HVAC System

CIRCUIT DESCRIPTION
The defroster valve actuator is a DC stepper motor by design. The stepper motor has four field windings that are controlled by the heater and A/C control. When the heater and A/C control needs to move the stepper motor, it energizes the proper field windings which rotates the armature a fixed amount in the proper direction. This fixed amount is called a step. Battery voltage for the stepper motor is supplied by the heater and A/C control when the ignition is in the ON position.

Attached to the stepper motor is a small gear. When the stepper motor is installed on the HVAC module, it meshes with a larger gear which is part of an assembly that drives the linkage of the air mix door.

CONDITIONS FOR SETTING THE DTC
When the ignition is in the ON position, the heater and A/C control measures the current of all four coil wires of the stepper motor. The measured current is averaged by the controller and a "fail threshold" is calculated. After the output current is measured it is compared with the threshold value. If the current is too low then the output from the controller to the stepper motor is presumed to be open or shorted to ground. This step is repeated 3 times within a 10 second time frame.

ACTION TAKEN WHEN THE DTC SETS
^ Stores a DTC 25 in the heater and A/C control memory.
^ The fault detection process is repeated 3 times at a fixed interval of 70 ms to allow for debouncing.
^ When the fault is set, the stepper motor can no longer be controlled.

CONDITIONS FOR CLEARING THE DTC
^ The conditions for the fault are no longer present.
^ The DTC can be cleared using a scan tool.
^ A history DTC will clear after 20 consecutive ignition cycles if the conditions for the fault are not present.

DIAGNOSTIC AIDS
^ If the DTC is a history or an intermittent, try performing the tests shown while moving the harness wiring and connectors. This can often cause the malfunction to appear.
^ Visually inspect the stepper motor connector and harness for damage, corrosion or water intrusion.
^ If condition is not present, refer to Testing for Intermittent and Poor Connections in Diagrams.

TEST DESCRIPTION

Steps 1-3:

Steps 4-11:

Steps 12-25:

The numbers below refer to the step numbers on the diagnostic table.
2. Tests to ensure that the heater and A/C control has the ability to command the defroster valve actuator.
5. Tests for proper operation of the circuit in the high voltage range when the actuator is at rest.
6. Tests for proper operation of the circuit in the high voltage range when the actuator is at rest.
7. Ensures the heater and A/C control has the ability to control the defroster valve actuator.
8. Ensures the heater and A/C control has the ability to control the defroster valve actuator.
9. Ensures the heater and A/C control has the ability to control the defroster valve actuator. Test lamp will be illuminated when connection is made. Test lamp increases intensity when commanded.
10. Ensures the heater and A/C control has the ability to control the defroster valve actuator. Test lamp will be illuminated when connection is made. Test lamp increases intensity when commanded.
11. Verifies the heater and A/C control is getting power from fuse CLAMP 15 Fuse 14.