The air temperature controls are divided into six areas:

HVAC Control Module

The HVAC control module is a device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The control module sends switch input data to the instrument panel module (IPM), and receives display data from the IPM through signal and clock circuits. The ignition 3 voltage circuit provides a device on signal. The control module does not retain any HVAC DTCs or settings.

Instrument Panel Integration Module

A function of the IPM operation is to process HVAC system inputs and outputs. Also, the IPM acts as the HVAC control module's Class 2 interface. The battery positive voltage circuit provides power that the IPM uses for keep alive memory (KAM). If the battery positive voltage circuit loses power, then all HVAC DTCs and settings will be erased from KAM. The ignition 3 voltage circuit provides a device on signal. The IPM supports the following features:

Air Temperature Actuator

The actuator is a 5 wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 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 IPM uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. 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.

Air Temperature Sensors

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

A signal and low reference 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 150°C (300°F). The sensor signal decreases as the resistance decreases. The sensor signal varies between 0-5 volts. The IPM converts the signal to a range between 0-255 counts.

The IPM uses the sensor signals to determine the air temperature door position. Left and right air temperature sensor inputs in a dual zone HVAC system are used to adjust the corresponding left and right air temperature door. In HVAC systems with upper and lower duct temperature sensors, the IPM uses the following sensor inputs with the indicated mode positions:

If the IPM detects a malfunctioning sensor, 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 condition is corrected.

The scan tool has the ability to update the displayed ambient air temperature. The ambient air temperature value is displayed or updated under the following conditions:

Sunload Sensor

The sunload sensor is a 2 wire photo diode. The vehicle uses left and right sunload sensors. The two sensors are integrated into the sunload sensor assembly. 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 IPM converts the signal to a range between 0-255 counts.

The sunload sensor provides the IPM 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 IPM detects a malfunctioning sensor, then the IPM software will use a defaulted sunload value. The default action ensures that the HVAC system can adjust the inside air temperature near the desired temperature until the condition is fixed.

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:

Heating and A/C Operation

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 IPM makes the following actions when an air temperature setting is selected:

The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The IPM receives an input from the HVAC control module. In order for the powertrain control module (PCM) to internally ground the A/C compressor clutch relay control circuit, the DIM and the PCM must communicate to each other over the Class 2 serial data circuits.

The DIM will request A/C operation from the PCM if these parameters are within normal operating limits and the ambient temperature is more than 3°C (37°F). If the A/C compressor is turned off due to low ambient temperatures, the A/C compressor will not be turned back on until the ambient temperature reaches 6°C (42°F). 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.

Automatic Operation

In automatic operation, the IPM will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower, the air temperature actuators, mode actuator and recirculation.

To place the HVAC system in Automatic mode, the following is required:

Once the desired temperature is reached, the blower motor, recirculation and temperature actuators will automatically be adjusted to maintain the temperature selected. The IPM performs the following functions to maintain the desired air temperature:

Dual Zone Operation

The right air temperature allows the passenger to offset air discharge temperatures on the right side of the vehicle by 3°C (5°F). To activate the dual zone, the passenger rotates the switch to the desired offset. The switch assembly has LEDs that will illuminate in order to inform the passenger of the temperature offset. The temperature offset is allowed as long as the driver's set temperature is not in the maximum hot or cold settings. The display will not show the passenger temperature selection.

Once the temperature offset request is made to the front passenger door module (FPDM), the signal is sent to the driver door module (DDM). The DDM is connected to the FPDM through the power door serial data circuit. The DDM will make a request to the IPM through the Class 2 serial data circuit. The IPM will position the right air temperature actuator, located on the right side of the HVAC module to a position to divert sufficient air past the heater core to achieve the desired passenger temperature.

Steering Wheel Controls

An additional temperature switch is mounted on the steering wheel in order to allow the driver to adjust the HVAC temperature. If the driver adjusts the temperature using the steering wheel temperature control switch, then voltage is sent through a series of resistors. That varied voltage is sent back through the inflatable restraint steering wheel module coil to the DIM through the steering wheel controls signal circuit. Once the DIM receives the varied voltage signal, the information is sent out over the Class 2 serial data circuit to the IPM, where the air temperature actuator is adjusted.

Engine Coolant

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

The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC module 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 an very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

The 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 orifice tube.

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

Refrigerant exiting the orifice tube 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.