Movement of flaps and valves can also be achieved by using electrical motors that respond to output signals from the electronic control unit.
An electrical servo motor is used to move the blend door. It incorporates limit switches to open circuit the motor when the maximum travel of the door has been reached. This stops the motor from burning out.
Some electrical servos also utilise a carbon film potentiometer to signal to the electronic control unit the position of the blend door and if it is moving the direction in which it is travelling.
A voltage from the electronic control unit is applied across the potentiometer. The wiper is connected by the signal wire to the electronic control unit.
For maximum cooling effect, the blend door is moved to stop air flow through the heater core, and in this position, the voltage signal back to the control unit is at its lowest value. As the blend door is moved to allow air flow through the heater core, the voltage at the signal wire progressively increases until at the maximum heat position the voltage signal is at its highest.
The control unit is thus aware of the blend door position, movement, and direction of travel.