The air delivery description and operation is divided into 4 areas:

HVAC Control Assembly

The HVAC control assembly is a non-class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The ignition 3 voltage circuits provide power to the control assembly. The control assembly 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 assembly 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 HVAC control assembly 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 HVAC control assembly. Once the position signal and the commanded value are the same, the HVAC control assembly changes the control signal to 2.5 volts.

The HVAC control assembly applies ground 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 ground directly to the blower motor.

OFF mode

When the OFF position is selected, the HVAC control module applies ground to the fan off input within the HVAC control assembly through the off blower motor control circuit. When the HVAC control assembly receives this input, any A/C request will be cancelled and the A/C request LED will turn OFF.

The HVAC control assembly 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 assembly will place the mode door in the last selected position.

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 coordinate the actuator movement. The HVAC control assembly reverses the polarity of the control circuits to move the actuator in the opposite direction. The recirculation operation can function with blower motor in either the ON or OFF position. The A/C compressor automatically comes ON when recirculation is selected. Recirculation is not available in Defrost and Mix-Blend mode.