I recently bought a used F150 supercab XLT 4x4 automatic with 5.4 I tested the a/c before I bought and felt satisfied that it was in good working order. Fan works, Max a/c works, heater and defroster works. I towed a small boat last week through the desert in 95 degree heat and noticed that the fan would stop blowing out the main vents as I accelerated and under a load uphill but on deacceleration or down hill the fan would blow cold air at full force. The defroster vents seemed to blow at a constant speed regardless of the main vents blowing or not. I know this can't be right! Has anyone had this problem and / or have any solutions or suggestions? Your help and advice would be appreciated.

 

behind the battery theres a small vacume hose from the engine to the firewall check to see if it doesnt have a leak

 

I've got a 94 f150 4x4 302 AT and the same thing is happening to me. Since I don't see any real obvious vacuum lines behind the battery on this rig, I suspect that what I need to look for is a vacuum leak of some kind. I'll do that, but now I'm curious. Why is a vacuum leak causing the fan motor to cut out? I can understand the flappers in the heater box moving around, but that would/should only give me cold defrost (if that's the default position). Right?
Or is the AC actually hooked into the brain somehow and so responds to the load that is on the motor?

 

Don’t know if this helps with your situtation but the compressor will shut down when your at WOT (wide open throttle).

 

Here's some info.

https://www.f150online.com/forums/sh...ghlight=vacuum


Your vacuum will drop with an increase in rpm. If you have a leak, you may not have enough reserve to prevent the airflow diverter from defaulting to defrost.