cold air blowing out defrost vents

[locobob]

Air conditioner blows out defrost vents, putting truck in 4wd will start air blowing out of ac vents ,but only lasts a few minutes, I havent tried leaving it in 4wd,Is this caused by a vacume leak?

[afdude88]

Sounds like it. The 4 wheel drive while electronically controlled is actuacted by vacuum. Might be drawing more vacuum for the 4 wheel drive switch-over then bleeding off once the demand is gone. The HVAC system vents are contolled by vacuum and defaults to defrost with the absence of vacuum.
Check all your vac lines. If you cant find it spray then lightly spray the lines with carb cleaner or starter fluid( i think thats right) while the engine is running and listen for an idle change. When the idle does change you have found your leak.

[jpopp]

My A/C does this too. Sometimes it blows through the vents instead of defrost if I am not running high RPMs or heavy on the gas. Other times it doesn't matter what I do, it just runs through the defrost. It's frustrating because I can't find the vacuum leak and the heat during the winter works on every setting.