Mode Door Operation

MODE DOOR OPERATION
The DRB trouble codes that indicate a stall failure or a feedback signal failure work together to show what is wrong with the HVAC system. The stall trouble codes mean that the body control module cannot force an actuator to the end of the mode door travel. Internal problems in the body control module will set other short-related trouble codes. Having only stall trouble codes indicates that there is a problem with an open or short circuit in the wiring, a bad actuator, a bad mode door or connecting linkage. It is important that the operation of the mode door be checked if an actuator is removed. This can be checked by rotating the door shaft to confirm that the door will stop at both ends of travel. The actuator itself has no stops. It must have the mode door stop travel to pass the stall test.

A feedback signal failure can occur on the mode door. The body control module monitors the feedback signal to check the position of the actuator. The body control module not only checks the level of the signal but also how much voltage changes.

A feedback failure can occur if there is a short or open circuit in the wiring, a bad body control module, a bad mode door or connector linkage. The easiest way to diagnose this is to use the DRB to actuate the mode door. Note the feedback voltage of the actuator should smoothly change as the actuator is moved. A sudden change in the feedback voltage to a 5.0-volt or a 0.0-volt level indicates the actuator is bad. A fixed feedback voltage that is less than 5.0-Volts or greater than 0.0 Volts without a stall failure, or a short failure indicates that the actuator, the mode door, or a connecting mechanical linkage is jammed preventing movement. A feedback signal voltage that stays on 5.0 Volts or 0.0 Volts indicates a wiring or body control module problem.

The feedback failure trouble code can also occur from a lack of actuator travel. This can be checked by confirming that the feedback signal smoothly changes when the actuator is moved with the DRB III. If the signal is OK and the door travel is not correct, the actuator must be removed and the mode door mechanically checked for proper operation. Typical problems that prevent door movement include screws dropped in the A/C unit or warped doors. Replace any part that is found defective.

Anytime the voltage to the BCM is interrupted for less than 2 minutes or voltage drops below approximately 10.0 Volts (battery disconnected, etc.), it may be necessary to recalibrate the mode door. This can be done by using the DRB, or removing the DRB III(R) (M1) fuse for 10 seconds, reinstall it and then start the engine.