I have a 2004 F250 Lariat with electronic climate control system. About a month ago I was driving with the A/C on and it was blowing through the normal vents on the dash. Then randomly it just started blowing out of the defrost vents. Did this for maybe 15 seconds, then back to normal operation. Going to manual fan control did not seem to change the issue, however I usually operate it in auto mode. One thing I did notice was when on the highway if it happens and I tap the brake, it goes back to normal operation. WEIRD.
Any thoughts on this???

 

Vacuum leak.

 

The system is controlled by vacuum. If there is not enough vacuum to operate the system the default is blow out the defrost vents.

 

First time lurker and user, great site. I'm having a similar problem with my 04 F350 diesel. Driving the freeway last week in Phoenix, with the ac on high, I noticed not a lot of air coming from the dash vents. Odd. Then, I started hearing a hum, than a loud clicking, than almost a grinding sound coming from the dash area. Trying to isolate the sound, I turned off the ac unit. Sound stopped. When I turned it back on a moment later, the air flow had shifted and was now blowing hard and cold from the defrost vents. I wondered if there were 2 blowers and the dash vent blower had blown a bearing. Continued trip with ac coming through defrost vents. Then, three days later, returning from trip, the ac again worked fine blowing through the dash vents. Next day, and ever since, back to defrost, no dash vents. Is this vacuum related? And if so, vacuum pump or vacuum line? And what the heck was the loud, almost grinding, sound prior to it diverting to defrost? Could that have been my vacuum pump eating itself? Sorry for all the dumb questions. Anybody recommend a site with a layman's description of the ac delivery system? I need my ac at 100% to get through the Phoenix summer...........

 

Hey folks, I know you didn't respond to me directly, but while reading the posts here and on another thread, I determined that it probably was the vacuum pump, how to test it, and where to find an inexpensive replacement pump. It is all good now, thank you. I'll survive another Phoenix summer.
Great site, I'll be back........

 

Well that sucks....the vacuum pump, that is.

Glad you got it fixed, bigtdriver.