Yes. The vacuum lines control all of them. I am not disputing that.

But, given no other problems, when the vacuum line under the hood is disconnected, the defroster vents fail closed and the front floor vents fail open.

In normal operation when the engine is running and the floor vents button is not depressed vacuum is present at the floor vent actuators keeping the flaps closed. When the floor vents button is pressed, vacuum is cut off and the flaps open.

Control of the defrost vents is the opposite. When the engine is running and the defrost vents button is not depressed no vacuum is present at the defroster vent actuators and they are closed. When the button is pressed, vacuum is introduced and the flaps open.

So when no vacuum is present because the supply is disconnected under the hood, the front floor vents are open, the defrost vents are closed and the buttons at the control unit have no effect one way or the other.


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'80 DL 2 door, '89 DL Wagon, '15 XC70 T6