No air from the defroster

[crawdad]

I can't get any air to come out of the defroster ducts in my truck. It feels like all of the air is coming from the heater (floor) duct. I saw what looks like a vacuum port right next to the floor duct, but there was no vacuum hose connected to it. Could this be the problem? If so, where does the line come from? I couldn't find one anywhere. Course its so tight up under the dash, I probably would have missed it any way. What is the best way to make some room under there to get to anything? Thanks.

[SlapFish]

Did you try switching it to defrost?

Actually you might be able to find some nifty things by taking the glove box out easily. Quite a big opening.

[hotrodhomi]

there is also a vacuum line under the hood near the upper heater hose.

[1976K5CHALET]

Most likely it is a vacum related issue as generally when you loose vacum to the system it seems to want to give you the floor only treatment.

And yes...if there is a silver vacum pod there with a small bent line dead center on it ...it requires a line to me hooked to it. Generally the feed line comes from beind the carb and or tbi unit from the vacum tree in the manifold...from there it goes into the firewall on the older stuff....the newer stuff uses a one way vacum valve as when your rpm's increase quickly there is not enough vacum to support everything and the vents being vacum operated have a tendency to fall off from vent to heater rather quickly....so thats why gm has the vacum resivor for the a/c system and the one way valve. From the firewall it goes directly to the vacum switch in the hvac control panel...at which point...depending on which mode you have selected vacumis routed to the appropriate vacum pot. The biggest pain to fix is the pot on the back side dead center of the interior heater box ....the vacum line goes thru the center of the box as the pot is a push pull typr dependant on which side gets vacum....and of course i had to drain the coolant and remove the whole enchilada for a small length of vacum hos because the piece installed by gm in 79 had dry rotted. Another way to find besides the obvious method that employs yourself...a flashlight...and a little bit of harry whodinni's moves is to find that rascal under the dash as gm has smart enough to make the end that comes off the vacum harness with a little loop built into it to prevent it from falling of and its made of a clear rubber. If this and the inspection of the main feed line turn up nothing the next method is a little work but will isolate the source of the problem. You'll need the factory gm manual as it will have a chart of the vacum harness....as gm identified each vacum circuit with a colored stripe on the black vacum tube...a mighty vac or there of harbor freight knock off to apply a vacum source to each circuit after you remove the dash bezel and remove the hvac control panel so you can work with the vacum harness....then circuit by circuit test and see if she holds....and that should isolate the source of your troubles. I'd also plug the feed line at the intake manifold so you can also confirm the feed source is trouble free.

Hope this helps....

Doug W.

[crawdad]

Thanks very much Doug. That was a very descriptive answer and it definitely helps. Appreciate it.

[1976K5CHALET]

Glad to help....if you need a/c help i can post another tome with tips and tricks to make the a/c work better also. If that interior hvac case has to come out its a damn fine time to do some tuning up while its apart....

BTW your avatar is a SF crest is it not?

Got a few month's to go and my 15 years in the USA is done....except for that next 6 years of IRR which i'm sure will earn me a vacation in Iraq since i missed the trip this april to mosul and LSA diamondback...

Doug W.

[crawdad]

Thanks again, Doug. Yes, my avatar is the SF Crest. You're one of the few people who have caught that. I retired from Special Forces back in '91. I was with them for 14 of my 21 years. Best part of my army career. Hopefully you want have to go to the big sand box, but a lot of the reserves and NG do. I teach school now and about 4 or 5 of my former students are now over there after having joined the reserves.
Thanks again for your help on the ac/defroster system.

[Phuloi68]

Had same problem on 85 C10. looked at anything I could think of under dash, top of engine. Went as far as tying flapper in direction of defrosters.
A little while went by, was changing air filter and noticed rubber hose (to 3 way vacuum fitting behind carb. )on top of intake manifold broke. It was either from getting old or air breather hitting it when lifted off carb.
Anyway, trimmed end, put it back on port, heard flapper shift, engine idle smooth out and other noises I haven't a clue what they were.
Heat and defrost work great now, best heater I ever had in a vehicle.
Truck runs a hole lot better,too
Amazing what happens when there is good vacuum pressure. Saw this same broken hose on friends 2 trucks and same thing happened. Might be a common problem, easy to fix.

[crawdad]

Originally Posted by Phuloi68

Had same problem on 85 C10. looked at anything I could think of under dash, top of engine. Went as far as tying flapper in direction of defrosters.
A little while went by, was changing air filter and noticed rubber hose (to 3 way vacuum fitting behind carb. )on top of intake manifold broke. It was either from getting old or air breather hitting it when lifted off carb.
Anyway, trimmed end, put it back on port, heard flapper shift, engine idle smooth out and other noises I haven't a clue what they were.
Heat and defrost work great now, best heater I ever had in a vehicle.
Truck runs a hole lot better,too
Amazing what happens when there is good vacuum pressure. Saw this same broken hose on friends 2 trucks and same thing happened. Might be a common problem, easy to fix.

That's good to know Phuloi68. The problem I seem to be having is finding the vacuum hose under the dash that goes to the flapper. I've been looking all under the dash and haven't found it yet. I guess I'm going to have to tear everything out just to try and find that sucker.