A/C air flow keeps switching from "vent" to "defrost"


#1

My 2002 F-150 AC air flow keeps switching back and forth between the “vent” setting and the “defrost” setting. It blows cold air just fine but I can’t really control which vents it comes out of. Why is it doing this?


#2

Did you notice if it switches from one to the other when you hit the accellerator or go up a hill? If it does, it’s almost certainly due to a vacuum leak.

Russ


#3

Acceleration does not affect it. If I set the blower on “high” or “Max AC” it will only blow out of the defrost vents.


#4

I suppose it could still be a vacuum leak. You could look under the hood for any obvious unattached or broken hoses, though I don’t know if that particular model has anything obvious out in the open.

Russ


#5

Replace the check valve between the intake manifold and the HVAC system, as well as the hoses.

-Matt


#6

there should be vacuum check valve going into fire wall this holds vacuum for vacuum motors under dash under hard exceleration. $6.00 should fix this.