Air Vent Ducts.."pop" sound

97 XJ
Air vents quit working cannot switch between dash/defrost, most times the floor works fine, ive read that it defaults back to defrost, if its running on 1 or 2 it stays blowing on the defrost setting in either dash/defrost control position but then if you bump it up to 3 or 4 you can hear a "pop" like a flap closing and the air does not come out anymore and gets much louder from the trapped air.

any ideas, cruise controls works fine, i checked all vacumm lines in engine bay, also can hear parts moving when changing vent control with engine off. just this "pop" keeps blocking the flow.

Look at the vacuum actuators under the dash. One of them may have disconnected.

Fairly common on the 97+'s. The arm for the defroster door actuator pops off its little shaft. So what's happening is that the defroster door is slamming back shut, thus not allowing the air to flow to the windshield. Matter of fact, I just popped mine back into place last week.

Take the center of the dash off, remove the radio, and just to the left of the foam you'll feel the arm. Push it back in towards the passenger side (you'll hear it and feel it sort of "pop" back in). In the past, mine would go back into place just by doing that. Last week however, I had to turn the defroster on and get the door to open up, then push the arm back into place. Seems like it went back in much better/more by doing it, so you might try that method.

Here's a video I shot over the summer to try and help out another member who asked about this. In the video, the arm is the little green dot that you'll see in the last 5 or so seconds (just to the left of the foam).

http://s173.photobucket.com/albums/w55/99xjsport06/?action=view&current=DefrosterActuator.flv

I refuse to tear the entire dash apart to fix that completely...not until I already have the dash apart for some other more important reason. For now, and the last couple of years, I just do the same way I have been...takes all of a couple of minutes and lasts a while depending on how much I run the defroster on 3 or 4.

thank you very much boys, i will try that tomarrow.
Already had the radio out and glove box off today but couldn't see anything. Its interesting you say its dependent on how much you run the fan on 3 or 4, because just until recently my blower resistor was f'd and would only work on 4 and i ran it like that for maybe 14 months.

Hope you were able to see what I was talking about in the video.

There's really no set time as to when it's going to pop back out again...it may not be so much how much you run it on 3 or 4, but how quickly you put it to 4 when you cut it on....if you're like me and cut it wide open immediately, I think that's what speeds up the process more.