I did some searching thru the threads but did not find this exact problem anywhere... About a year ago I had the issue where under load the air flow would redirect from the vents to the defrost and floor vents. I went to the primary vacuum line at the intake and it was dust. Replaced, and problem was fixed. This morning, I'm driving in to work and I hear a hissing sound behind the HVAC control, just inside the dash. I once again am unable to get air to come out of any of the vents besides defrost and floor vents. I checked under the hood at the primary supply lines, and they are all good. Last time, all of my different functions worked correctly; heat and A/C. Now, when I select the A/C controls, the A/C compressor is not kicking on (fresh recharge last week). The temperature selection is working fine, getting both cool outside and warm air from the heater core, but the actual thermostat selections seemingly make no difference in terms of Max A/C, A/C, dash/floor, floor, defrost, defrost/floor; it just blows air. With the thermostat set to off, you can hear the hissing literally right behind the selector in the dash. What am I looking for when I pull the dash apart? Has anyone had this particular issue? Thanks.

 

I have the same problem. Did you end up with a solution?

 

There are a series of vac lines that attach to the controls on the dash. You can remove those with some screws to make sure they are all attached properly.

 

Not sure what year your truck is but go to these links (in order) and it they should give you what you need:

GMC Sonoma & Chevy S-10 Transfer Case Vacuum Switch

GMC Sonoma & Chevy S-10 Transfer Case Vacuum Switch

 

Are you referring to the hvac controls and just checking the input there? It all seemed fine, and I didn't notice a leak in that exact spot.

 

I was, its extremely hard to get those to seat properly onto the hvac controls if you have removed them at any point.

I removed them several times when I had my first gens and getting them back on properly was extremely hard.

 

I'll have to mess around with it and make sure it is on perfect. At the moment my car won't start, so this problem moved to the back burner..

 

I hope you get it figured out.