The air temperature controls are divided into three primary 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 body control module (BCM), which is the vehicle power mode master, provides a device on signal. The control module supports the following features:

Personalization

The HVAC control module 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 three unique drivers:

This information shall be stored inside the HVAC control module memory. When a different driver identification button is selected the HVAC control module will recall the appropriate driver settings. When the HVAC control module 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 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 12 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 two 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 0-255 counts. The HVAC control module uses a range of 0-255 counts to index the actuator position.

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 to +57.5°C (+20.3 to +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 detects that one of the duct temperatures must change, the HVAC control module 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 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 to +57.5°C (+20.3 to +135.5°F). If the sensor is shorted to ground, an open, or shorted to voltage, the HVAC system will use a default value.

The outside temperature sensor operates within a temperature range between -30 to +51°C (-22 to +123.8°F). The OAT valve is received and displayed through the instrument panel cluster (IPC). If the IPC has determined that the outside temperature sensor has failed, the IPC shall display, 75, in place of the outside air temperature. If the sensor is shorted to ground, an open, or shorted voltage, the IPC will use a default value.

The IPC displays the outside air temperature value that it receives through a class 2 or GMLAN message . The scan tool has the ability to update the displayed ambient air temperature. The outside 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 2 sensors are integrated into the sunload sensor assembly along with the ambient light sensor. 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 or shorted, no sunload condition occurs.

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 engine control module (ECM) disables the compressor clutch under the following conditions:

If the ECM detects a failure in the A/C refrigerant pressure sensor or circuit, the class 2 message sent to the HVAC control module will be invalid. The HVAC control module will display A/C OFF on the module as long as the condition is present.

The purpose of the heating and air conditioning (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 except the, A/C OFF switch. The A/C switch will illuminate, A/C OFF, when the A/C switch is selected. The HVAC control module sends a class 2 A/C request message to the body control module (BCM) for A/C compressor clutch operation. The BCM must communicate with the engine control module (ECM) in order for the A/C clutch to be engaged. The HVAC system uses a compact variable swash plate compressor. The following conditions must be met in order for the ECM to turn ON the compressor clutch from the HVAC control module 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 setting is turned off then the driver setting controls both driver and passenger temperature actuators. The passenger setting can not be used without the driver setting also being ON. The passenger setting can be turned ON or OFF by pressing the temperature switch on the passenger side of the HVAC control module. When the passenger setting is ON, the passenger temperature can be adjusted independently from the driver setting and the passenger temperature is displayed on the passenger 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 setting is not turned ON.

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, 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 performs the following functions to maintain the desired air temperature:

When the warmest position is selected in automatic operation the blower speed, based on coolant temperature, 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.

Remote Start Activation

The following describes the HVAC control head functionality upon receiving the remote start active serial data message and a power mode status is set to off a wake. Outside ambient temperature sensor, OAT- on automatic systems or intake air temperature (IAT) on manual systems, are utilized to determine the moding of the following features: Blower Mode (Bi-level, floor, Defrost, Recirc and rear Defrost. These features as determined by the OAT/IAT during activation sequence are maintained for the duration of engine run time. Class 2 supplies the ambient temperature. Calibratable temperatures are utilized to provide low, mid and high temperature ranges for feature selectable. Blower and mode have the range to select any valid feature positions. When the remote start active serial data is received ignition-off loop HVAC algorithms will act the same as if a run power mode received (i.e. re-calibration, after blow, ignition-off motor positioning etc.) off timer and ignition on timer have the functionality as in Run mode.

Remote Start De-activation

When the remote start is exited (class 2 power mode = run mode or remote start engine) all displays will revert to that based on actual settings on the vehicle. In the case of manual settings, displays will revert to actual state of operation of the features on the basis of their normal control algorithms.

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.