Air Handling
Based on the climate control system temperature settings, the temperature door actuator directs airflow through the evaporator and heater cores as needed. The air distribution door actuator controls the airflow to the defrost, floor or panel register vents. The air source is from outside air or recirculated passenger compartment air as determined by the air inlet door position. The air inlet door actuator contains a reversible electric motor with no potentiometer and is either fully open or fully closed.
The air distribution door and temperature door actuators are stepper motors. There is no feedback circuit or potentiometer for these actuators. The pins labeled Door A, B, C and D are for the different phases (coils) of the motor. All the phases for a given motor are fed from a common power lead, the pin labeled Door Power and the HVAC control module controls each phase by turning a low side output on or off for the different phases. The motor turns, or steps, through a series of motions to position the actuator. The HVAC control module tracks the actuator positions by counting steps and periodically calibrates itself so it knows how many steps there are for the full range movement of the actuator. The HVAC control module sets Diagnostic Trouble Codes (DTCs) by monitoring the current flow through a given phase. If it sees too much current it identifies it as a short circuit. If it does not see any current it identifies it as an open circuit.
The blower motor is controlled using a blower motor resistor. The blower motor resistor uses 3 resistance elements that are wired in series on the ground side of the blower motor. The blower motor has 4 speeds:
- for low speed, all 3 resistors are used
- for medium-low speed, resistors 1 and 2 are used
- for medium-high speed, resistor 1 is used
- for high speed, the blower motor is provided a path directly to ground and the blower motor resistors are not used
The HVAC control module grounds 1 of the 4 circuits to control the blower motor speed.