Posted this in the Superduty forum a few weeks ago and didn't get so much as a peep.

My 2002 is acting really strange. The a/c quits blowing once you get up to 65 mph or so. It looks to me to be a damper problem, because the temperature stays pretty cool, it just doesnt blow as hard (like, almost not at all). You can hear the fan rip roarin away and it sounds like it is dead heading in the lower part of the passenger side in the climate control system. I checked all the dampers and they seem to have full range of motion, it is not the main damper that goes between hot and cold, that one works at any speed, the other two seem to be working also, but for some reason whenever i get up to high speeds, the air just poops out. If you slow back down, rotate your climate control **** from ac to vents and back again, it starts working fine. speed up again, and it poops back out. Is my vacuum pump out?

Any thoughts?

 

When it does this, pop the hood and see if the AC lines are iced up at the firewall. If so, change the cycling switch.

 

Nope, no ice. I thought that too, I will say that when it finally cuts loose, it seems that a TON of water comes out of the water drain.

 

Blend door????

 

Checked the blend door, has full range of motion, and goes from hot to cold just fine. Is there a door that goes to the outside air? It really seems like there is something totally obstructing the air when it does it.

 

I will say that when it finally cuts loose, it seems that a TON of water comes out of the water drain.

That's your clue, the evaporator is freezing up.Replace the cycling switch on the accumulator and it should be fine. It's letting the evaporator temp drop too low allowing the condensation to freeze. Very common failure mode for that switch.

There is no combination of air door positions that will totally block the airflow.

 

I thought that switch only had to do with pressure, not temperature

 

There is a direct correlation between the evaporator temperature and pressure. At ~28psi, the evaporator will be at ~33*. If the pressure drops, so does the temperature of the evap. and freezing occurs. The switch cuts power to the compressor at 28psi to prevent it from going lower and freezing the condensation.
Your switch is probably stuck closed letting the compressor pull the low side down below the freezing point of the evaporator.

Take a look at this Pressure/Temperature chart for R134a: http://www.csgnetwork.com/r134apresstempconv.html

 

Thanks, Ill give that a try. Pressure switches are cheap.

 

Just thought I would report back in. I replaced the pressure switch and it seems to have fixed the problem. I just went to houston and back with no issues(3 hours each way). Thanks for all the help guys. Rep points given.