HVAC Vent/Defrost/Floor Mode of Operation Will Not Change
 

I have a 2002 Blazer 4WD 4-Door with 116,000k. My problem that just started is with my HVAC. When HVAC are on, it blows through the vents and no other mode of operation. It will not change to Defrost or Floor mode of operation. However, when HVAC is turned off, vent air still blows through vents when driving, like the vent mode is open. I found two vacuum lines one driver side firewall/brake area. That did not help. There are two other vacuum lines in the same general area, one goes to the engine which is good, and one that goes across firewall and down behind the distributor on the passenger side. It looks very dry rotted and looks like it needs replaced, but cannot find where it goes to yet. Could this be my problem or am I looking in wrong area. I have to get this fixed as in Virginia we have State Inspections and vehicle will fail inspection and will be unable to drive, and of course mine expires next month. Any help would be appreciated.

Short answer is yes. The vacuum controls the operation of the HVAC.
Try to do a search for a vacuum diagram so you can see where the hoses are connected.
I have a 96 Blazer which has a vacuum reservoir on the hood. Yours is in the fender
on the drivers side I think. All of the hoses need to be in good condition to make
the vacuum work. I am still fighting some vacuum issues on my blazer.

Vacuum Schematics: https://blazerforum.com/forum/showth...7_hvac_sch.gif
May help some

I had a similar thing happen to my 02. Mine had automatic trans fluid in it from a faulty switch on my transfer case (I think that's where it was or auto trans anyway). The vacuum pulled the fluid into my dash controls through the vacuum lines. I had to flush/clean the vacuum lines from the reservoir all the way to the controls. I also had to replace the dash control and the switch on the tranny. There is a thread on it with more details. Search and you'll find it.

i seriously doubt your ac not working right will make you fail inspections.

HVAC Vent/Defrost/Floor Mode of Operation Will Not Change
 

So you folks are telling me the vacuum reservior is on the fender, which means I have to pull the wheel well out to inspect it? Not sure what to look for, but I assume I will see something. If bad, is this a dealer item? If this isn't it, what about the switch on transfer case? Where is it loacted and is it easy to access or an aamco job? If bad, is this also a dealer item? Thanks for the help info thus far.

Do not pull the fender. Get a hand vacuum pump with a gauge on it. They are cheap enough. Attach the pump to the fitting on the reservoir and pump it down a bit. If it holds vacuum, then it is fine.

I doubt your reservoir is bad, but the vacuum hose may have come unplugged. The vacuum switch in the transmission is not bad to replace, just is in a tight spot. Check all of the hoses you can to see if they are cracked, unplugged, etc.