The air temperature controls are divided into 4 areas

HVAC Control Module

The HVAC control module is a Class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The battery positive voltage circuit provides power that the control module uses for keep alive memory (KAM). If the battery positive voltage circuit loses power, all HVAC DTCs and settings will be erased from KAM. The control module supports the following features:

Air Temperature Actuator

The Buehler actuator is a 2 wire bi-directional electric motor. Two control circuits enable the actuator to operate. The control circuits use either a 0 or 12-volt value to co-ordinate the actuator movement. When the actuator is at rest, both control circuits have a value of 0 volts. In order to move the actuator, the HVAC control module grounds the appropriate control circuit for the commanded direction. The HVAC control module reverses the polarity of the control circuits to move the actuator in the opposite direction.

The HVAC control module determines the door position by counting motor pulses on one of the control circuits. These pulses are small voltage fluctuations that occur when the brush is shorted across 2 commutator contacts as the motor rotates. As the actuator shaft rotates, the HVAC control module monitors the voltage drop across an internal resistance to detect the pulses. The HVAC control module converts the pulses to counts with a range of 0 to 255 counts. The HVAC control module uses a range of 0-255 counts to index the actuator position.

A/C Refrigerant Pressure Sensor

The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5-volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts.

The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high or low pressure condition exists. The PCM disables the compressor clutch under the following conditions:

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

The A/C system can be engaged by either pressing the A/C switch or by selecting the following modes:

When the A/C switch is pressed, the HVAC control assembly applies B+ to the A/C request signal circuit. The body control module (BCM) receives this input and sends a class 2 message to the powertrain control module (PCM) for an A/C request. This input will request the PCM to ground the A/C compressor clutch relay control circuit, which will switch the A/C CMPR relay. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch assembly. The following conditions must be met in order for the PCM to turn on the compressor clutch:

Once engaged, the compressor clutch will be disengaged for the following conditions:

When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.

Dual Zone Operation

The right air temperature adjustment allows the passenger to offset the air discharge temperatures on the right side of the vehicle. To activate the dual zone, the passenger slides the switch to the desired offset.

The HVAC control module will position the right air temperature actuator located on the right side of the HVAC module to a position to divert sufficient air through the heater core or around the heater core to achieve the desired passenger temperature.

Engine coolant is the key element of the heating system. The thermostat controls engine operating coolant temperature. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation. Coolant enters the heater core through the inlet heater hose, in a pressurized state.

The heater core is located inside the HVAC module assembly. The heat of the coolant flowing through the heater core is absorbed by the ambient air drawn through the HVAC module assembly. Heated air is distributed to the passenger compartment, through the HVAC module assembly, for passenger comfort.

The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC module assembly air temperature door. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.

Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved refrigerant for automotive use. R-134a is a very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

A Delphi model CVC7 piston type compressor is used on this model year vehicle. The A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant, which adds heat to the refrigerant. The refrigerant is discharged from the compressor through the discharge hose and is forced to flow to the condenser and then through the balance of the A/C system. The A/C system is mechanically protected with the use of a high pressure relief valve. If the high pressure switch were to fail or if the refrigerant system becomes restricted and refrigerant pressure continues to rise, the high pressure relief will pop open and release refrigerant from the system.

Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state.

The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum tubing and aluminum cooling fins, which allows rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser and flows through the liquid line to the TXV.

The TXV is located at the evaporator inlet. The TXV is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the TXV, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to boil at the TXV. The TXV also meters the amount of liquid refrigerant that can flow into the evaporator.

Refrigerant exiting the TXV flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the HVAC module and passes through the evaporator core. Warm and moist air causes the liquid refrigerant to boil inside of the evaporator core. The boiling refrigerant absorbs heat from the ambient air and draws moisture onto the evaporator. The refrigerant exits the evaporator through the suction line and back to the compressor in a vapor state, and completes the A/C cycle of heat removal. At the compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.

The conditioned air is distributed through the HVAC module for passenger comfort. The heat and moisture removed from the passenger compartment condenses and is discharged from the HVAC module as water.