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so my 93 f150 will only blow on defrost. i can grab the little arm under the dash and can make it go to defrost or vents. if i turn the selector **** to ac the ac will come on but i have to manually move the arm under the dash to either defrost or vent. that's the only 2 selections i can use. i cant find any vacuum leaks or any obvious issues. but i really dont understand the system fully either.
You either have a vacuum leak or the vacuum line for the system is no longer connected to the intake.
Most of the time this problem is from something under the hood and most of the time it is the white hose for the outside air motor as in the photo below.