HVAC - Manual
Manual HVAC Description and Operation
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). If the battery positive voltage circuit loses power, all HVAC DTCs and settings will be erased from KAM. The control module supports the following features:
Air Speed
The blower motor forces air to circulate within the vehicles interior. The vehicle operator determines the blower motor's speed by placing the blower motor switch in a desired speed position. The blower motor will only operate if the blower motor switch is in any position other than OFF, and the ignition switch is in the RUN position, with the exception of afterblow.
Blower Motor Control Processor
The blower motor control module is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive and ground circuits enable the control module to operate. The HVAC control module provides a pulse width modulation (PWM) signal to the blower motor control module in order to command the blower motor speed. The control processor uses the blower motor ground as a low side control to adjust the blower motor speed.
Afterblow
Afterblow is a feature that dries the evaporator core by operating the blower motor after the engine is turned OFF. This reduces the amount of microbial growth that can create undesirable odors. The vehicle does not come equipped with the afterblow feature turned ON. If the afterblow feature is required due to an odor concern, it must be turned ON by the scan tool.
The following conditions must be met for afterblow to operate:
* The A/C compressor operated for more than 10 seconds.
* The A/C compressor, after operating for more than 10 seconds has not been deactivated for more than 10 minutes.
* The system voltage is at least 11 volts to start and 10 volts to continue to run.
* The ignition has been in the OFF position for at least 20 minutes.
Once the above conditions have been met, the following sequence of events will occur:
* The blower motor will RUN for 20 seconds.
* The blower motor will be OFF for 10 minutes.
The blower motor will repeat this sequence for 2 more cycles.
Mode and Air Temperature Actuators
The actuators are 5-wire bi-directional electric motors that incorporate 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.
Recirculation Actuator
The recirculation actuator is a 2 position actuator. In recirculation mode the actuator moves to recirculation door to close off the outside air inlet. When not in recirculation mode it opens the outside air inlet. The recirculation actuator has B+ voltages circuit and two control circuits. At rest both of the control circuits are open. When a command is made one of the control circuits will be grounded until the door reaches the desired position. To move in the opposite direction the other control circuit is grounded.
Ambient Air Temperature Sensor
The ambient 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.
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 HVAC 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 2 210 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.
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/awake. When the outside temperature is cold, the HVAC control system will start up the rear window defogger.