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

Important: Anytime a repair or the HVAC control module connection is opened, the control head will not display correct temperature information for approximately 5-10 minutes after reconnecting.

HVAC Control Module - Automatic Climate Control

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. To change the HVAC display from degrees Fahrenheit to degrees Celsius press the Auto, Econ and Mode buttons together for 3 to 5 seconds. The battery positive voltage and ignition 3 voltage circuits provide power to the control module. The Body Control Module (BCM), which is the vehicle mode master, provides a device on signal. The control module supports the following features:

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.

Recirculation Actuator

The recirculation actuator is a 2-wire bi-directional electric motor. Two control circuits enable the actuator to operate. The control circuits use either ground or a 12-volt value to co-ordinate the actuator movement. The HVAC control assembly reverses the polarity of the control circuits to move the actuator in the opposite direction.

Blower Motor Control Processor

The blower motor control processor is an interface between the HVAC control module and the blower motor. The blower motor speed control, battery positive voltage and ground circuits enable the control processor to operate. The HVAC control module provides a PWM signal to the control processor in order to command the blower motor speed. The HVAC control processor provides a constant ground to the blower motor and controls the blower motor speed by varying the supply voltage to the blower motor.

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

OFF Mode

Press the Off switch to turn off the HVAC control module. 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 into the HVAC module and out through the floor and windshield outlets. Since the A/C compressor is not running, the incoming air may be warmed but not cooled. Recirculation is not adjustable in OFF mode.

Purge Operation

If the ambient air temperature is less than 2°C (35°F) and the coolant temperature is less than 10°C (50°F), then a 75-second purge will occur. The purge will begin as soon as the coolant temperature reaches 4°C (40°F). The mode door will move to the defrost position and the blower motor will be near half speed.

The HVAC control module controls the mode actuator in order to distribute airflow to a desired outlet. The mode switch provides the vehicle operator with the ability to override the automatic setting. When the mode door is moved to the 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. This vehicle may be equipped with a passenger compartment air filter.

After a malfunction occurs to the mode actuator it is driven to the Defrost position. On startup, the HVAC control module will place the mode door in the last selected position.

The HVAC control module controls the air intake through the recirculation actuator. Recirculation is only available when the HVAC control module is in Panel or Bi-level. The recirculation mode will stay on and the recirculation indicator will illuminate until either outside air or a non-recirculation mode is selected. When the recirculation actuator sees an open in the control circuit, the actuator will default to the recirculation position.

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 you must set the temperature and press the drivers temperature knob. 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: