The automatic HVAC description and operation is divided into 6 primary areas:

HVAC Control Module

The HVAC control module is a 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 serial data. 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 and Personalization

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 RCDLR module through serial data 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. The full auto mode and 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). Upon a battery reset, the HVAC control module shall also set the driver ID to Driver#1. The HVAC system will memorize the following system configurations for up to 2 unique drivers:

If the remote start personalization feature for heated and vented seat control is selected through radio, if the inside air temperature is below 12°C (54°F) the heated seat will be commanded to high. If the inside air temperature is above 26°C (80°F) and the ambient air temperature is above 21°C (70°F) the vented seat will be commanded to high. When the HVAC control module is first turned On or key On, the last stored settings for the current driver will be activated.

OnStar® Blower Control ON/OFF

During OnStar® audio control ON, the HVAC control module receives signals from OnStar® to reduce blower speed. OnStar® blower control becomes active in auto or manual blower operation. During this OnStar® 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® 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, Air Temperature Actuators

The actuators are 5-wire bi-directional electric motors that incorporates 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 -32.64  percent to 134.89  percent to index the actuator position. The door position signal voltage is converted to a percentage 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:

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 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 - - - - 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 35 km/h (22 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 serial data 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:

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 0-3906.6 W/m². 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 vehicle's 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.

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 message to the engine control module (ECM) to stop requesting the A/C compressor 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 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 serial data 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. 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 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 display will also display A/C OFF on the module as long as the condition is present.

Blower Motor Relay

The blower motor relay provides a supply voltage to the blower motor and blower motor control processor. The HVAC control module commands the blower motor relay ON anytime the commanded blower speed is not OFF.

Blower Motor Control Module

The blower motor control module is an interface between the HVAC control module and the blower motor. The blower motor speed control, blower motor supply voltage and ground circuits enable the control module 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 control module uses the blower motor ground as a low side control to adjust the blower motor speed.

The blower motor forces air to circulate within the vehicle's 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.

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

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

A/C Request Signal and A/C Control

The compressor is an externally controlled variable displacement (ECVD) A/C compressor. The HVAC controls the A/C compressor displacement with a pulse width modulated (PWM) signal. When the A/C switch is pressed, the HVAC control module requests permission to engage the A/C compressor from the ECM. The ECM must grant this permission before the HVAC control module can engage the A/C compressor. The BCM acts as a gateway to pass these messages on serial data between the HVAC control module and the ECM. If the conditions listed are not met, the ECM will not grant this permission and the A/C compressor will remain at approximately 4 cc of displacement (OFF). The A/C indicator LED will remain ON. If A/C pressure is below 196 kPa the A/C compressor will be reduced back to approximately 4 cc of displacement (turned OFF) and the A/C indicator LED will turn OFF. This is a normal condition in very cold ambient temperatures. While the A/C compressor is running the HVAC control module transmits the compressor torque value to the ECM. The ECM uses this value to manage engine torque requirements and maintain idle stability. The A/C compressor will be reduced back to approximately 4 cc of displacement (turned OFF) if the HVAC blower is turned to OFF.

Compressor

The A/C compressor can match the air conditioning demand under all conditions without cycling a clutch. The basic A/C compressor mechanism is a variable angle swash-plate with 6 axially oriented cylinders. The A/C compressor has a pumping capacity of 160 cc of displacement. The electronic control valve is installed in the A/C compressor rear head. The swash-plate angle of the A/C compressor and the resultant A/C compressor displacement, are determined by the A/C compressor crankcase to suction pressure differential which is governed by the control valve. When the A/C capacity demand is low, the crankcase pressure behind the pistons is equal to the pressure in front of the pistons. This forces the swash plate to change its angle to towards vertical which reduces the stroke of the pistons and reduces the output of the A/C compressor to approximately 4 cc of displacement. The evaporator cooling load is reduced, ambient temperature or blower fan speed is reduced and therefore, the suction pressure is reduced until it reaches the control point. To reach the control point, the electronic control valve assembly allows discharge pressure to bleed past the control valve ball valve seat and into the A/C compressor crankcase. This crankcase pressure acts as an opposing force behind the A/C compressor pistons to cause the swash plate to change its angle towards vertical and therefore, reduce piston stroke. When the A/C capacity demand is high, the crankcase pressure behind the pistons is less than the pressure in front of the pistons. This forces the swash plate to change its angle away from vertical which increase the stroke of the pistons. The A/C compressor will then have a corresponding increase in its displacement.

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:

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:

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.

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 extremely cool temperature will not cool the vehicle any faster.

Dual Zone Operation

The HVAC control module has temperature settings for the driver and the passenger. If the passenger setting is turned OFF then the driver setting controls both driver and passenger temperature actuators. The passenger setting cannot be used without the driver setting also being ON. The passenger setting can be turned ON or OFF by pressing the power button in the center of the passenger temperature rocker switch. When the passenger setting is ON, the passenger temperature can be adjusted independently from the driver setting and the passenger temperature is displayed on the passenger side of the control module. A different sunload on one side of the vehicle may cause different discharge air temperatures, even when the passenger setting is not turned ON.