Automotive Heating Diagnosis – Verify Blend Door Operation

Another important step in diagnosing the heating system is to make sure that the blend door is functioning properly. The blend door is the part in the duct box system that controls airflow through or around the heater core. The operation of the door can be checked by moving the temperature control from full-cool mode to full-heat mode while monitoring to see if the temperature changes to the desired selections.

Systems that use a cable to move the temperature door will make a distinctive sound as the temperature selector reaches the full-heat and full-cool settings. Many systems use electric actuators that do not make an audible sound to indicate that the door is reaching the correct positions. These systems will require a calibration to be performed if possible.

The calibration procedure can be performed on most systems by pressing a sequence of buttons on the control head that forces the actuator to move to the limits of travel. The doors can be calibrated with a scan tool as well. This action will sometimes cause the system to function correctly after being performed. It may be necessary to remove the actuator and move the blend door by hand to make sure the door has full range of motion.

The temperature door can be actuated using pliers to physically move the

The temperature control electric actuator is being removed to inspect the temperature door during diagnosis. This actuator is held in place with three small

Removing the temperature control electric actuator will allow the temperature door to be inspected as well as the door to be physically actuated by

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