Servo motors are used to control flaps and valves in the heater and air-conditioning systems. By re-positioning the flaps and valves, the output temperatures can be adjusted to the temperature desired by the occupants of the vehicle.
Servos can be one of two types: Vacuum servos (top) and electrical servos (bottom).
Vacuum servos consist of a sealed housing divided by a flexible diaphragm. The diaphragm seals the housing into two separate chambers.
One of the chambers is opened to the atmosphere; the other side is sealed and has a fitting for a vacuum hose connection. A spring forces the diaphragm towards one side when no vacuum is applied to the diaphragm.
A linkage from the diaphragm connects it to the component to be moved.
When vacuum is supplied to the sealed side of the diaphragm a difference in air pressure occurs across the diaphragm. The force developed overcomes the spring force and the diaphragm moves, compressing the spring and moving the linkage.
Admitting air to the vacuum side of the diaphragm allows the spring to return the diaphragm to the rest position.
Some diaphragms use two sealed chambers with neither end open directly to the atmosphere.
When movement is desired vacuum is applied to one side of the diaphragm and atmospheric air pressure to the other.
The vacuum delivered to the diaphragm chamber can be controlled by a solenoid valve.
When the electronic control unit wants to move a damper flap, it grounds the electrical circuit for the solenoid winding. Vacuum from a storage tank is applied through the solenoid valve to the diaphragm.
To release the diaphragm, the electronic control unit open-circuits the solenoid, and the diaphragm returns to the rest position.