Heater and Vent Work, But not Defrost? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

LBJ · 01-06-2008, 02:44 PM

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

Ivan D. · 01-06-2008, 02:47 PM

A/C or not?

LBJ · 01-06-2008, 03:05 PM

Factory A/C.

swb85 · 01-06-2008, 03:29 PM

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!

LBJ · 01-06-2008, 03:32 PM

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.

Pyrotechnic · 01-06-2008, 03:58 PM

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.

LBJ · 01-06-2008, 06:09 PM

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.

knightlife · 01-06-2008, 08:57 PM

welcome to my world lol...

kevinr1970 · 01-06-2008, 10:20 PM

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?

killthewabbit · 01-07-2008, 05:40 AM

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.

jokersbowtie · 01-07-2008, 07:45 AM

on my 84 it was a vacum hose under the hood

LBJ · 01-07-2008, 02:36 PM

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.

Mickey_D · 01-07-2008, 04:35 PM

Quote:

Originally Posted by LBJ

Well, there is an electrical switch position for Defrost. (snip) I have to wonder about that switch and if it is somehow faulty.

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

)).

**Phuloi68** · 01-07-2008, 07:46 PM

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.

LBJ · 01-08-2008, 01:57 PM

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.

01-06-2008, 02:44 PM	#1
LBJ Registered User Join Date: Jun 2007 Location: TEXAS Posts: 378	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

01-06-2008, 02:47 PM	#2
Ivan D. glamoros piece o' lowlife Join Date: Mar 2007 Location: Redford, MI Posts: 1,164	Re: Heater and Vent Work, But not Defrost? A/C or not? __________________ The more I drink, the more I drink, the more I drink... diesel, reg cab, long bed, 4 across the rear, single stack, wooden stakes, and lotsa lights - the Hay Express

01-06-2008, 03:05 PM	#3
LBJ Registered User Join Date: Jun 2007 Location: TEXAS Posts: 378	Re: Heater and Vent Work, But not Defrost? Factory A/C.

01-06-2008, 03:29 PM	#4
swb85 On a budget like Fred Sanford Join Date: Oct 2005 Location: Topeka, KS Posts: 2,031	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! __________________ '85 Silverado swb: 383 stroker, 10.5:1, vortec heads, 232/238 roller cam, RPM air gap, performer 750 carb, stainless longtubes, 3" duals/super 44's, T56/4.11 383ci build / exterior refresh thread '98 Camaro z28: 370ci twin turbo 370ci build '01 Tahoe LT 4x4: 5.3, longtubes/ory, magnaflow duals, custom tune....wife's DD

01-06-2008, 03:32 PM	#5
LBJ Registered User Join Date: Jun 2007 Location: TEXAS Posts: 378	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.

01-06-2008, 03:58 PM	#6
Pyrotechnic Registered User Join Date: Feb 2007 Location: Austin, TX Posts: 3,930	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.

01-06-2008, 06:09 PM	#7
LBJ Registered User Join Date: Jun 2007 Location: TEXAS Posts: 378	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.

01-06-2008, 08:57 PM	#8
knightlife Dark Heart Motorsports Join Date: Apr 2007 Location: 29404 DKFX Posts: 683	Re: Heater and Vent Work, But not Defrost? welcome to my world lol... __________________ How Deep Can YOU Bleed..? 1985 Chevy Blazer K5- 6.2 N/A, 400 turbo trans CUCV, 12v converted... a new chapter 2005 GMC Sierra Crew Cab 6.0 powwwaa!!! custom jumpseat install w/ captain seats, Hypertech tuner, MSD Blaster Coils, CB and twin antenna's, all electric interior, touch screen in dash. custom center console, with jump seat replacing original console. 1500 HD!

01-06-2008, 10:20 PM	#9
kevinr1970 Senior Member Join Date: Aug 2004 Location: Everett, WA Posts: 2,316	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? __________________ 2009 Honda Fit CfC (bsf 44.9 mpg) 2000 Tahoe Limited 1991 GMC CrewCab Dually 2wd, will end up swb, not dually and replace CCswb below 1991 GMC CrewCab Dually 4x4, just going to fix things up for now 1982/1989 K5/GMC Jimmy 2wd 1987 GMC 1/2 ton swb 2wd Crew (sold)

01-07-2008, 05:40 AM	#10
killthewabbit Spear and magic helmet! Join Date: Oct 2006 Location: Nashville, TN Posts: 1,226	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.

01-07-2008, 07:45 AM	#11
jokersbowtie Registered User Join Date: Jul 2007 Location: Louisiana Posts: 74	Re: Heater and Vent Work, But not Defrost? on my 84 it was a vacum hose under the hood

01-07-2008, 02:36 PM	#12
LBJ Registered User Join Date: Jun 2007 Location: TEXAS Posts: 378	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.

01-07-2008, 07:46 PM	#14
Phuloi68 Senior Member Join Date: Dec 2003 Location: Valparaiso, In Posts: 1,114	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. __________________ Phuloi68 Indy survivor Norwood survivor Big Sandy survivor for those who fight for it... F R E E D O M has a price the protected will never know.

01-08-2008, 01:57 PM	#15
LBJ Registered User Join Date: Jun 2007 Location: TEXAS Posts: 378	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.



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