Modern cars and light trucks use small electronic actuators to control the temperature and mode selections. These electronic controls replaced the vacuum operated actuators, used through the 80's and into the 90's, to pivot the blend door and the mode door into the selected position. Unlike the vacuum controls that are either "ON" or "OFF", the electronic actuator is adjustable to an infinite number of positions.With the introduction of the infinitely adjustable "Electronic Actuator" it became possible to have computer controlled climate control as well as dual zone climate control.
The electronic actuator uses a reversible electric motor that is attached to the blend or mode door pivot point by gears inside the actuator. By varying the voltage supplied to the actuator motor, the door is moved to the requested position. In vehicles with Computerized Climate Control Systems, the blend door actuator is controlled by an on board computer that adjusts the temperature output to achieve the exact temperature the operator requests...and then keeps adjusting the blend door actuator to maintain that temperature when it is achieved.
Symptoms of a faulty actuator include a mode door that seems to be stuck in the defrost position or a blend door that seems to be stuck in heat or cool position. Occasionally this is also accompanied by an audible clicking or thumping sound from behind the dash. Replacement of either the blend door actuator or the mode door actuator may require the removal of the dash.