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 HVAC control module uses for keep alive memory (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:

Remote Start and Personalization 

When the Remote Start feature is activated through the key FOB, it starts up the vehicles automatic HVAC control system according to the previous settings of the system before turning the vehicle off. When the outside temperature is cold, the automatic HVAC control system will start up the rear window defogger and if the vehicle has heated seats, they will also turn on. Cooled seat are activated during a remote start. Normal operation of the automatic HVAC control system will return after the key is turned to RUN position.

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 Remote Keyless Entry via GMLAN. 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. When the HVAC control module is first turned ON, the last stored settings for the current driver will be activated. Upon a battery reset, the HVAC control module shall also set the driver ID to Driver#2. The HVAC system will memorize the following system configurations for up to 2 unique drivers:

• Driver set temperature
• Passenger set temperature
• Mode
• Blower motor speed
• A/C compressor request, auto ON or A/C OFF

OnStar® Blower Control ON/OFF 

During OnStar® audio control ON, the HVAC control module receives signals from OnStar® to reduce blower speed level. When OnStar® audio control is active, the blower control runs the fan at a reduced speed to enable audio communications. This ensures 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 driver air temperature actuator is a reverse polarity motor. 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:

• Ambient
• Inside

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 -40°C to +150°C. 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 -40°C to +125°C (-40°F to +257°F). If the HVAC control module has determined that the ambient temperature sensor has failed, the driver information center (DIC) display shall display (-°F) 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 sensor reading when the vehicle is turned on less than the sensor reading before the vehicle was turned off, 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 16 km/h (10 mph), the ambient air temperature displayed may be allowed to increase, but only at a slow, filtered rate. The HVAC control module displays the ambient air temperature value. 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 while holding down the Left Temperature Up and the PASS enable button, press the Rear Defog button.

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 sunlight 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 sensor is open or shorted, no sunload adjustment occurs and the SERVICE A/C SYSTEM message is displayed in the instrument panel cluster (IPC).

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 backside of 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 GMLAN message to the engine control module (ECM) to stop requesting the A/C compressor clutch operation if the temperature drops below -4°C (25°F). The sensor must be above 2°C (36°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 GMLAN message sent to the ECM will not submit the A/C ON request. The HVAC control module will then send a request to instrument panel for display of the SERVICE A/C SYSTEM that will be displayed on the DIC. The HVAC control module will also turn off the A/C LED on the HVAC control module as long as the condition is present, if the A/C button is pushed the LED will flash 3 times and remain OFF.

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 GMLAN message sent to the HVAC control module will be invalid. The HVAC control module will then send a request to the IPC for display of the SERVICE A/C SYSTEM that will be displayed on the DIC. The HVAC control module will also turn off A/C OFF LED display on the HVAC control module as long as the condition is present, if the A/C button is pushed the LED will flash 3 times and remain OFF.