This may seem a little long, but the problem is not that much in detail. I just like to be thorough when I tell people my car trouble.

I have a 2004 4X4 Blazer. I have asked this question in the past, but have not had much luck. My top air vents are always on. The heat and A/C work fine, but if I put the selector on just the floor, it will come out of the floor and the top vents, if I put the selector on just the front windshield defogger, it will come out both the top vents and the windshield defogger. But the front windshield defogger is not as strong as it used to be before this problem. I guess because it is also coming out of the top air vents.
*Even if I have the selector on the Off position, it will still come out of the air vents. I am thinking it is a vacuum hose that popped off somewhere, because I can hear the "hissing" when I put the selector on any position except the "off" position. I have taken off the panels underneath the dashboard but I do not see any hoses there. I looked in the engine compartment and every vacuum hose I can see is connected.
Please help, my front windshield does not defrost as good as it used to, so I can't see that great out of it. Thank you.

 

Did ya take the HVAC unit right out? if's possible that it might have split or cracked where it goes into the unit.

 

EDIT:

i realized that the "hissing" sound is coming from the dashboard from the climate control *****. if i listen for it anywhere else on the dashboard, it is the loudest where the climate control ***** are

 

how do i take apart the dash, or where do i look to check the vacuum hoses that are inside the dashboard? are there even vacuum hoses for the climate control inside the dashboard? i have taken off the plastic panels below the dash which covers the computer in front of the center console. all i saw was wires, no hoses. are they above the ashtray? thanks

 

There's 5-8 vacuum hoses attached to the back of the A/C control unit. Red, blue, green, orange, etc... You'll have to pull the front of the dash off to get the control unit out. Undo two 7mm screws on each side/below the steering wheel and pull the lower dash down a bit. That will reveal two more 7mm screws that must be removed to remove the front of the dash, which should pop off. The control unit is also held by two 7mm screws and 4 "clips" that must be undone.

I just did this Friday, so I'm pretty sure those instructions should be accurate.

 

I have a 95 blazer the air from both the heat and ac only comes out of the defroster and floor vents

 

I have a 95 Blazer and is having the same issue as Beachbum. Thoughts anyone?????

 

Just check out the stuff that everyone else above was suggesting. That is all there is to stuff like this