The air temperature controls are divided in 5 primary areas:

HVAC Control Module

The HVAC control module works in a dual role operating both air temperature and air conditioning (A/C) operation from the drivers side switch. 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 body control module (BCM), which is the vehicle mode master, provides a device on signal. The control module supports the following features:

Air Temperature Actuator

The air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition voltage low reference control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal between 0-5 volts. The HVAC control module uses a range of 0-255 counts to index the actuator position. When the module sets a commanded, or targeted, value, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module changes the control signal to 2.5 volts.

Inside Air Temperature Sensor

Replacement of the inside air temperature sensor means replacing the HVAC control module. The inside air temperature sensor is an integral part of the HVAC control module. The HVAC control module monitors the passenger compartment inside air temperature through an inside air temperature sensor. When the air is cold, the sensor resistance and the signal voltage are high. When the air is warm, the sensor resistance and the signal voltage are low. The HVAC control module requests A/C compressor clutch engagement and controls the air temperature actuator door positions in order to maintain the selected air temperature. The HVAC control module sends out a 5-volt reference signal to the inside air temperature sensor over the 5-volt reference circuit. A thermistor varies the reference voltage back to the HVAC control module through the low reference circuit.

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:

Evaporator Temperature Sensor

The A/C System is protected by the evaporator temperature sensor. The sensor located on the evaporator core provides the HVAC control module with the surface temperature of the evaporator core. If the evaporator temperature sensor reads a temperature of 0°C (32°F), the HVAC control module will turn off the A/C compressor clutch until evaporator temperatures reach 2°C (36°F).

Ambient Air Temperature Sensor

The ambient air temperature sensor is a 2-wire negative temperature co-efficient thermistor. The vehicle uses the following air temperature sensors:

A 5-volt reference and signal circuit enables the sensor to operate. As the air temperature surrounding the sensor increases, the sensor resistance decreases.

The sensor operates within a temperature range of -40°C (-40°F) to 60°C (140°F). The HVAC control module receives a class 2 message from the instrument panel cluster (IPC). The ambient air temperature sensor is mounted in the forward engine area of the vehicle. In this position, it is exposed to the airflow through the grill before it reaches the radiators. The sensor signal varies between 0 to 5 volts. The HVAC control module converts the signal to a working temperature range.

If the HVAC control module detects a faulty class 2 signal, then the control module software will use a defaulted air temperature value.

The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the system is fixed.

The ambient air temperature is updated and displayed under the following conditions:

Sunload Sensor

The sunload sensor is a 2-wire photo diode. Low reference and signal circuits enable the sensor to operate. As the light shining upon the sensor gets brighter, the sensor conductance increases. The sensor signal decreases as the conductance increases. The sensor operates within an intensity range between completely dark and bright. The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a range between 0-255 counts.

The sunload sensor provides the HVAC control module a measurement of the amount of light shining on the vehicle. Bright, or high intensity, light causes the vehicles inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle. If the HVAC control module detects a malfunctioning sensor, then the control module software will use a defaulted sunload valve. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is fixed. A resistance check of the sunload sensor should not be performed as it will damage the sensor.

The purpose of the heating and air conditioning (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 control module makes the following actions when automatic operation is not selected, and an air temperature setting is selected:

The following shows what blower motor, recirculation actuator, and mode actuator adjustments are made for each air temperature switch selection:

The A/C system can be engaged by either pressing the driver temperature switch or during automatic operation. The A/C display will illuminate on the HVAC control module display board when the driver temperature switch is pressed. The control module sends a class 2 A/C request message to the powertrain control module (PCM). The HVAC system uses a scroll compressor that incorporates a thermal switch that opens once the compressor temperature is more than 155°C (311°F). The thermal switch is a non-serviceable item. 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.

The HVAC control module uses dual rotary temperature switches. The dual zone controls allows for maximum temperature offset and comfort between the driver and passenger. It is possible to select maximum airflow over the evaporator core with one dual zone switch along with maximum airflow over the heater core with the other dual zone switch. Each air temperature actuator is independent from the other and the passenger side is not limited in it's range of temperature offset.

In automatic operation, the HVAC control module will maintain the comfort level inside of the vehicle by controlling the air conditioning (A/C) compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation. To place the HVAC system in Automatic mode you must set the temperature and press the drivers temperature knob. Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators will automatically be adjusted to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature:

Engine coolant is the essential element of the heating system. The thermostat controls the normal operating temperature of the engine. 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. The ambient air drawn through the HVAC module absorbs the heat of the coolant flowing through the heater core. The HVAC module distributes heated air to the passenger compartment for consistent passenger comfort. Opening or closing the HVAC module temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and is 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 Mitsubishi scroll compressor is used on this model year vehicle. The air conditioning (A/C) compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant is discharged from the compressor, through the discharge hose, and 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 continued 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 will cause the liquid refrigerant 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 completing 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 will also change form, or condense, and is discharged from the HVAC module as water.