Defrost Problems - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72YellowBird · 10-29-2007, 06:23 PM

I have an 86 GMC pick up with factor A/C that blows very little defrost air out of the vents when the defrost switch is selected. It seems to work fine on any of the A/C positions with air blowing from the vents and the defrost vent is cut off. When defrost is selected the air still blows from the A/C vents and out of the defrost vent. It doesn't shoot all the air through the defrost vents. Seems to work properly on heat air is coming out of the heat vent. You can hear the vaccum hiss when seletor switch is moved.Can't seem tofigure it out.

j hannah · 10-29-2007, 06:59 PM

you might have to take your ducts apart and clean them out. I pulled pencils, and popcicle sticks out of mine,but thats when the kids were little.

crawdad · 10-29-2007, 08:27 PM

I had a problem similar to this. If I remember correctly there is a flapper door that is opened and closed depending on what your selector is set to. It is controled by a vacuum pod that opens and closes it. In my case, the vacuum line had come loose and all I had to do what put it back on. Yours could be something similar, not allowing the door to close or open all the way. I think the vacuum pod is behind the glove compartment or underneath the dash, right in the middle, I forget. You may have to take off some of the duct work, but you'll see it if you look. Hope I haven't confused you too much.

72YellowBird · 10-29-2007, 10:19 PM

Thanks, I did see that pot under dash and it looked like the line is attached but I will take a closer look or maybe that pot is weak. I don't know if this is related or not but I thought that I read somewhere if the cruise is acting up that there is another vac control by the firewall that could be causing problems with the defrost control and the cruise. The cruise would surge when it was on indicating a vaccum problem of some kind. These maybe to separate issues.

77gmcjoe · 10-29-2007, 10:21 PM

I've had the same problems with a couple of different trucks. My '77 ended up having a bad vacuum pod that wouldn't allow any air to come out of the defroster ducts. My Grandpa had bought this truck new and said it had been that way since day one. When he'd taken it back to the dealer for them to fix it, they told him they didn't know what was wrong with it!

I also had another old 'Burb that ended up having a dry-rotted vacuum hose inside of the ductwork that kept the defrost from working properly. Unfortunately, in both cases I had to pull out the whole ductwork/heater core assembly out to find the problem.

What j hannah said is also a possibility. One time in a junkyard I saw a truck that had a fishing bobber wedged in one of the doors inside the ductwork that would have caused problems.

crawdad · 10-29-2007, 10:44 PM

Check the vacuum line under the hood that runs into the dash. It runs in over near the heater/ac cover. Make sure that it is hooked up to its vacuum source. On mine it is connected to a T fitting that comes from behind the carb. Just trace it out and make sure it is connected to a good vacuum source. About the cruise, mine didn't work either until I got this line hooked up. Defroster and cruise all started working at the same time.

72YellowBird · 10-30-2007, 05:31 PM

I checked the vaccum pot that operates the door that opens and closes when heat and defrost are selected and it is operating fine. It opens to deflect heat to the floor when heat is selected and closes when defrost is selected. There is still air blowing out of the vents when the door is closed. Is there another door somewhere else that shuts off the air to the vents when the defrost is selected?? It shuts the air off to the defrost when any of the vent postions are slected.

crawdad · 10-30-2007, 07:00 PM

There is a second one I believe. It is near the center of the dash in between the glove box and the radio, somewhere in that area. It is concealed by the duct work. Take the bottom duct work off and look up under there and you'll see it. It's near or on the end of the main heater box, for lack of a better way of describing it. If I remember correctly, that is the one that controls the defroster. At least it was on mine. If I can find a schematic, I'll post it for you. HTH

**Phuloi68** · 10-30-2007, 07:43 PM

there are two things, assuming all your vacuum is intact and original, that causes most defroster issues.
The rubber hose coming off the t right behind the carb cracks with age and heat and the heater flapper door has a little white plastic piece that always breaks on the older trucks. My guess it is one or both of those items.
Do a search on the general forum (I think) and you will see a couple of pics.

10-29-2007, 06:23 PM	#1
72YellowBird Registered User Join Date: Mar 2006 Location: Airdrie,Alberta Canada Posts: 41	Defrost Problems I have an 86 GMC pick up with factor A/C that blows very little defrost air out of the vents when the defrost switch is selected. It seems to work fine on any of the A/C positions with air blowing from the vents and the defrost vent is cut off. When defrost is selected the air still blows from the A/C vents and out of the defrost vent. It doesn't shoot all the air through the defrost vents. Seems to work properly on heat air is coming out of the heat vent. You can hear the vaccum hiss when seletor switch is moved.Can't seem tofigure it out.

10-29-2007, 06:59 PM	#2
j hannah comfortably numb Join Date: Nov 2006 Location: Tulsa,oklahoma Posts: 175	Re: Defrost Problems you might have to take your ducts apart and clean them out. I pulled pencils, and popcicle sticks out of mine,but thats when the kids were little. __________________ 1974 gmc 3/4 t. c/s 1974 chevy k-10 1973 pontiac grandville(turnpike cruiser)

10-29-2007, 08:27 PM	#3
crawdad Senior Member Join Date: Sep 2004 Location: Georgia Posts: 614	Re: Defrost Problems I had a problem similar to this. If I remember correctly there is a flapper door that is opened and closed depending on what your selector is set to. It is controled by a vacuum pod that opens and closes it. In my case, the vacuum line had come loose and all I had to do what put it back on. Yours could be something similar, not allowing the door to close or open all the way. I think the vacuum pod is behind the glove compartment or underneath the dash, right in the middle, I forget. You may have to take off some of the duct work, but you'll see it if you look. Hope I haven't confused you too much. __________________ 2012 Silverado LT 1500, 5.3, 6 speed auto '86 SWB C-10 Silverado

10-29-2007, 10:19 PM	#4
72YellowBird Registered User Join Date: Mar 2006 Location: Airdrie,Alberta Canada Posts: 41	Re: Defrost Problems Thanks, I did see that pot under dash and it looked like the line is attached but I will take a closer look or maybe that pot is weak. I don't know if this is related or not but I thought that I read somewhere if the cruise is acting up that there is another vac control by the firewall that could be causing problems with the defrost control and the cruise. The cruise would surge when it was on indicating a vaccum problem of some kind. These maybe to separate issues.

10-29-2007, 10:21 PM	#5
77gmcjoe What do we get for $10? Join Date: Oct 2004 Location: Beardstown, Illinois Posts: 340	Re: Defrost Problems I've had the same problems with a couple of different trucks. My '77 ended up having a bad vacuum pod that wouldn't allow any air to come out of the defroster ducts. My Grandpa had bought this truck new and said it had been that way since day one. When he'd taken it back to the dealer for them to fix it, they told him they didn't know what was wrong with it! I also had another old 'Burb that ended up having a dry-rotted vacuum hose inside of the ductwork that kept the defrost from working properly. Unfortunately, in both cases I had to pull out the whole ductwork/heater core assembly out to find the problem. What j hannah said is also a possibility. One time in a junkyard I saw a truck that had a fishing bobber wedged in one of the doors inside the ductwork that would have caused problems. __________________ 1977 GMC K25 400/400/205 1989 Chevy Suburban R1500 350/700R4 (work beater) 1994 Chevy K3500 dually 454/4L80 1996 Corvette LT4 350/ZF 6 speed 2000 Bonneville SSEI Supercharged 3.8 Liter V6 1966 Chevelle 2 door sedan (Needs major repair )

10-29-2007, 10:44 PM	#6
crawdad Senior Member Join Date: Sep 2004 Location: Georgia Posts: 614	Re: Defrost Problems Check the vacuum line under the hood that runs into the dash. It runs in over near the heater/ac cover. Make sure that it is hooked up to its vacuum source. On mine it is connected to a T fitting that comes from behind the carb. Just trace it out and make sure it is connected to a good vacuum source. About the cruise, mine didn't work either until I got this line hooked up. Defroster and cruise all started working at the same time. __________________ 2012 Silverado LT 1500, 5.3, 6 speed auto '86 SWB C-10 Silverado

10-30-2007, 05:31 PM	#7
72YellowBird Registered User Join Date: Mar 2006 Location: Airdrie,Alberta Canada Posts: 41	Re: Defrost Problems I checked the vaccum pot that operates the door that opens and closes when heat and defrost are selected and it is operating fine. It opens to deflect heat to the floor when heat is selected and closes when defrost is selected. There is still air blowing out of the vents when the door is closed. Is there another door somewhere else that shuts off the air to the vents when the defrost is selected?? It shuts the air off to the defrost when any of the vent postions are slected.

10-30-2007, 07:00 PM	#8
crawdad Senior Member Join Date: Sep 2004 Location: Georgia Posts: 614	Re: Defrost Problems There is a second one I believe. It is near the center of the dash in between the glove box and the radio, somewhere in that area. It is concealed by the duct work. Take the bottom duct work off and look up under there and you'll see it. It's near or on the end of the main heater box, for lack of a better way of describing it. If I remember correctly, that is the one that controls the defroster. At least it was on mine. If I can find a schematic, I'll post it for you. HTH __________________ 2012 Silverado LT 1500, 5.3, 6 speed auto '86 SWB C-10 Silverado

10-30-2007, 07:43 PM	#9
Phuloi68 Senior Member Join Date: Dec 2003 Location: Valparaiso, In Posts: 1,114	Re: Defrost Problems there are two things, assuming all your vacuum is intact and original, that causes most defroster issues. The rubber hose coming off the t right behind the carb cracks with age and heat and the heater flapper door has a little white plastic piece that always breaks on the older trucks. My guess it is one or both of those items. Do a search on the general forum (I think) and you will see a couple of pics. __________________ 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.



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