mode doors, which are operated by vacuum actuators.
On air conditioned vehicles, the outside air intake can be shut off by selecting the Recirculation Mode with the mode control knob. This will operate a vacuum actuated recirculation door that closes OFF the outside fresh air intake and recirculates the air that is already inside the vehicle.
The optional air conditioner for all models is designed for the use of non-CFC, R-134a refrigerant. The air conditioning system has an evaporator to cool and dehumidify the incoming air prior to blending it with the heated air. This air conditioning system uses a fixed orifice tube in the middle of the liquid line to meter refrigerant flow to the evaporator coil. To maintain minimum evaporator temperature and prevent evaporator freezing, the a/c low pressure switch on the accumulator cycles the compressor clutch.
Cooling System Requirements
To maintain the performance level of the 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 condenser will reduce the performance of the air conditioning and engine cooling systems. The engine cooling system includes the heater core and the heater hoses. Refer to Cooling for more information before the opening of; or attempting any service to the engine cooling system.
Refrigerant System Service Port
The two refrigerant system 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 refrigerant system is not accidentally contaminated by the use of the wrong refrigerant (R-12), or refrigerant system service equipment.
The high pressure service port is located on the liquid line between the condenser and the evaporator, near the front of the engine compartment. The low pressure service port is located on the suction line, near the accumulator outlet.
Each of the service ports has a threaded plastic protective cap installed over it from the factory. After servicing the refrigerant system, always reinstall both of the service port caps.
Fig.4 Compressor Clutch - Typical:
A/C COMPRESSOR CLUTCH
The compressor clutch assembly consists of a stationary electromagnetic coil, a hub bearing and pulley assembly, and a clutch plate. The electromagnetic coil unit and the hub bearing and pulley assembly are each retained on the nose of the compressor front housing with snap rings. The clutch plate is mounted to the compressor shaft and secured with a nut.
The compressor clutch assembly provides the means to engage and disengage the compressor from the engine serpentine accessory drive belt. When the clutch coil is energized, it magnetically draws the clutch into contact with the pulley and drives the compressor shaft. When the coil is not energized, the pulley freewheels on the clutch hub bearing, which is part of the pulley. The compressor clutch and coil are the only serviced parts on the compressor.
The compressor clutch engagement is controlled by several components: the a/c heater mode control switch, the a/c low pressure switch, the a/c high pressure switch, the compressor clutch relay, and the Powertrain Control Module (PCM). The PCM may delay compressor clutch engagement for up to thirty seconds. Refer to Electronic Control Modules for more information on the PCM controls.
A/C COMPRESSOR CLUTCH RELAY
The a/c compressor clutch relay is a International Standards Organization (ISO) micro-relay. The terminal designations and functions are the same as a conventional ISO relay. However, the micro-relay terminal orientation (footprint) is different, the current capacity is lower, and the relay case dimensions are smaller than those of the conventional ISO relay.
The compressor clutch relay is a electro mechanical device that switches battery current to the compressor clutch coil when the Powertrain Control Module (PCM) grounds the coil side of the relay. The PCM responds to inputs from the A/C heater control, the a/c low pressure switch, and the a/c high pressure switch.
The compressor clutch relay is located in the Power Distribution Center (PDC) in the engine compartment. Refer to the PDC label for relay identification and location.
The compressor clutch relay cannot be repaired and, if faulty or damaged, it must be replaced.
A/C HEATER CONTROL
Both the heater-only and a/c heater systems use a combination of electrical and vacuum controls. These controls provide the vehicle operator with a number of setting options to help control the climate and comfort within the vehicle. Refer to the owner's manual in the vehicle glove box for more information on the features, use, and suggested operation of these controls.
The heater-only or a/c heater control panel is located to the right of the instrument cluster on the instrument panel. The control panel contains a rotary-type temperature control knob, a rotary-type mode control switch knob, and a rotary-type blower motor speed switch knob. On models with the optional heated mirror system, a momentary push button switch and indicator lamp are located near the bottom of the a/c heater control panel. Refer to Heated Mirrors for more information on this feature.
The heater-only or a/c heater control panel cannot be repaired. If faulty or damaged, the entire unit must be replaced. The control knobs and the illumination lamps are available for service replacement.
A/C HIGH PRESSURE SWITCH
The a/c high pressure switch is located on the discharge line near the compressor. The switch is screwed onto a fitting that contains a Schrader-type valve, which allows the switch to be serviced without discharging the refrigerant system. The discharge line fitting is equipped with an O-ring to seal the switch connection.
The a/c high pressure switch is connected in series electrically with the a/c low pressure switch between ground and the Powertrain Control Module (PCM). The switch contacts open and close causing the PCM to turn the compressor clutch on and OFF. This prevents compressor operation when the discharge line pressure approaches high levels.
The a/c high pressure switch contacts are open when the discharge line pressure rises above about 3100 to 3375 kPa (450 to 490 psi). The switch contacts will close when the discharge line pressure drops to about 1860 to 2275 kPa (270 to 330 psi). When checking refrigerant system pressures with a manifold gauge set, keep in mind that the indicated pressures will be about 172 kpa (25 psi) below the actual switch pressure values due to the pressure drop that occurs in the refrigerant system between the switch and the high pressure service port.
The a/c high pressure switch is a factory-calibrated unit. The switch cannot be adjusted or repaired and, if faulty or damaged, it must be replaced.
A/C LOW PRESSURE SWITCH
The a/c low pressure switch is located on the top of the accumulator. The switch is screwed onto an accumulator fitting that contains a Schrader-type valve, which allows the switch to be serviced without discharging the refrigerant system. The accumulator fitting is equipped with an O-ring to seal the switch connection.
The a/c low pressure switch is connected in series electrically with the a/c high pressure switch and the a/c heater control, between ground and the Powertrain Control Module (PCM). The switch contacts open and close causing the PCM to turn the compressor clutch ON and OFF. This regulates the refrigerant system pressure and controls evaporator temperature. Controlling evaporator temperature prevents condensate water on the evaporator fins from freezing and obstructing air conditioning system air flow.
The a/c low pressure switch contacts are open when the suction pressure is about 152 - 165 kPa (22 - 24 psi) or lower. The switch contacts will close when the suction pressure rises to about 255 - 296 kPa (37 - 43 psi) or above. Lower ambient temperatures, below about -1 °C (30 °F), will also cause the switch contacts to open. This is due to the pressure/temperature relationship of the refrigerant in the system.
The a/c low pressure switch is a factory-calibrated unit. It cannot be adjusted or repaired and, if faulty or damaged, it must be replaced.
BLOWER MOTOR RELAY
The blower motor relay is an International Standards Organization (ISO)-type relay. The relay is an electromechanical device that switches battery current from a fuse in the Power Distribution Center (PDC) directly to the blower motor. The relay is energized when the relay coil is provided a voltage signal by the ignition switch. This arrangement reduces the amount of battery current that must flow through the ignition switch.
The blower motor relay control circuit is protected by a fuse located in the junction block. When the relay is de-energized, the blower motor receives no battery current.
The blower motor relay is located in the PDC in the engine compartment. Refer to the PDC label for blower motor relay identification and location.
The blower motor relay cannot be repaired and, if faulty or damaged, it must be replaced.
BLOWER MOTOR RESISTOR BLOCK
The blower motor resistor is mounted to the bottom of the HVAC housing, under the instrument panel and just inboard of the blower motor. It can be accessed without removing any other components.
The 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.
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.
The blower motor resistor cannot be repaired and, if faulty or damaged, it must be replaced.
BLOWER MOTOR SWITCH
The heater-only or a/c heater blower motor is controlled by a four position rotary-type blower motor switch, mounted in the HVAC control panel. The switch allows the selection of one of four blower motor speeds, but can of only be turned OFF by selecting the OFF position with the heater-only or a/c heater control switch knob.
The blower motor switch directs the blower motor ground path through the mode control switch to the blower motor resistor, or directly to ground, as required to achieve the selected blower motor speed.
The blower motor switch cannot be repaired and, if faulty or damaged, the entire heater-only or a/c heater control unit must be replaced. The blower motor switch knob is serviced separately.
VACUUM CHECK VALVE
On models with a this engine, a vacuum check valve is installed in the accessory vacuum supply line near the vacuum tap on the right side of the engine intake manifold.
The use of a vacuum check valve helps to maintain the system vacuum needed to retain the selected HVAC mode and vehicle speed control settings. On this engine models, it prevents the engine from bleeding down system vacuum through the intake manifold during extended heavy engine load (low engine vacuum) operation.
On this engine models, a second vacuum check valve is installed in the accessory vacuum supply line at the tee fitting near the dash panel in the engine compartment. This check valve also helps to maintain the system vacuum needed to retain the selected HVAC mode settings, but isolates the HVAC vacuum circuit from the vehicle speed control vacuum circuit. It prevents the vehicle speed control servo from bleeding down the HVAC system vacuum during extended heavy engine load operation.
The vacuum check valve cannot be repaired and, if faulty or damaged, it must be replaced.