The air temperature controls are divided into 3 primary areas:

HVAC Control Module

The HVAC control module (IPM) 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 dash integration module (DIM), which is the vehicle power mode master, provides a device on signal. The control module (IPM) supports the following features:

Personalization

The HVAC control module (IPM) will receive information that defines the current driver of the vehicle from the driver door module (DDM) through class 2 communication. The HVAC system will memorize the following system configurations for up to two unique drivers:

This information shall be stored inside the HVAC control module (IPM) memory. When a different driver identification button is selected the HVAC control module will recall the appropriate driver settings. When the HVAC control module (IPM) is first turned on, the last stored settings for the current driver will be activated except for the rear defrost and heated seat settings.

Air Temperature Actuators

The air temperature 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.

Air Temperature Sensors

The air temperature sensors are 2 wire negative temperature co-efficient thermistors. The vehicle uses the following air temperature sensors:

The upper and lower duct sensors are divided into left and right zone operation. The left side upper and lower duct sensors will only effect the operation of the left air temperature actuator and the right side upper and lower duct sensors will only effect the operation of the right air temperature actuator. The duct sensors are used to measure the temperature of the air exiting the ducts. The sensors operate within a temperature range between -6.5°C (20.3°F) to 57.5°C (135.5°F). When temperature increases the sensor signal decreases. The HVAC control module converts the 0-5 volt sensor signal to a range between 0-255 counts. If the HVAC control module (IPM) detects that one of the duct temperatures must change, the HVAC control module (IPM) will adjust the appropriate air temperature actuator. The following list shows the duct sensors that are monitored by the HVAC control module in each mode position:

If the HVAC control module (IPM) detects a malfunctioning sensor, the HVAC system will only malfunction when the sensor is monitored.

The inside temperature sensor operates within a temperature range between -6.5°C (20.3°F) to 57.5°C (135.5°F). A fan located internally to the inside air temperature sensor housing continually draws passenger compartment air over the sensor. If the internal fan is inoperative the inside air temperature will change very slowly. If the sensor is shorted to ground, the system will operate in the maximum cooling mode. If the sensor is shorted to voltage or open, the system will operate in the maximum heating mode.

The ambient sensor operates within a temperature range between -30°C (-22°F) to 51°C (123.8°F). If the HVAC control module (IPM) has determined that the outside temperature sensor has failed, the driver information center (DIC) display shall display "59°F" in place of the outside air temperature. If the sensor is shorted to ground, the system will operate in the maximum cooling mode. If the sensor is shorted to voltage or open, the system will operate in the maximum heating mode. If the engine coolant temperature is not more than 28°C (50°F) above the sensor reading, or if the engine has not been started in two hours, then the actual ambient air temperature sensor reading is displayed. The ambient air temperature displayed may be allowed to increase after 80 seconds as long as the vehicle speed is greater than 32 Km/h (20 mph), but only at a slow, filtered rate. If the sensor reading is ever less than the displayed value or if the vehicle speed is 72 Km/h (45 mph) or greater, then the ambient air temperature changes are displayed as rapidly as possible.

Sunload Sensor

The sunload sensor is a 2 wire photo diode. The vehicle uses left and right sunload sensors. Low reference and signal circuits enable the sensor to operate. As the sunload increases, the sensor signal decreases. 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 sensor is open, no sunload condition occurs. If sensor is shorted, maximum sunload condition occurs with increased cooling mode.

A/C Refrigerant Low Temperature Sensor

The dash integration module (DIM) monitors the sensor to determine low side pressure based on the pressure to temperature relationship of the R134a. This sensor is located on the low side line. The sensor is used to cycle the A/C compressor ON and OFF to prevent the evaporator core from freezing. A thermistor inside the sensor varies its resistance to monitor the temperature of the freon. The DIM monitors the voltage drop across the thermistor when supplied with a 5 volt reference signal. The DIM will send a class 2 message to the PCM to stop requesting the A/C compressor clutch operation if the temperature drops below 2°C (28°F). Vehicles operating above 32 km/h (20 mph) will be controlled to a minimum ON time of 46 seconds before the DIM will allow the clutch to disengage. The sensor must be above 10°C (50°F) before the DIM will request A/C compressor clutch operation again.

The sensor operates within a temperature range between -40°C (-40°F) to 215°C (-355°F). If the DIM detects an open in the A/C refrigerant low temperature sensor or circuit, the class 2 message sent to the PCM will not allow A/C operation. The HVAC control module (IPM) will then send a request to the Radio for display of the "SERVICE A/C SYSTEM" that will be displayed on the DIC. The HVAC control module (IPM) will also display A/C OFF on the module as long as the condition is present.

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 prevents 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:

If the PCM detects a failure in the A/C refrigerant pressure sensor or circuit, the class 2 message sent to the HVAC control module (IPM) will be invalid. The HVAC control module (IPM) will then send a request to the Radio for display of the "SERVICE A/C SYSTEM" that will be displayed on the DIC. The HVAC control module will also display A/C OFF on the module as long as the condition is present.

The purpose of the heating and A/C system is to provide the following:

Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve a desired temperature:

The HVAC control module commands or monitors the following actions when an air temperature setting is selected.

WARMEST POSITION - The air temperature actuator door position directs maximum air flow through the heater core.

COLDEST POSITION - The air temperature actuator door position directs maximum air flow around the heater core.

BETWEEN THE WARMEST AND COLDEST POSITION - The following sensors are monitored to direct the appropriate amount of air through the heater core to achieve the desired temperature:

The A/C system is engaged by selecting any switch on the HVAC control module (IPM) except the "A/C OFF" switch. The A/C switch will illuminate "A/C OFF" when the A/C switch is selected. The control module (IPM) sends a class 2 A/C request message to the powertrain control module (PCM) for A/C compressor clutch operation. The dash integration module (DIM) must communicate with the PCM in order for the A/C clutch to be engaged. The HVAC system uses a scroll compressor that incorporates a thermal switch that opens once the compressor temperature is more than 125°C (257°F). The following conditions must be met in order for the PCM to turn on the compressor clutch from the IPM request:

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

If there is a malfunction in the A/C system, the driver information center will read "SERVICE A/C SYSTEM" to alert the driver.

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

Dual Zone Operation

The HVAC control module has temperature settings for the driver and the passenger. If the passenger's setting is turned off then the driver's setting controls both driver and passenger temperature actuators. The passenger's setting can not be used without the driver's setting also being ON. The passenger's setting can be turned ON or OFF by pressing the temperature knob on the passenger's side of the HVAC control module (IPM). When the passenger's setting is ON, the passenger temperature can be adjusted independently from the driver's setting and the passenger temperature is displayed on the passenger's side of the control module. A different sunload or duct temperature on one side of the vehicle may cause different discharge air temperatures even when the passenger's setting is not turned ON.

In automatic operation, the HVAC control module (IPM) will maintain the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, 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, mode, recirculation and temperature actuators will automatically adjust to maintain the temperature selected (except in the extreme temperature positions. The HVAC control module (IPM) performs the following functions to maintain the desired air temperature:

When the warmest position is selected in automatic operation the blower speed will increase gradually until the vehicle reaches normal operating temperature. When normal operating temperature is reached the blower will stay on high speed and the air temperature actuators will stay in the full heat position. When the coldest position is selected in automatic operation the blower will stay on high and the air temperature actuators will stay in the full cold position.

In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. The vehicle operator can select an extreme temperature setting but the system will not warm the vehicle any faster. In warm temperatures, the automatic HVAC system will also provide air conditioning in the most efficient manner. Selecting an extreme cool temperature will not cool the vehicle any faster.

The HVAC control module receives class 2 messages that the driver has activated a steering wheel control switch from the radio interface. The HVAC steering wheel control button controls driver set temperature increase and driver set temperature decrease.

The HVAC system interprets the set temperature switch on the steering wheel as if the driver had activated the same switch function on the HVAC control module (IPM).

Engine coolant is the key element of the heating system. The engine thermostat controls the normal engine operating coolant temperature. Coolant pumped out of the engine enters the heater core through the inlet heater hose. The air flowing through the HVAC module absorbs the heat of the coolant flowing through the heater core. The coolant then exits the heater core through the heater outlet hose and returns back to the engine block.

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.