My 01 just started doing this. When I turn on air it blows out of defrost vents only.Sometimes it will blow out of dash vents then starts blowing out of defrost again.What do I check.

 

My 96 had that problem at one time. In it's case, the vacuum hose(very small) that is routed across the top of the firewall fell onto the turbo and melted. My youngest daughter's 96 sport with a GAS engine has a similar problem. Her's will go to defrost when you shower down on it hard, climbing a steep hill, etc. I figure it's probably a vacuum hose collapsing, I'm not sure yet.

 

Don't know about the diesel's but on most gas engine's, yes, a vaccum leak will cause it to distribute the air to the defroster vents, usually a busted hose or the vaccum cannister is bad.

 

i had the same problem on my 96... it was the vacuum line running across the firewall, it fell and melted on the turbo there was enough slack in the line to trim it and hook it back up. then i took and zip tied all of the wires and what-not running up there to help prevent this from happening again.

 

Can you get it to blow out of the floor vents? IF not, check the vacuum line as described above. If so, you have a bad selector switch, actuator under the dash, or problem with a vacuum line going from the selector to the actuator.
I found this problem only to the vents on my 96 and it turned out there was a TSB stating that oil could seep up the vacuum lines into the actuator and destroy the rubber diaphram over time. I think this only affected 96 trucks though.


Chris