I would like to post this im a broader forum bc it doesn't pertain only to Broncos. My ac cuts out when I am driving. The air switches from the dash to defrost and since there are no refrigerant coils, it blows hot air.
I thought maybe it had something to do with engine rpm but i van rev the engine in P and it does fine . It' seems to be only when the engime is under load.
Helpme, I'm melting

 

Our ducting and blend doors operate on a vacuum. When you are under load at open throttle, the vacuum being created by the engine is at a minimum(it's getting all the air it needs from the throttle body or carburetor. For that reason, We have a small vacuum canister to maintain some vac during these conditions(On mine, it's a round mini-coffee can looking thing on the driver's side fender). There's also a check valve on the firewall that should only allow the vacuum to flow in one direction so you don't lose the vacuum required by the AC to the engine.

If your system is changing how it works under load, you have a vacuum leak. First, find that check valve and suck on both sides. If the air flows in both directions, replace it. Then do the same for the vacuum canister to make sure it doesn't have a leak. If those are both tight, you have the unenviable job of finding the leak(s) by checking the miles of tubing and vacuum pots under the dash. You will want to look up the vacuum diagram for your particular year and you will need to start checking the different legs of the system for leaks.

I have a slower leak on mine that causes the doors to shut off dash vents when under heavy acceleration. I've not found my particular leak yet but can say the process is an incredible pain in the ***.

 

The most common cause of your problem is the white vacuum hose for recirculation door disintegrates over time. It's located on top of the heater/AC plenum on the passenger side of the engine compartment.Looking at it too long will cause it to crack/break if you are not careful.

 

Got it fixed. Thanks for the belp guys. Definitely pointed me in the right direction.