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!
Mine does the same except my air conditioner works normal but my heat when i use switch to heat it automatically switches to dedrost and will max airflow at my windshield and you feel just a little bit out vent very very little. Do you know why it would do this? I took it to the dealer and they said it had an aftermarket windshield that is why. It doesnt show a windshield was even replaced and it is a 2018 and had only 60000 miles on it. The car fax showed a regular maintainance work. But no windshield. Please help if you can.
Carfax only has what is reported to them . The windshield could have been replaced but not reported . Your vehicle is out of warranty and I think the dealer just does not want to mess with the problem. You can try another dealer or a shop that does this kind of work but expect to pay for it.
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I have a hard time believing a replacement windshield has anything to do with it. Did they give an explanation or just that flimsy excuse? If they figured out it’s the windshield, then they should know how to correct it. If you refused a repair becasue you have to pay for it then you are wrong. As @VOLVO-V70 said, your truck is out If warranty.
Since there is no warranty remaining, take it to a good shop and ask them for an analysis. Expect to pay for the evaluation in addition to the repair if you want them to fix it. They should provide a repair estimate clearly explaining what they will do in addition to an estimated cost.
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+1
And, I would be willing to bet at least the price of a cup of coffee that replacing the windshield has nothing to do with the current problem.
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Although I agree, nothing surprises me anymore with modern vehicles… 
I had to look real hard to make sure my disability parking thingy didn’t affect anything while hanging from the mirror on my latest ride… lol
Your issue was interfering with sensors, not the AC system. The defroster vents are in the dashboard, but are mostly below the windshield.
Yes of course I know all that, my point was with todays vehicles, nothing would surprise me anymore, including something stupid like the wrong windshield made my whatever stop working…
Can’t just make a general statement on here without having to give a detailed description with it… 
An open circuit in the heated windshield grid could cause a HVAC system fault.
Does a 2002 F150:have an electrically heated windshield? I checked the features list at motortrend.com and it says there are heated wipers, but not the Quiickclear option available on some Fords.
The person in post # 10 with the same problem only different (heater defaults to defrost) stated the vehicle is a 2018. Could be an F150, BMW, Tesla etc.
The first domestic vehicle with a wiper heating grid in the windshield was 1996.