Air Delivery Description and Operation

The air delivery controls are divided into two primary areas. The first, air speed, is dependent upon blower motor speed. The second, air distribution, is related to various single or multiple air outlets.

Air Speed
The blower motor forces ambient air into the vehicle's interior. The vehicle operator determines the blower motor's speed by toggling the blower motor switch manually or by using the automatic mode. The blower motor will always operate at some speed as long as the HVAC control module is in any position other than OFF, and the ignition switch is in the RUN position.

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 rear fuse block through the battery positive voltage circuit. Ground to the blower motor is provided by the blower motor control processor and ground circuit.

The HVAC control module is the interface between the vehicle operator and the Instrument Panel Integration Module (IPM). The HVAC control module receives power from the rear fuse block on the ignition 3 voltage circuit. The IPM is also powered from the rear fuse block through the ignition 3 voltage circuit along with the battery positive voltage circuit. Both modules are grounded by the ground circuit.

When any blower speed is selected, whether manual or automatic, the IPM receives an input from the HVAC control module. The IPM and HVAC control module communicate back and forth over the HVAC control module data signal and the HVAC control module clock signal circuits. Once received by the IPM, a 5 volt Pulse Width Modulated (PWM) signal is sent to the blower motor control processor on the blower motor speed control circuit. When a low blower speed is requested, the PWM signal is reduced. When a higher blower speed is requested, the PWM signal is increased. A 12 volt signal is then sent to the blower motor from the blower motor control processor on the blower motor supply voltage circuit. The blower motor control processor interprets the PWM signal and varies the ground on the blower motor control circuit internally. An open, 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. The blower motor flange ground circuit is used to limit radio frequency interference and does not affect blower operation. In automatic operation, the IPM will determine what blower speed is necessary in order to achieve or maintain a desired temperature.

When in manual mode, the driver can change the blower speed by toggling the FAN switch. If the driver toggles the FAN switch once, the blower speed will increase/decrease 1 level. Holding the FAN switch will increase/decrease the blower speed to the maximum/minimum speed.

Off Mode
When the Vehicle is moving, air flowing over the vehicle increases the air pressure just ahead of the windshield. This forces air into the ambient air inlet, into the HVAC module and out through the floor and windshield outlets. The IPM attempts to match the inside air temperature and driver selected temperatures. Since the A/C compressor is not running. The incoming air may be warned but not cooled.

If the right air temperature switch assembly has been turned ON, it can be turned off by pressing the OFF switch once on the HVAC control module. Pressing the OFF switch a second time will turn off the entire HVAC system.

Recirculation Mode
When the Recirculation (RECIRC) switch is pressed, a signal is sent from the HVAC control module to the IPM and on to the recirculation actuator through the recirculation door control circuit. When the logic circuit receives the signal, the recirculation actuator moves into the recirculation mode. This brings air from inside the vehicle instead of air from the outside.

The rear fuse block provides power to the recirculation actuator through the ignition 3 voltage circuit. Ground is provided by the low reference circuit, the IPM and ground circuit.

The recirculation actuator is an electronic stepper motor with feedback potentiometers. Zero volts drives the actuator in one direction while 5 volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, actuator rotation stops. A 5 volt reference signal is sent out over the 5 volt reference circuit, from the IPM, to the recirculation actuator. When a desired recirculation setting is selected, whether manual or automatic, the logic circuit is used to determine the recirculation door position signals value. A separate 5 volt reference is sent from the IPM to the logic circuit. The IPM software uses this reference voltage to determine the recirculation actuators position through the recirculation door position signal circuit. The motor moves the recirculation door to the desired position. Ground for the recirculation actuator are provided by the low reference circuit and IPM

The recirculation mode can be used in both automatic and manual operation. The only time recirculation is not available is when FRONT DEFROST or VENT is selected. The RECIRC LED will flash to alert the driver that recirculation mode is not available. When manually selected, recirculation will stay on until either the vehicle operator selects VENT or the recirculation mode has been on for approximately 10 minutes. Recirculation will come on automatically when maximum cooling is required in the automatic mode. When the HVAC system is off, pressing the recirculation switch will set the blower, mode and A/C compressor clutch into automatic mode.

Automatic Modes
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 IPM. The IPM will always come back to the last settings after an ignition cycle or, if equipped, to the last settings of the driver by pressing the UNLOCK button on the remote keyless entry fob and placing the ignition in RUN.

Moisture tends to accumulate in the HVAC module assembly after the engine has been shut off. The IPM will purge the HVAC module of stale air and moisture to prevent window fogging. The mode actuator will move to the floor mode and run the low speed blower for 10 seconds in order to purge out the air and moisture. Purge mode can be bypassed if any manual modes are selected during the 10 second purge mode. In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. On cold engine start-up, in cold weather, the IPM moves the mode outlets to the windshield and operates the blower for 3 minutes. As coolant temperature increases, blower speed is increased. This is done to prevent windshield fogging. The purge can be bypassed if any manual modes are selected. To warm the interior quickly, maximum heat mode is used where the blower is at maximum speed, floor mode, air temperature actuator is in full hot and ambient air is being drawn in. The vehicle operator can select the extreme warm setting of 32°C (90°F), 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 IPM.

In warm temperatures, the automatic HVAC system will provide A/C in the most efficient manner. On vehicle start up, the IPM will monitor for conditions that indicate the possible presence of moisture in the HVAC module. To avoid blowing hot air and moisture out at the driver, the IPM moves the mode door toward the floor and runs the blower for 3 seconds. This purge mode can be bypassed if any manual modes are selected. To cool the interior quickly, maximum cool mode is used where the blower is at 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 of 16°C (60°F), 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 IPM.

Air Distribution
When the mode switch is toggled, a signal is sent from the HVAC control module to the IPM and on to the mode actuator through the mode door control circuit. When the logic circuit receives the signal, the mode actuator moves into the desired position.

The rear fuse block provides power to the mode actuator through the ignition 3 voltage circuit. Ground is provided by the low reference circuit, the IPM and ground circuit.

The HVAC control module is the interface between the vehicle operator and the IPM. The HVAC control module receives power from the rear fuse block on the ignition 3 voltage circuit. The IPM is also powered from the rear fuse block through the ignition 3 voltage circuit along with the battery positive voltage circuit. Both modules are grounded through the ground circuit.

The mode actuator is an electronic stepper motor with feedback potentiometers. Zero volts drives the actuator in one direction while 5 volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, actuator rotation stops. A 5 volt reference signal is sent out over the 5 volt reference circuit, from the IPM, to the mode actuator. When a desired mode setting is selected, whether manual or automatic, the logic circuit is used to determine the mode door position signals value. A separate 5 volt reference is sent from the IPM to the logic circuit. The IPM software uses this reference voltage to determine the mode actuators position through the lower mode door position signal circuit. The actuator moves the mode door to the desired position. Ground for the mode actuator are provided by the low reference circuit and IPM.

Vent Mode
When the driver presses the VENT switch, ambient air is brought into the vehicle and the A/C compressor clutch is turned off. The VENT button has no effect on the HVAC system when the FRONT DEFROST mode is selected. If the VENT button is pressed while system is in FRONT DEF, the VENT LED will flash 3 times to remind the driver that VENT is not available. When the HVAC system is off, pressing the VENT switch will put the HVAC system into automatic mode.

Front Defrost
The front defrost mode is the only setting that is not controlled by the automatic HVAC system. When defrost is selected, by the vehicle operator, the A/C compressor is activated. The blower motor will be activated, regardless of coolant temperature and air will be directed toward the windshield. The rear window defogger does not affect the HVAC system at all.