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:

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:

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.

Once a blower speed is selected, the blower speed remains constant, until a new speed is selected.

As the requested blower speed increases, the following conditions occur:

As the requested blower speed decreases, the following conditions occur:

Blower Motor Control Processor

The blower motor control processor is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive voltage and ground circuits enable the control processor to operate. The HVAC control module provides a pulse width modulation (PWM) signal to the control processor in order to command the blower motor speed. The processor supplies 12 volts to the blower motor through the blower motor voltage supply circuit. 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:

Once the above conditions have been met, the following sequence of events will occur:

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.

Mode, Recirculation 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.

Mode Valve Assembly

The film valve system has several operating characteristics:

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 (-°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:

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

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:

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.

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. The intake air temperature (IAT) on manual systems, is utilized to determine the following features: Blower Mode (Bi-level, Floor, Defrost, Recirc and Rear Defrost. These features are determined by the IAT during activation sequence and are maintained for the duration of engine run time. Calibratable temperatures are utilized to provide low, mid and high temperature ranges for the selectable feature. Blower and mode have the range to select any valid feature positions. When the remote start active serial data is received, ignition-OFF loop HVAC algorithms will act the same as if a run power mode was received, i.e. re-calibration, afterblow, ignition-OFF motor positioning etc. The OFF timer and the ignition ON timer have the functionality as in the Run mode.