The duct box doors located inside the duct box can be moved by a cable, a vacuum actuator, or an electric actuator. Diagnosis of all of these types of mechanisms will be covered later in this chapter.
Below is a quick description of how these systems operate. Cable-Operated A cable is used to connect a moveable arm on the control head to a moveable arm on the duct door. When the control head arm moves, it causes the duct-door arm to move, which moves the duct door.
Vacuum Actuator Vacuum is supplied to the control head, which directs the vacuum to vacuum actuators through colored vacuum hoses. The vacuum actuators are connected to the duct doors and are typically spring loaded to a fixed position. When vacuum is directed to the vacuum actuator, the device moves the linkage, which moves the duct door.
Electric Actuator The duct doors are operated by small electric motors that are directly mounted on the doors. These actuators are reversible and move the doors back and forth as directed by the control head.
