General Information

INTRODUCTION
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 Heater A/C unit housing assembly

SYSTEM AIRFLOW
The system draws outside air through the cowl opening at the base of the windshield. Then it goes into the plenum chamber above the Heater A/C unit housing and passes through the evaporator. At this point airflow 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. After the air passes the blend air door, the air flow is then directed from the PANEL, BI-LEVEL (panel and floor), and FLOOR - 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 rotating the RECIRC. knob on the control head. Rotating the MODE control knob to the Defrost/Floor or Defrost setting on the control head 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. The compressor can also be engaged by depressing the A/C button on the control head.

A/C PRESSURE TRANSDUCER
The switch is located on the discharge line near the compressor. The pressure transducer functions as the refrigerant system pressure sensor. It supports the condenser/radiator fans and compressor functions.

CLUTCH, PULLEY AND COIL
They are mounted on the compressor providing a way to drive the compressor. The compressor clutch and coil are the only serviced parts on the compressor. When the compressor is not in operation, the pulley free wheels on the clutch hub bearing. When the coil is energized the clutch plate is magnetically engaged with the pulley and turns the compressor shaft.

The clutch assembly consists of a stationary electromagnetic coil, hub bearing pulley assembly, and clutch plate.

The electromagnetic coil and pulley are retained on the compressor with snap rings. The clutch plate is mounted on the compressor shaft and secured with a nut.

Fig 1 TRS90 Compressor:

COMPRESSOR
The TRS9O is a fixed displacement type compressor.

The compressor compresses the low pressure refrigerant vapor from the evaporator into a high pressure, high temperature vapor. The Scroll TRS-90 Compressor is used on all models. The system uses polyalkylene glycol synthetic wax-free refrigerant oil SP-15 PAG.

CAUTION: Cleanliness is extremely important. Clean the surfaces around the suction and discharge ports of the compressor before opening the system. If compressor is removed from vehicle, apply tape to the opened ports to prevent any contamination.

CONDENSER
It is located in front of the engine cooling radiator. Its function is to cool the hot high pressure refrigerant gas. This causes it to condense into a high pressure liquid refrigerant.

EVAPORATOR COIL
The coil removes heat and dehumidifies the air before it enters the vehicle. The coil is located in the A/C housing.

EVAPORATOR PROBE
The evaporator probe is a temperature sensing element located at the coldest point on the face of the evaporator. Output from the probe is sampled by the Body Control Module (BCM). It is used to switch the A/C compressor clutch OFF before evaporator freeze up occurs. The clutch is switched OFF when the probe temperature reaches O.94°C (33.7°F.). It is allowed to switch ON when the probe temperature reaches 2.05°C (35.7°F).

The evaporator probe is located on the lower right side of the evaporator housing.

EXPANSION VALVE
The valve is used to meter refrigerant into the evaporator in accordance with cooling requirements. The valve is located in front of the evaporator coil.

The expansion valve is located on the engine side of the dash panel, near the right shock tower. The expansion valve can fail in three different positions (open, closed or restricted).

In an Open Position: this will result in a noisy compressor or no cooling. The cause can be a broken spring, broken ball or excessive moisture in the A/C system. If the spring or ball are found to be defective, replace the expansion valve. If excessive moisture is found in the A/C system, recycle the refrigerant.

In a Closed Position: There will be low suction pressure and no cooling. This may be caused by a failed power dome or excessive moisture in the A/C system. If the power dome on the expansion valve is found to be defective replace the expansion valve. If excessive moisture is found recycle the refrigerant.

A Restricted Orifice: There will be low suction pressure and no cooling. This may be caused by debris in the refrigerant system. If debris is believed to be the cause, recycle the refrigerant and replace the expansion valve and receiver/drier.

HIGH PRESSURE RELIEF VALVE
The valve is located at the rear of the compressor. The valve is used to prevent excessive high system pressure. The valve vents the system when a pressure of 3445-4135 kPa (500-600 psi) and above is reached. This prevents damage to the compressor and other system components. The valve closes with a minimum pressure of 2756 kPa (400 psi).

FILTER/DRIER
The drier is used to remove any traces of moisture from the refrigerant system. The filter is used to separate any foreign particles.

REFRIGERANT LINES
The lines are used to carry the refrigerant between the various system components.

SERVICE GAUGE PORT
The high pressure gauge port is located on the compressor discharge line. The low pressure gauge port is located on the suction line.

THERMAL LIMITER SWITCH
The switch is used to measure compressor surface temperature. If compressor surface temperature is excessive the switch will cut battery feed voltage to the compressor clutch. The switch will then reset once compressor surface temperature returns to normal.

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.