The air delivery description and operation is divided into 6 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 control module supports the following features:

Door Actuators

The actuators are 2-wire, bi-directional electric motors that share the same common control circuit. When the operator adjusts the actuators, the control module can only operate one actuator at a time due to the shared common control circuit.

The actuator control priority sequence when the MODE is set to VENT or RECIRC or BI-LEVEL from other modes shall be:

1. Mode Actuator
2. Defrost Actuator
3. Left Temperature Actuator
4. Right Temperature Actuator
5. Recirculation Actuator
6. Auxiliary Mode Actuator
7. Auxiliary Temperature Actuator

The actuator control priority sequence when the MODE is set to HEATER, DEFOG, or DEFROST from other modes shall be:

1. Recirculation Actuator
2. Mode Actuator
3. Defrost Actuator
4. Left Temperature Actuator
5. Right Temperature Actuator
6. Auxiliary Mode Actuator
7. Auxiliary Temperature Actuator

The control circuits use either a 0 or 12-volt value to coordinate the actuator movement. The HVAC control module reverses the polarity of the circuits to move the actuator in the opposite direction. The HVAC control module determines the door position by counting motor pulses on one of the control circuits. These pulses are small voltage fluctuations that occur when the brush is shorted across 2 commutator contacts as the motor rotates. As the actuator shaft rotates, the HVAC control module monitors the voltage drop across an internal resistance to detect the pulses. The HVAC control module converts the pulses to counts with a range of 0-255 counts. The HVAC control module uses a range of 0-255 counts to index the actuator position.

The HVAC control assembly applies voltage to the blower motor control circuit that corresponds to the selected blower speed. The resistors and the blower motor are in a series circuit. The following list represents the number of resistors in series with the blower motor per particular speed request:

When the operator requests High speed, the HVAC control assembly applies voltage to the blower motor relay through the high blower motor control circuit. The voltage energizes the blower motor relay, connecting the blower motor to battery positive voltage.

OFF Mode

When the OFF position is selected, the HVAC control module applies voltage to the fan off input within the HVAC control module through the off blower motor control circuit. When the HVAC control module receives this input, any A/C request will be cancelled and the A/C request LED will turn OFF. The auxiliary blower motor, recirculation, mode, and temperature actuators are adjustable in OFF mode.

The auxiliary blower motor is controlled by the front auxiliary blower motor switch. If the vehicle is equipped with rear DVD (U32), the auxiliary blower motor can be controlled by the overhead console, vehicle entertainment system (VES). The front auxiliary blower switch must be in the AUX position in order to control blower speed from the VES. The VES will send a class 2 message to the HVAC control module requesting a blower motor speed.

The auxiliary blower motor control module is an interface between the HVAC control module and the auxiliary blower motor. The auxiliary blower motor speed control, auxiliary blower motor speed signal, blower motor speed control, and ground circuits enable the control processor to operate. The HVAC control module sends the commanded blower motor speed to the control processor through a 0 to 8-volt linear ramped blower motor speed control circuit. As the blower motor speed control voltage increases, the blower motor speed increases. The control processor sends the actual blower speed to the HVAC control module through a 0 to 12-volt blower motor speed signal circuit. The control processor uses the blower motor ground as a low side control to adjust the blower motor speed. The underhood fuse block supplies 12 volts to the blower motor through the blower motor voltage supply circuit.

The HVAC control module controls the mode actuators in order to distribute airflow to a desired outlet. When the mode door is moved to the mix-blend or defrost position, the A/C compressor clutch engages and the recirculation actuator will be moved to the outside air position. Regardless of the mode setting, a small amount of air will be diverted to the defrost ducts to reduce windshield fogging. On startup, the HVAC control module will place the mode door in the last selected position.

The optional auxiliary HVAC system allows the driver to adjust the amount of air directed to the rear of the vehicle using the front mode switch. To direct air to the rear of the vehicle using the floor vents, set the mode switch on the HVAC control module to the BI-LEVEL, FLOOR, DEFROST, or MIX-BLEND modes. To direct air through the headliner vents, set the mode switch on the HVAC control module to the VENT or MAX modes.