Heater and Vent Work, But not Defrost?
 

I can get lots of heat in the cab, but not to the windshield. I don't depend on this truck in bad weather (or any time) so am not in a panic to get it fixed. But, like everything else on the truck, I'd like to get it in working order. Is it the selector switch on the panel? Missing vacuum hose? Stuck diffuser door?

LBJ

Re: Heater and Vent Work, But not Defrost?
 

A/C or not?

Re: Heater and Vent Work, But not Defrost?
 

Factory A/C.

Re: Heater and Vent Work, But not Defrost?
 

Mine does the same thing and so does my buddy's '85......and while I haven't had time to determine the cause/fix, I'll place my bet on this: Remove your glove box door and you'll see the top of the hvac "box". On top of that is a metal rod attached to a flapper door inside the "box". With your defrost running, actuate that rod with your hand and I'll bet your defrost starts to work. I haven't figured out why the door won't work on it's own (I think it's got to be a bad vacuum line/pod somewhere), maybe you can figure it out and tell me the answer so I can fix mine!:lol:

Re: Heater and Vent Work, But not Defrost?
 

SWB85,

That'll be easy to do. I still have the whole glove box out from putting the right door on. If I find the cause, I'll let you know.

Re: Heater and Vent Work, But not Defrost?
 

I have just the opposite issue. I get heat out of my defrost vents but not the main vents. Thats fine with me because I don't like the heat blowing directly on me and I've been plenty warm on my trips this winter, but I'd like to get it working anyway.

I'll mess with it and see what I can come up with.

Re: Heater and Vent Work, But not Defrost?
 

Well, there is an electrical switch position for Defrost. The other switch is really a slide that closes a door in the plenum and regulates the temp of the air. It's purely mechanical. I have to wonder about that switch and if it is somehow faulty.

Re: Heater and Vent Work, But not Defrost?
 

welcome to my world lol...

Re: Heater and Vent Work, But not Defrost?
 

On my '82 K5 when I switch it over to defrost I get more through the AC vents (? the ones pointed towards you) then through the defroster vents.
I close those vents best I can and it helps but there is still a lot of "spillage" through the AC vents.

If I could get it locked into defrost I could live with that better than the way it is now?

Re: Heater and Vent Work, But not Defrost?
 

This is a valuable topic to me too. I can toast the rest of my cab, but not the windshield. I'm going to try the rod behind the glove box door also.

Re: Heater and Vent Work, But not Defrost?
 

on my 84 it was a vacum hose under the hood

Re: Heater and Vent Work, But not Defrost?
 

Here's a page from a parts book I got at a swap meet. Shows the heater layout. Might be of help in figuring out the various problems with the heaters.

http://img.photobucket.com/albums/v2...n/Kitty491.jpg
http://img.photobucket.com/albums/v2...n/Kitty492.jpg

Re: Heater and Vent Work, But not Defrost?
 

That switch is to override the temperature controls for the air conditioning and turn the compressor on to help dehumidify the air going to the defroster.

All GM products since the early 70's have done this and still do today. It's why if you have A/C, you shouldn't drive with the selector in the defroster position as you're cycling the A/C and wasting gas (unless you're currently USING the defrost, which means you shouldn't be driving anyways due to limited vision (had a cop inform me of this :hm: )).

Re: Heater and Vent Work, But not Defrost?
 

It's either a vacuum problem(most likely) or the little white plastic part on the rod operating the flapper door is broke(common).
Look at the tee fitting on top of intake right behind carb. Seems like those little rubber hoses on that become brittle and crack.

Re: Heater and Vent Work, But not Defrost?
 

I think the main problem is the firewall pass thru on that vacuum line. It was just about in two pieces at that point. I also had a loose fit on the accumulator ball on the left of the A/C unit there. You can see the hole in the firewall where the line goes to the heater-A/C controls. Also, there is at least one vacuum line inside that crumbled and I don't know where it hooked to. I tested several of the vacuum cans and they seem to work. They just have not been getting any vacuum due to open lines and cracked fittings. I am going to war on this on Monday.

I also discovered why my engine was missing and a bit uneven on idle - vacuum leak at that accumulator ball on the firewall. I plugged the line and it smoothed out. All the lines in the engine compartment probably need to be replaced. Most are old and brittle.

You can also see the overspray from the paint shop. He also failed to paint under that black grille below the windshield like I asked him to. The wipers had been taken off and replaced to paint and both were in the wrong position. the right wiper was jammed on and had the fine serrations cross-threaded. Really pXXXXd me off. I was lucky to get it back on straight without having to buy another shaft assembly.

http://img.photobucket.com/albums/v2...n/Kitty493.jpg

Re: Heater and Vent Work, But not Defrost?
 

On my 91 suburban i have heat at the vents and floor but not at the defrost, the actuator does work but the plastic keeper for the rod is broken , but even with me moving the leaver on the top of the heat box I still don't get defrost , so my question is does the defrost operate on a electric actuator located somewhere else? If anyone can tell me if it does it would appreciate that , I'd go buy a service book but it's Thanksgiving day .
Thank you

Re: Heater and Vent Work, But not Defrost?
 

My 84 Sierra 2500 is in the same boat. I'm looking at replacing the defrost actuator. I've fixed the vacuum line in the engine bay and everything else seems ok.

Re: Heater and Vent Work, But not Defrost?
 

Well about to take the dash pad off so I can look from the top and see what's going on

Re: Heater and Vent Work, But not Defrost?
 

Check the vacuum at PS kick panel