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?
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
Acceleration does not affect it. If I set the blower on “high” or “Max AC” it will only blow out of the defrost vents.
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
Replace the check valve between the intake manifold and the HVAC system, as well as the hoses.
-Matt
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.
I have noticed mine does the same thing. If I am sitting still it works fine. As soon as I accelerate or put strain on the motor it switches to defrost. Any ideas?
Yeah, read the previous posts. Find where you are losing the vacuum.
Assuming that your mystery vehicle’s Blend Doors are controlled by vacuum from the engine, I think that you have a vacuum problem. Time to check those vacuum lines!