Finally got my AC up and running in my 93 F150 302. Now when I'm accelerating around 2000 to 2500 RPM the airflow switches to the cabin vents to the defroster. When you let off of the accelerator for cruise it goes back to the cabin vents. Any ideas?

 

You have a vacuum leak. It can be a bad hose, the control panel in the dash but more than likely it's a vacuum check valve. There should be one on the vacuum line going into the firewall. They look like this:


If it's this check valve, it'll be the cheapest thing you'll ever fix on yer truck. Cost is under 5 bucks. This valve only allows vacuum to go one way only. When you accelerate, the vacuum falls off and this valve is suppose to hold the vacuum in the control panel for the environmentals until you get back off of the gas and vacuum goes back up. They CAN be installed backwards so be aware.

 

A bad check valve would be awesome! It might fix a surging problem I'm having. Thanks for the info!

 

OK, Did the "oral" vacuum test on the pictured valve. Also did the same on the white line to the valve that's shown in pic 2 but couldn't find the other end of that line since it goes into a harness that's taped up. The valve in pic 1 seems good. No flow one direction and flow in the other.

 

Looks like the control unit in the dash is bad if the valve is working correctly. Check the vacuum lines as close as you can for any leaks. If you can't find one, the dash unit is bad. Might wanna check on Fleabay for a used one. Should cost you less than 25 bucks for a reconditioned one.

 

Found a crack in the line to the actuator in the second pic! Works like a champ now. Now if I could only fix my surging issue I'll be 100% with this truck! Thanks to everyone for your help.

 

Just wanted to add my fix to the same issue. My vac lined failed at the actuator too. See my photos at https://www.f150online.com/forums/me...cuum+line.html