I have a 2010 jeep liberty and suddenly the heat and A/C stopped working by the control dials and settings. It just blows hot air constantly through defroster
RESPONSE
What you have described is a faulty mode door actuator.
The heating and Air Conditioning system uses a reversible, 12-volt direct current (DC) servo motor which mechanically positions the mode-air door. The mode door actuator (1) is located on the driver side of the HVAC housing.
The mode door actuator is interchangeable with the actuators for the blend-air door and the recirculation-air door. Each actuator is contained within a black molded plastic housing. Each with an integral wire connector receptacle (2) and integral mounting tabs (3). This allows the actuator to be secured to the HVAC housing. The mode door actuator output shaft (4) is connected to the linkage that drives the mode-air door. The mode door actuator does not require mechanical indexing to the mode-air doors, as it is electronically calibrated by the A/C-heater control.
How the Mode Door Actuator Works
The mode door actuator is connected to the A/C-heater control. It is connected through the vehicle electrical system by a dedicated two-wire lead and connector of the HVAC wire harness. The floor, defrost and the panel-air doors can be moved in two different directions by the mode door actuator. When the A/C-heater control pulls the voltage on one side of the motor connection high and the other connection low, the mode-air doors will move in one direction. When the A/C-heater control reverses the polarity of the voltage to the motor, the mode-air doors moves in the opposite direction. Furthermore when the A/C-heater control makes the voltage to both connections high or both connections low, the mode-air doors stop and will not move.
The A/C-heater control uses a pulse-count positioning system. This is to monitor the operation and relative position of the mode door actuator and the mode-air doors. The A/C-heater control learns the mode-air doors stop position during the actuator calibration procedure. And will store a diagnostic trouble code (DTC) for any problems it detects in the mode door actuator circuits.
The mode door actuator cannot be adjusted or repaired. It must be replaced if inoperative or damaged because of its design.