The air temperature controls are divided into 4 primary areas:

HVAC Control Components

HVAC Control Module

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

Feature	Availability
Afterblow	Available if reprogrammed by the technician
Purge	No
Personalization	Yes
Actuator Calibration	No
Heated Seat Control	Optional

Remote Start and Personalization

If the remote start personalization feature for climate control is selected through radio, the HVAC control module will receive information that defines the current driver of the vehicle from the Ezkey module through GMLAN communication. This information shall be stored inside the HVAC control module memory. When a different key fob is selected, the HVAC control module will recall the appropriate driver settings. The full auto mode and previous set temp will be commanded if ambient air temperature is above 0°C (32°F). Defrost mode will be commanded when the ambient air temperature is below 0°C (32°F). Upon a battery reset, the HVAC control module shall also set the driver ID to Driver#1. The HVAC system will memorize the following system configurations for up to 2 unique drivers:

If the remote start personalization feature for heated and vented seat control is selected through radio, if the inside air temperature is below 12°C (54°F) the heated seat will be commanded to high. If the inside air temperature is above 26°C (80°F) and the ambient air temperature is above 21°C (70°F) the vented seat will be commanded to high. When the HVAC control module is first turned On or key On, the last stored settings for the current driver will be activated, heat/vented seat control is carried over from remote start.

OnStar® Blower Control ON/OFF

During OnStar® audio control ON, the HVAC control module receives signals from OnStar® to reduce blower level to a level 4. OnStar® blower control becomes active in auto or manual blower operation. During this OnStar® blower control, if in manual or auto blower, the display will show AUTO for blower display. This blower reduction is to ensure that all incoming and outgoing voice calls will be recognized by the OnStar® module. The driver is allowed to override the OnStar® auto blower level by increasing or decreasing it manually to any desired level. When OnStar® no longer sends the request for blower control the HVAC control module will cancel OnStar® blower control and resume previous blower level if no manual override of the blower was selected.

Air Temperature Actuators

The air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Low reference, 5-volt reference, position signal, and 2 control circuits enable the actuator to operate. The control circuits use either a 0 or 12-volt value to coordinate 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 one of the control circuits while providing the other with 12 volts. The HVAC control module reverses the polarity of the control circuits to move the actuator in the opposite direction. When the actuator shaft rotates, the potentiometers 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. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted value, one of the control circuits is grounded. 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 removes power and ground from the control circuits.

Air Temperature Sensors

The air temperature sensors are 2-wire negative temperature co-efficient thermistors. 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 signal decreases as the resistance decreases. The sensor signal varies between 0-5 volts. The HVAC control module converts the signal to a range between 0-255 counts.

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, voltage, or an open, the system will operate using an estimated default value to allow the system to operate. The ambient sensor operates within a temperature range between -30 to +51°C (-22 to +123.8°F). If the HVAC control module has determined that the ambient temperature sensor has failed, the driver information center (DIC) display shall display - - - - in place of the outside air temperature. If the sensor is shorted to ground, voltage, or an open, the system will operate using an estimated default value to allow the system to operate. If the engine coolant temperature is not more than 3°C (5.4°F) above the sensor reading, or if the engine has not been started in 3 hours, then the actual ambient air temperature sensor reading is displayed. Also at vehicle speeds greater than 35 km/h (22 mph), the ambient air temperature displayed may be allowed to increase, but only at a slow, filtered rate. The DIC displays the ambient air temperature value that it receives from the HVAC control module through a GMLAN message. The ambient air temperature value can be updated by an outside air instant update feature.

To use this feature, press the following switches on the HVAC control module simultaneously:

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 vehicle's inside temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle. If the sensor is open or shorted, no sunload adjustment occurs and the SERVICE A/C SYSTEM message is displayed.

Evaporator Temperature Sensor

The HVAC control module monitors the temperature of the air passing through the evaporator by the A/C evaporator air temperature sensor. This sensor is located on the evaporator core. The temperature 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 evaporator air temperature. The HVAC control module monitors the voltage drop across the thermistor when supplied with a 5-volt reference signal. The HVAC control module will send a class 2 message to the engine control module (ECM) to stop requesting the A/C compressor clutch operation if the temperature drops below 3°C (37°F). The sensor must be above 4°C (39°F) to request the A/C compressor clutch again.

The sensor operates within a temperature range between -40 to +215°C (-40 to +355°F). If the HVAC control module detects an open in the evaporator temperature sensor or circuit, the class 2 message sent to the ECM will not submit the A/C ON request. The HVAC control module 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.

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.

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 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.

Air Quality Sensor (RPO K14)

Warning - The air quality sensor recognizes carbon monoxide but is not designed as a safety feature to protect you from carbon monoxide. The air quality sensor is designed to limit your exposure to gasoline exhaust fumes and or diesel exhaust fumes that could enter into the passenger compartment from nearby vehicles for a limited amount of time.

The HVAC control module monitors the outside air quality while in AUTO mode via the air quality sensor assembly. You can activate the air quality sensor on your vehicle, if equipped, by pressing the Auto button on the HVAC module. The air quality sensor is a 5-wire sensor, 1 ignition voltage circuit, 2 ground circuit, and 2 control circuits. There are 2 sensors inside the air quality sensor assembly. A hydro carbon (HC) sensor that is used for detecting gasoline exhaust fumes and a Nox sensor that is used for detecting nitrous oxide, diesel exhaust fumes. The HVAC control module outputs a 5-volt signal on the HC and Nox control circuits and operates in a range between 0-5 volts.

The HVAC module updates the air quality sensor calibration approximately 4 times per second. The air quality sensor input to the HVAC module is compared to the previously updated air quality sensor calibration. If the voltage difference is great enough the recirc actuator will be controlled to recirc, outside air, or partial recirc outside air position based on the air quality sensor input.

When air quality around the front of the vehicle is poor, the air quality sensor system operates as follows:

When air quality around the front of the vehicle is normal, the air quality sensor system operates as follows:

The following are conditions that will prevent the air quality sensor from controlling the recirc actuator:

The following describes the recirc actuator function after the air quality sensor has canceled the command to recirc

The instrument panel module (IPM) shall provide the GMLAN Option Information for "Climate Air Quality Sensor Present" and "Climate Control Air Quality Sensitivity Selection" to the HVAC control module. Based on the option information sent to the HVAC control module by the IPM, the HVAC control module will choose 1 of 7 different sensitivity calibration settings stored in the HVAC module. The IPM sends the signal that tells the HVAC control module which sensitivity setting to use. From production the IPM sends a GMLAN message of 0 which tells the HVAC control module to use the default level 4 sensitivity calibration. The air quality sensitivity message sent to the HVAC module can be changed through the IPM with a TECH 2 and has 7 different air quality sensor calibration settings to choose from, 1 being the most sensitive to exhaust fumes and 7 being the least sensitive to exhaust fumes.

The driver information center (DIC) will display the SERVICE A/C SYSTEM message if the HVAC system has detected a short to ground in the air quality sensor circuit. An open circuit or a short to voltage in the air quality sensor circuit will be diagnosed through symptom based diagnostics because both of these conditions will not turn on the SERVICE A/C SYSTEM message or set a trouble code.

Heating and A/C Operation

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.

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 control module sends a GMLAN A/C request message to the engine control module (ECM) for A/C compressor clutch operation. The following conditions must be met in order for the ECM 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 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 cannot be used without the driver setting also being ON. The passenger setting can be turned ON or OFF by pressing the power button in the center of the passenger temperature rocker switch. 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 on one side of the vehicle may cause different discharge air temperatures, even when the passenger setting is not turned ON.

Automatic Operation

In automatic operation, the HVAC control module 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. 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 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 extremely cool temperature will not cool the vehicle any faster.

Auxiliary HVAC Control Module

The auxiliary HVAC control module is a non-GM LAN device that interfaces between the operator and the auxiliary HVAC system to maintain air temperature and air distribution settings. Five volts and ground are supplied to the rear HVAC control module. The front HVAC control module monitors the position of the rear air temperature actuator and the rear mode actuator. The front HVAC module will move the actuators to the proper position when it receives a request from the rear HVAC control module on which position to place the actuator.

Auxiliary Heating and A/C Operation

The auxiliary HVAC control module provides airflow direction and temperature control for the back seat passengers. Passengers can operate the rear HVAC control module in both manual or automatic modes. Auxiliary HVAC temperatures can be set cooler or warmer than the front primary HVAC setting. The front HVAC module provides power and ground to the auxiliary air temperature actuator. The front HVAC module receives power through the ignition 1 and battery positive voltage circuits from the underhood fuse block. The auxiliary air temperature actuator is a reverse polarity motor. Each circuit provides both power and ground to the auxiliary air temperature actuator. When the auxiliary air temperature actuator is being held in position, both of the auxiliary air temperature door control circuits have 0 volts applied to both sides of the actuator motor. This holds the actuator stationary. When a cooler temperature is requested, one of the auxiliary air temperature door control circuits will ground, driving the auxiliary air temperature actuator to the desired temperature. When a warmer temperature is requested, the other auxiliary air temperature door control circuit will ground. This moves the auxiliary air temperature actuator into the desired position.

Auxiliary Air Temperature Actuator

The auxiliary air temperature actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Low reference, 5-volt reference, position signal, and 2 control circuits enable the actuator to operate. The control circuits use either a 0 or 12-volt value to coordinate 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 one of the control circuits while providing the other with 12 volts. The HVAC control module reverses the polarity of the control circuits to move the actuator in the opposite direction. When the actuator shaft rotates, the potentiometers 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. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted value, one of the control circuits is grounded. 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 removes power and ground from the control circuits.

Steering Wheel Controls

The voice recognition has the ability to control the HVAC output air temperature, fan speed and defrost selections. The radio will recognize the following verbal commands when the steering wheel switch is pressed for controlling specific HVAC controls:

For heater or A/C On the following commands will be recognized:

For heater or A/C Off the following commands will be recognized:

For heater or A/C fan high the following commands will be recognized:

For heater or A/C fan low the following commands will be recognized:

For heater or A/C fan auto the following commands will be recognized:

For heater or A/C temperature change the following commands will be recognized:

For heater or A/C Defrost the following commands will be recognized:

For heater or A/C Defrost the following commands will be recognized:

For heater or A/C Front Defrost the following commands will be recognized:

For heater or A/C Front Defrost the following commands will be recognized:

For heater or A/C Rear Defrost the following commands will be recognized:

For heater or A/C Rear Defrost the following commands will be recognized:

A/C Cycle

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.

The Denso 7SBU16 variable displacement swash plate 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 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 and aluminum cooling fins, which allow rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser and flows through the liquid line, to the thermal expansion valve.

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

Refrigerant exiting the thermal expansion valve 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 back through the thermal expansion valve and into the suction line and back to the compressor, in a vapor state 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 under the vehicle.