Air Temperature
Air Temperature Description and Operation
The air temperature controls are divided into 3 primary areas:
- The Heating and Air Conditioning System
- The A/C Cycle
- Auxiliary heating and A/C system
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. The control module supports the following features:
Remote Start and Personalization
If the remote start personalization feature for climate control is selected through the HVAC control module will receive information that defines the current driver of the vehicle from the RKE 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. 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 level between M1 and M2. OnStar blower control becomes active in auto or manual blower operation. 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 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 -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 (-°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 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 16 km/h (10 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:
- LEFT TEMPERATURE UP
- PASS ENABLE
- REAR DEFOG ENABLE
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 (257°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 the I/P for display of the SERVICE A/C SYSTEM that will be displayed on the DIC. The HVAC control module will also turn off the AC LED display on the ECC module as long as the condition is present, if the AC 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 I/P 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 module as long as the condition is present, if the AC button is pushed the LED will flash 3 times and remain off
Heating and A/C Operation
The A/C system is engaged by selecting the A/C button on the ECC control module. The A/C will illuminate LED when the A/C button 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:
- HVAC control module
- Evaporator Temperature more than 4�C (39�F)
- Control module operating range 9-16 volts
- ECM
- Engine coolant temperature (ECT) is less than 125°C (257°F).
- A/C pressure is between 3 137 kPa (455 psi) and 210 kPa (210 psi).
Once engaged, the compressor clutch will be disengaged for the following conditions:
- Throttle position is 90 percent.
- A/C pressure is more than 3 137 kPa (455 psi).
- A/C pressure is less than 193 kPa (28 psi).
- ECT is more than 125°C (257°F).
- Transmission shift
- ECM detects excessive torque load.
- ECM detects insufficient idle quality.
- ECM detects a hard launch condition.
When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.