The air delivery description and operation is divided into 5 areas:

HVAC Control Module

The HVAC control module is a class 2 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 body control module (BCM), which is the vehicle mode master, provides a device on signal. The control module supports the following features:

Auxiliary HVAC Control Module

Auxiliary HVAC Control functions are integrated into the Rear Seat Entertainment Module. The Rear Seat Entertainment Module communicates Rear HVAC settings over serial data. The control switches enable the module to send a GMLAN serial data message to the from auxiliary HVAC control module to the HVAC Control Module for the change request. The Auxiliary HVAC Control module does not store auxiliary HVAC DTCs.

Auxiliary HVAC Control Functions

All Auxiliary functions and DTCs are handled by the HVAC Control Module. There are two ways the rear functions can be controlled.

Control from the HVAC Control Module: If the AUX button on the HVAC Control Module is pressed, the auxiliary HVAC system will be enabled. The settings for the auxiliary will mimic the driver settings on the HVAC control module.

Control from the Auxiliary HVAC Control Module: If at any time any of the three auxiliary controls are adjusted, control of the rear HVAC system will transfer to the auxiliary controls. If the Aux button is not currently enabled, adjusting the auxiliary controls will enable it, even if the front system is OFF.

Mode Actuator

The mode actuator is a 5 wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5 volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5 volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometer's 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, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. 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 changes the control signal to 2.5 volts.

Auxiliary Mode Actuator

The auxiliary mode actuator is a 5-wire bi-directional electric motor that incorporates a feedback potentiometer. Ignition 3 voltage, low reference, control, 5-volt reference and position signal circuits enable the actuator to operate. The control circuit uses either a 0, 2.5 or 5-volt signal to command the actuator movement. When the actuator is at rest, the control circuit value is 2.5 volts. A 0 or 5-volt control signal commands the actuator movement in opposite directions. When the actuator shaft rotates, the potentiometers adjustable contact changes the door position signal between 0-5 volts.

The auxiliary 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, the control signal is changed to either 0 or 5 volts depending upon the direction that the actuator needs to rotate to reach the commanded value. 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 changes the control signal to 2.5 volts.

Blower Motor Control Processor

The blower motor control processor controls the speed of the blower motor by increasing or decreasing the voltage drop on the ground side of the blower motor. The HVAC control module provides a linear pulse modulated signal to the blower motor control processor over the blower motor speed control circuit. As the requested blower speed increases, the HVAC control module increases the amount of time that the speed signal is modulated to ground. As the requested blower speed decreases, the HVAC control module decreases the amount of time that the signal is modulated to ground.

Auxiliary Blower Motor Control Processor

The auxiliary blower motor control processor controls the speed of the blower motor by increasing or decreasing the voltage drop on the ground side of the auxiliary blower motor. The auxiliary HVAC control module or rear seat audio module (RSA) provides a low side pulse width modulated signal to the auxiliary blower motor control processor over the auxiliary blower motor speed control circuit. As the requested blower speed increases, the auxiliary HVAC control module or rear seat audio module (RSA) increases the amount of time that the speed signal is modulated to ground. As the requested blower speed decreases, the auxiliary HVAC control module or rear seat audio module (RSA) decreases the amount of time that the signal is modulated to ground.

The blower control switch is integrated into the HVAC control module. The two rocker type switches provide the vehicle operator the ability to select several blower speeds. The HVAC control module uses a bar graph type display to indicate the selected blower speed. The HVAC control module provides a linear pulse modulated (LPM) signal to the blower motor through the blower motor speed control circuit. The blower motor changes speed based on the received LPM signal from the HVAC control module. Power and ground are provided to the blower motor through the battery positive voltage and ground circuits. When the HVAC control module is operating in AUTO mode, the system automatically controls the blower speed. Power and ground are provided to the HVAC control module by the ignition 3 voltage and the ground circuits.

Air Speed - Auxiliary

A rocker type switch on the RSA control module provides the operator the ability to select several blower speeds. The RSA control module uses a bar graph to indicate the selected blower speed. The auxiliary blower motor control processor supplies a 5-volt reference to the auxiliary HVAC control module on the auxiliary blower motor speed control circuit. The auxiliary HVAC control module pulls the 5-volt reference low towards ground to provide a blower motor speed signal. As the voltage decreases the blower speed increases. Power and ground are provided to the auxiliary blower motor control processor through the battery positive voltage and ground circuits. The auxiliary HVAC control module can not request A/C operation from the powertrain control module (PCM).

The HVAC control module controls the distribution of air by the use of a mode actuator. The modes that may be selected are:

Mode Actuator

The mode actuator is connected to the mode door by a cam type linkage system. Depending on the position of the door, air is directed through the HVAC module and distributed through various ducts leading to the outlets in the dash. If the HVAC control module detects a fault with the mode door the HVAC control module will try to drive the actuator for a predetermined amount of time, to defrost, which is the defaulted position for the mode door actuator. When the mode switch is placed in the defrost or defog positions the A/C is commanded ON and the recirculation door is moved to the outside air position to help reduce window fogging. A/C is available in all modes and recirculation is only available in the panel and bi-level modes.

Front Defrost

When defrost is selected, the A/C compressor is activated. The A/C compressor clutch will engage when ambient temperatures are above 3°C (38°F). The blower motor will be activated, regardless of the coolant temperature. The HVAC control module will override the auxiliary HVAC control module so a high volume of air is delivered to the front defrost vents. The rear window defogger does not affect the HVAC system.

Air Distribution - Auxiliary Control

The auxiliary HVAC system provides ventilation for the rear seat occupants. The rear seat occupants will exercise control of the auxiliary air delivery modes, air speed and air temperature setting.

When the auxiliary mode switch is toggled, a signal is sent to the auxiliary mode actuator through the auxiliary mode door control circuit. Power and ground are supplied to the auxiliary mode actuator through the ignition 3 voltage and ground circuits.

A rocker type switch on the RSA control module provides the operator the ability to select several blower speeds. The RSA control module uses a bar graph to indicate the selected blower speed.

The HVAC control module controls the air intake through the recirculation actuator. The recirculation switch closes the recirculation door in order to circulate the air within the vehicle. The outside air switch opens the recirculation door in order to route outside air into the vehicle. Regardless of the blower motor switch position, recirculation is available only in the panel and bi-level mode switch positions. The mode switch must be placed in either the panel or bi-level position and the recirculation switch pressed before the blower motor switch is placed in the OFF position to achieve recirculation with the blower in the OFF position. In order to reduce windshield fogging, outside air is circulated when the mode switch is in the defrost or defog positions. If the recirculation switch is pressed into the ON position when the mode switch is in an unavailable mode position, then the recirculation switch LED will flash 3 times. If the HVAC control module detects a fault with the recirculation door the HVAC control module will try to drive the actuator for a predetermined amount of time, to outside air, which is the defaulted position for the recirculation actuator.

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 be adjusted to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature: