HVAC - Automatic
Automatic 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. Operator interface dials and buttons are remote from the HVAC Control Module and are part of the Integrated Center Stack. Button presses are communicated from the Integrated Center Stack to the Radio via a dedicated communication line. The Radio then communicates the information to the HVAC Control Module over GMLAN communications. The battery positive voltage circuit provides power that the 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
During remote start, full auto mode and the 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) and inside temperature sensor is below 10°C (50°F). When the key is turned ON, the HVAC control module will return to last stored settings.
Air Quality Sensor
The HVAC control module monitors the outside air quality while in AUTO mode via the air quality sensor. 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 3-wire sensor, ignition, ground and a control circuit enable the sensor to operate. The sensor outputs a PWM Signal to the HVAC control module with different levels of pollution as detected outside.
Sensor outputs PWM signal between 15% (clean) to 85% (very polluted) to HVAC control module.
When the sensor has detected poor air quality. Then the recirculation actuator is forced to the recirculation position and the recirculation indicator will illuminate.
The following are conditions that will prevent the air quality sensor from controlling the recirculation actuator:
* The HVAC control module is in manual mode.
* Outside air temperature less than 5°C (41°F).
* 60 seconds sensor warm-up time after ignition is turned ON.
* Customer manually selects recirculation or outside air position.
If the HVAC system has detected a short to ground or open is in the air quality sensor circuit or if invalid PWM data is output by sensor, a diagnostic trouble code will be set.
Temperature, Recirculation and Mode 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 co-ordinate 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°C and 57.5°C (20.3°C-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°C to 51°C (-22 to 123.8°F). If the HVAC control module has determined that the ambient temperature sensor has failed, the radio display will not display the ambient temperature reading. The radio displays the ambient air temperature value that it receives from the HVAC control module through a GMLAN message. The ambient air temperature sensor mounts underhood and can be affected by city traffic, by idling, and by restarting a hot engine. Therefore, the HVAC control module filters the value of the ambient air temperature sensor for temperature display. The outside air temperature reading is always instantly updated if ambient air temperature has decreased. The ambient air temperature value is 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 the sensor is open or shorted, no sunload adjustment occurs.
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 GMLAN message to the 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°C 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 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 read by engine control module (ECM). 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 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 Speed
The blower motor forces air to circulate within the vehicles interior. The vehicle operator determines the blower motors speed by placing the blower motor switch in a desired speed position or by selecting automatic operation. In manual operation, once a blower speed is selected, the blower speed remains constant until a new speed is selected. In automatic operation, the HVAC control module will determine what blower speed is necessary in order to achieve or maintain a desired temperature.
OnStar(R) Blower Control ON/OFF
During OnStar(R) audio control ON, the HVAC control module receives signals from OnStar(R) to reduce blower level to a level 4. OnStar(R) blower control becomes active in auto or manual blower operation. During this OnStar(R) 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(R) module. The driver is allowed to override the OnStar(R) auto blower level by increasing or decreasing it manually to any desired level. When OnStar(R) no longer sends the request for blower control the HVAC control module will cancel OnStar(R) blower control and resume previous blower level if no manual override of the blower was selected.
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 variable voltage signal to the blower motor control module to request the selected blower speed. The blower motor control module provides a pulse width modulation (PWM) signal to the blower motor in order to control the blower motor speed. The module supplies 12 volts to the blower motor through the blower motor voltage supply circuit.
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.
Use the following procedure in order to enable the afterblow mode:
1. Connect the Scan Tool.
2. Turn ON the ignition.
3. Select HVAC Control Module.
4. Select Module Setup.
5. Select Afterblow Option.
After the HVAC control module has been programmed for afterblow, the following conditions must be met for afterblow to operate:
* The engine has been turned off for at least 30 minutes.
* The ambient air temperature is at least 21°C (70°F).
* The A/C compressor operated for more than 2 minutes before shut down.
* The system voltage is at least 12 volts.
Once the above conditions have been met, the following sequence of events will occur:
1. The blower motor will RUN for 20 seconds.
2. The blower motor will be OFF for 10 minutes.
3. The blower motor will RUN for an additional 20 seconds.
Heating and A/C Operation
The purpose of the heating and A/C system is to provide the following:
* Heated air
* Cooled air
* Remove humidity from the interior of the vehicle
* Reduce windshield fogging
Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve a desired temperature:
* Recirculation actuator setting
* Difference between inside and desired temperature
* Difference between ambient and desired temperature
* Blower motor speed setting
* Mode setting
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:
* Sunload
* Ambient temperature
* Inside 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 control module sends a 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 and 16 volts.
* ECM
- Engine coolant temperature (ECT) is less than 125°C (257°F).
- Engine RPM is between 0 RPM and 6000 RPM.
- A/C pressure is between 3137 kPa (455 psi) or 2850 kPa (413 psi).
Once engaged, the compressor clutch will be disengaged for the following conditions:
* Throttle position is 100 percent.
* A/C pressure is more than 3137 kPa (455 psi).
* A/C pressure is less than 193 kPa (28 psi).
* Engine coolant temperature (ECT) is more than 125°C (257°F).
* Engine speed is more than 5,500 RPM.
* 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.
Dual Zone Operation
The HVAC control module has temperature settings for the driver and the passenger. If the passengers setting is turned off then the drivers setting controls both driver and passenger temperature actuators. The passengers setting can not be used without the drivers setting also being ON. The passengers setting can be turned ON or OFF by using passengers temperature switch. When the passengers setting is ON, the passenger temperature can be adjusted independently from the drivers setting and the passenger temperature is displayed on the passengers side of the control module. A different sunload on one side of the vehicle may cause different discharge air temperatures even when the passengers setting is not turned ON.
Air Distribution
The HVAC control module controls the mode actuator in order to distribute airflow to a desired outlet. The mode switch provides the vehicle operator with the ability to override the automatic setting. When the mode door is moved to the defrost position, the A/C compressor clutch engages and the recirculation actuator will be moved to the outside air position. In the outside air position, the incoming air is filtered by the passenger compartment air filter.
The HVAC system allows air to the outboard panel outlets in all air distribution modes. This allows air to be circulated higher in the cabin under any operating condition. The panel outlets have three operating positions, open, side window defogger and closed. To eliminate air from these outlets, the individual outlet thumb wheel must be turned to the OFF or side window defogger position.
Recirculation
The HVAC control module controls the air intake through the recirculation actuator. Recirculation is not available when the mode is in defrost. When the mode is in defog, Recirculation will only be available for 10 minutes. In the outside air position, the incoming air is filtered by the passenger compartment air filter.
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:
* The blower motor switch must be in the AUTO position.
* The air temperature switch must be in any other position other than 60 or 90 degrees.
* The mode switch must be in the AUTO position.
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:
* Regulate blower motor speed
* Position the air temperature actuator
* Position the mode actuator
* Position the recirculation actuator
* Request A/C operation
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. The mode actuator will remain in the panel position and the recirculation actuator will remain in the recirculation 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.