A/C vent controls?

[85 Jimmy]

Since I've been driving my blazer more often I noticed the PO had some duct tape covering up the vent for the floor. I took the tape off and realized why it was taped up. It doesn't matter what position the vents are set to max, normal, defrost, etc. the majority of the air/heat is coming out of the bottom vent above the trans tunnel.

Where do I need to start with diagnosing this? Or is this normal and I just never knew it b/c it was taped up?

 

[paratrooper307]

Mine is that way. I think it's a path of least resistance thing. I don't get a whole lot of airflow throw the upper vents, although it's much better when using A/C instead of heat (I'm guessing that the heater core is a restriction). Frequently the ducting comes undone, but I fixed all mine and still had the same issue. If you want to start diagnosing, look at your directional controls, ducting, and make sure your blower fan is working properly. I think it's just how these trucks are. I don't worry about it because it definitely has no problem heating up the cabin.

 

[blazer74]

Max a/c you should have minimum flow from the lower floor vents, bi level if you have it or vent will have flow to both upper and lower vents, defrost should be mostly defrost.

 

[85 Jimmy]

I guess I need to figure out how these blend doors work. I get most of the air, whether it's heat or A/c, through the floor vents. Max and normal are about the same at the vent, bi level is the same as max and normal, and defrost redirects most of the air that would be coming from the vents, to the defrost, but still is getting blasted out at the floor.

I know it's all vacuum operated, I just need to see why the blend door for the bottom isn't closing.

 

[blazer74]

No vac from control, bad diaphragm valve, broken vac line, or something stuck or stuck in it