Air Handling: Notes

The defrost mode door, panel/floor mode door, temperature blend door and air inlet mode door actuators are stepper motors. There is no feedback circuit or potentiometer for these actuators. The pins labeled 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 Power and the Heating, Ventilation, and Air Conditioning (HVAC) module controls each phase by turning on/off a low side output for the different phases. The motor turns or "steps" through a series of motions to position the actuator. The Heating, Ventilation, and Air Conditioning (HVAC) 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 Heating, Ventilation, and Air Conditioning (HVAC) 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.

When an airflow mode, desired driver or passenger temperature or fresh air or recirculation mode is selected the Heating, Ventilation, and Air Conditioning (HVAC) module drives the actuator motor in the direction necessary to make the actuator position agree with the expected Heating, Ventilation, and Air Conditioning (HVAC) module position to be obtained.

The Heating, Ventilation, and Air Conditioning (HVAC) module sends a Pulse Width Modulated (PWM) signal to the blower motor speed control to control the blower speed. The blower speed control provides variable ground feed for the blower motor. A delay function provides a gradual increase or decrease in blower motor speed under all conditions.