1991 Blazer 4.3 Tahoe. Just doesnot want to defrost. Air blows out the bottom vents. AC, Bilievel & heat setting seem to work fine. If I play with it some I can get a little air to come up the defrost. Also I hear a clicking sound just to the left of the steering column under the dash whenever the AC compressor should be kicked in--AC, BiLevel, & Defrost. Whats going on?? I will apreciate any help! Thanks

 

Sounds like you have either vacuum actuator problem (I think yours is vacuum powered) for the defrost actuator or a sticking door that is controlled by the defrost actuator. Does it click when you push it to defrost? If so, maybe try to get a good look under the dash at what is trying to happen and possibly help it along. Sometimes the doors like to get stuck over time and the actuators cannot open/close them properly.

I don't have my references at work to look up component locations... Hope this helps.

 

Played around with the vacuum lines a little, unhooking & rehooking - made it work a little better. Got pissed and removed the outer-under the dash cover and the main bottom deflector vent. Found that the flapper door was working but was not adjusted properly. Took the two adjusting screws out and could make it defrost perfectly outside the lines of the screws. Weird. Anywhere inside the adjustment was only partial - half defrost and half floor heat was the best I could get. Can't figure out what happened to it unless someone perhaps a previous owner screwed with it.........had signs of a heater core replacement. I have just owned the truck a few weeks. THANKS for your help....its what got me to looking and found the problem!