The blend door on an F-150 Ford pickup controls the airflow through the vents. When you select which vents you want air coming from, the blend door switches to the setting you chose in order to re-route the incoming air.
The ETC, the control head in the Ford F-150's dashboard, provides the first step of operational control over the blend door. By selecting the different vent settings, you're sending a signal to operate the blend door.
The ETC sends a signal to the blend door actuator. The actuator has a small arm on it that pulls a lever on the blend door assembly, which moves the blend door itself.
Older Ford F-150 ETCs use vacuum control to send the operational intent from the ETC to the blend door assembly, while newer models use electrical circuitry to command the blend door to operate the assembly.
