ford f150 ac vent problem

[bay_slayer]

the selector for defrost, vents or floor (the knob) is vacuum controlled. two vacuum cylinders pull doors to direct the air up or down to these vents. the vacuum line starts in the engine compartment right side, rear of the motor. it is a small black line about 3/16 diameter. has a TEE at the firewall about where the glove box is. Goes thru a rubber gromet at that point. Normally the TEE has a bad rubber fitting on it. the TEE also goes to an accumulator tank between the battery and the fender. Look for the TEE and inspect it. Pull the TEE apart and check that you have vacuum at that point. If so, you might have a leak on the backside of the control knob.--- the default position is to defrost (no vacuum) so you can defrost the windows if it fails.