The air delivery description and operation are divided into five 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 ignition 3 voltage circuit provides a device on signal. Three integrated potentiometers control mode and air temperature door positions and blower motor speed. The control assembly communicates the mode door position to the vacuum control assembly through five solenoid control circuits. The control module supports the following features:

The blower motor forces outside air into the vehicle's interior. The vehicle operator determines the blower motor's speed by turning the blower motor switch manually or by using the automatic mode. The blower motor will only operate if the blower motor switch is in any position, as long as the ignition switch is in the RUN position and the mode switch is in any position other than OFF. The blower motor and mode switches are located within the HVAC control module.

Power is provided to the blower motor from the blower motor control processor through the blower motor supply voltage circuit. The blower motor control processor receives power from the underhood fuse block through the battery positive voltage circuit. Ground is provided by the blower motor control processor and ground circuit.

The HVAC control module receives power from the instrument panel fuse block on the ignition 3 voltage circuit along with the battery positive voltage circuit. The module is grounded by the ground circuit and splice pack. The HVAC control module communicates directly to the powertrain control module (PCM) on the class 2 serial data and the ECM/PCM/VCM class 2 serial data circuits through the splice pack.

When any blower speed is selected, whether manual or automatic, the blower motor control processor will control blower motor speeds based on voltage signals from the HVAC control module. A 5 volt signal is sent from the blower motor control processor to the HVAC control module on the blower motor speed control circuit. When the driver manually selects a blower speed or the automatic HVAC system determines a needed speed, the HVAC control module will provide a pulse width modulated (PWM) ground. The remaining voltage, at the blower motor control processor, is used to provide a blower motor speed signal. A 12 volt signal is sent to the blower motor from the blower motor control processor on the blower motor supply voltage circuit. The blower motor control processor varies the ground on the blower motor control circuit internally with a separate PWM signal. An open circuit, short to ground or short to battery on the blower motor speed control circuit will disrupt the PWM signal and cause the blower motor to not operate. In automatic operation, the HVAC control module will determine what blower speed is necessary in order to achieve or maintain a desired temperature.

Off Mode

Turn the FAN switch to the OFF position to turn off the HVAC system. When the vehicle is moving, air flowing over the vehicle increases the air pressure just ahead of the windshield. This forces air into the HVAC air inlet and out through any desired mode setting. The HVAC control module attempts to match the inside air temperature and HVAC control module selected temperatures. Since the A/C compressor is not running, the incoming air may be warmed but not cooled.

When the mode switch is turned, a signal is sent from the HVAC control module to the vacuum control assembly. The HVAC control module provides a ground for the necessary mode actuator solenoid. This connects the desired mode actuator to vacuum. The instrument panel fuse block provides power to the vacuum control assembly through the ignition 3 voltage circuit. Ground is provided by the HVAC control module.

The HVAC control module receives power from the instrument panel fuse block on the ignition 3 voltage circuit along with the battery positive voltage circuit. The module is grounded by the ground circuit and splice pack. The HVAC control module communicates directly to the powertrain control module (PCM) on the class 2 serial data and ECM/PCM/VCM class 2 serial data circuits through the splice pack.

Mode Switch

Use the MODE switch in order to change the air delivery mode in the vehicle. The air flow direction will sequence through the following modes:

When in AUTO mode, the HVAC control module will deliver air through the following modes:

Off Mode

Turn the FAN switch to the OFF position to turn off the HVAC system. When the vehicle is moving, air flowing over the vehicle increases the air pressure just ahead of the windshield. This forces air into the HVAC air inlet and out through any desired mode setting. The HVAC control module attempts to match the inside air temperature and HVAC control module selected temperatures. Since the A/C compressor is not running, the incoming air may be warmed but not cooled.

Panel

When VENT is selected, air is delivered through the instrument panel outlets with a small amount of air delivered to the floor. Vacuum is applied to the mode actuator through the Brown vacuum line, and to the defrost actuator through the Red vacuum line. The mode actuator will retract, opening the Vent Door. The defroster actuator will retract, closing the Defroster Door and open the Heater Door through mechanical linkage.

Floor

When Floor is selected, air is delivered to the floor outlets. Vacuum is applied to the mode actuator through the Blue vacuum line, and to the defrost actuator through the Red vacuum line. The mode actuator will retract, closing the Vent Door. The defroster actuator will retract, closing the Defroster Door and open the Heater Door through mechanical linkage.

Defog

When Mix-Blend is selected, air is delivered to the windshield outlets and the floor outlets. Vacuum is applied to the mode actuator through the Blue vacuum line, and to the defrost actuator through the Red and the Yellow vacuum lines. The mode actuator will retract, closing the Vent Door. Applying vacuum to both sides of the defroster actuator will hold the Defroster Door stationary in the half open position. The Heater Door will also be held stationary in the half open position through mechanical linkage.

Defrost

When Defrost is selected, air is delivered to the windshield outlets. Vacuum is applied to the mode actuator through the Blue vacuum line, and to the defrost actuator through the Yellow vacuum line. The mode actuator will retract, closing the Vent Door. The defroster actuator will retract, opening the Defroster Door and close the Heater Door through mechanical linkage.

Outside Air

When the OUTSIDE AIR switch is pressed, outside air is brought into the vehicle. This mode has no effect on the system when FRONT DEFROST mode is selected. OUTSIDE AIR and RECIRCULATION are separate modes and are not available together. When OUTSIDE AIR is selected, when in automatic mode, the HVAC system will stay in this mode until AUTO is pressed again. The outside air LED will not illuminate in automatic mode.

Recirculation

When the recirculation mode is requested, whether manual or automatic, a solenoid inside the vacuum control assembly connects the recirculation actuator to the vacuum source. The HVAC control module grounds the recirculation mode valve solenoid control circuit. The recirculation actuator retracts, closing the recirculation door. This draws air from inside the vehicle instead of fresh air from the outside.

The recirculation mode can be used in both automatic and manual operation. The only time recirculation is not available is when FRONT DEFROST or MIX-BLEND is selected. The recirculation LED will flash to alert the driver that recirculation mode is not available. The recirculation LED will not illuminate in automatic mode. When in automatic mode, recirculation will stay on until either the vehicle operator selects OUTSIDE AIR or the automatic system has cooled the vehicle sufficiently. The recirculation mode has a 10 minute timer. After the timer expires, the HVAC control module will command OUTSIDE AIR. Any time the recirculation button is pressed the A/C compressor clutch automatically engages.

Automatic

The automatic HVAC system will warm up/cool down and maintain the interior temperature of the vehicle by controlling the A/C compressor clutch, blower motor, air temperature, mode and recirculation actuators to achieve the desired temperature. For fully automatic operation, both the blower and mode switches must be in the AUTO position. Blower speeds will change automatically based on inputs to the HVAC control module. The HVAC control module will always come back to the last settings after an ignition cycle.

In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. When the ambient air temperature is below 2°C (35°F) and engine coolant is below 10°C (50°F), the HVAC control mode will go into a purge mode. The HVAC control module will into a 60-second purge to the front defrost outlets. To warm the interior quickly, maximum heat mode is used where the blower is at maximum speed, floor mode, air temperature is in full hot and outside air is being drawn in. The vehicle operator can select the extreme warm setting, but the system will not warm the vehicle any faster. Once the desired temperature is reached, the blower motor, mode, recirculation and temperature will be adjusted automatically by the HVAC control module.

In warm temperatures, the automatic HVAC system will provide A/C in the most efficient manner. To cool the interior quickly, maximum cool mode is used where the blower is maximum speed, air temperature actuator is in full cold and the recirculation actuator is drawing air from inside the vehicle. The vehicle operator can select the extreme cool setting, but the system will not cool the vehicle any faster. Once the desired temperature is reached, the blower motor, mode, recirculation and temperature will be adjusted automatically by the HVAC control module.