As the title states...
Last Oct. I noticed that my ac/heater started to intermittently stop blowing from AC fresh air/recir/vent/etc. mode and stick to defrost only.
Do I have a bad switch and or a vacuum issue here? It's a '01 F350 CC 7.3

 

I have the same problem. Just bought my '05 SD truck and knew that the climate controls and ESOF actuation were wonky. So far have confirmed vacuum pump is not operating.. Found this video on you tube that will try first.. Might be helpful for you.

 

on a 2005 fuse f2.27 controls vacuum pump and esof solenoid

 

See HVAC FAQs, link is in my signature.

 

its pobably the one on the fender well on a 01, i have replaced that several times before, get's moisture in it, i got the same rig as you,so it's different from a 05-06

 

It's looking like the vacuum pump has stopped working. Will not engage at all when vacuum isn't present. I opened it and physically saw that that piston wasn't doing anything.
Well..looks like I need to buy a new unit. It does after all has 245xxx miles on it. I'd be wore out too.
Thanks for all your replies.

 

On the 01, it could be the BLEND DOOR..... if they still had one in 01. Ford products in the late 90's had that problem.