Hi, I own a 2002 Ford F150XL 4.2L. I swore I changed the air cabin filter before but I can not find it now. So I may be wrong. But problem is When AC is on you can hear the blower going strong. But the air (Which is cold) The air coming out of Vents are weak. During the use of it the vents air does get strong then weaker on its own. Not strong enough to get the cabin cold but you can hear and feel it get stronger and weaker . Like high and low coming from vents on its own. But never does it come out of vents strong enough for the cabin of truck to get cold. Mind you the air is cold. Put AC on max and fan speed is on high. at this point you here the blower going fast and hard. But air coming out of vent is feeling low but air coming out of vent though weak will fluctuate to a little higher or harder coming out then lower. But never strong enough for cooling truck cabin. I replaced the control panel and all switches behind it. But nothing has changed.

 

https://www.fordf150.net/forums/viewtopic.php?t=5069

Sounds like the filter is full. Above is a post about its location.

"Well, it turns out that the '02s do have a cabin air filter. The local Ford parts dude has them and he says its located adjacent to the heater box and accessible from the hood-windshield cowl area."

 

Cabin air filters were NOT factory installed on that era of trucks. That old thread from the other forum is flat out wrong. Ford did offer kits that were dealer-installable, though. Owner-enthusiasts also came up with their own method of installing one.

Most likely, the OP has a dirty evaporator core if no filter was ever installed. Pull the blower motor and inspect. Other possibility is an obstruction in the plenum or a squirrel cage that is loaded with dirt.