Defroster Duct Doesn't Blow - The 1947 - Present Chevrolet & GMC Truck Message Board Network

75sierraclassic · 09-25-2009, 02:30 PM

Well its starting to get a little chilly in the mornings and I recently noticed by defrost vent isn't working. What do you guys think is the most likely culprit? The hot/cold lever works, the A/C still turns on, it just seems that some of the dampers aren't working that redirect the air to the defroster vents. Vacuum leak somewhere, bad diaphragm, vacuum switch? If the hot / cold lever still works does this mean that the vacuum switch is good and its a diaphragm or vacuum leak somewhere?

STOCKISH · 09-25-2009, 02:38 PM

There is a vacuum line under the hood that is most likely unhooked or split. Mine is right behind the carb and hooks to a manifold vacuum source. Take care.

77gmcjoe · 09-25-2009, 02:49 PM

I had trouble with no air blowing out of the defrosters on a couple of different trucks, one time it was a bad vacuum pod and the other was a rotted hose going to a vacuum pod. Unfortunately I had to pull the whole plenum assembly out from under the dash to get to both of the problems.

75sierraclassic · 09-28-2009, 09:46 AM

Thanks for the info guys, found a rotted vacuum line under the hood. She's blowin again

STOCKISH · 09-28-2009, 10:28 AM

Sweet, glad you got it fixed. Missouri will turn to Misery real quick with no heat.

kelsfine69 · 09-28-2009, 11:28 AM

I was watching this thread hoping for ideas on mine. Mine all works but air bleeds from the a/c vents when on heat and defrost. Mayby I will wait till the core craters then I will pull it all out. i was hoping for an easy fix to a similer problem

632bbc · 10-19-2009, 01:05 AM

this may be a dumb question. . . but where does the vacuum pod get it's vacuum from? I found the line coming out of the cab but that's all. . . my heater is the same way. . no defrost or floor just blows out the vents

77nomad · 10-20-2009, 12:06 AM

There is a tiny kind of hard plastic vac line under the hood that gets engine vacuum. Mine is teed into carb base.

09-25-2009, 02:30 PM	#1
75sierraclassic Registered User Join Date: Jun 2002 Location: Kansas City, MO Posts: 186	Defroster Duct Doesn't Blow Well its starting to get a little chilly in the mornings and I recently noticed by defrost vent isn't working. What do you guys think is the most likely culprit? The hot/cold lever works, the A/C still turns on, it just seems that some of the dampers aren't working that redirect the air to the defroster vents. Vacuum leak somewhere, bad diaphragm, vacuum switch? If the hot / cold lever still works does this mean that the vacuum switch is good and its a diaphragm or vacuum leak somewhere?

09-25-2009, 02:38 PM	#2
STOCKISH Formerly- 1972SuperCheyenne Join Date: Oct 2008 Location: Wake Forest NC Posts: 5,782	Re: Defroster Duct Doesn't Blow There is a vacuum line under the hood that is most likely unhooked or split. Mine is right behind the carb and hooks to a manifold vacuum source. Take care. __________________ --Josh My Build Thread:http://67-72chevytrucks.com/vboard/s...d.php?t=799218 A good crossthread is better than Loctite any day. Life is not about what you have, but who you have to share it with.

09-25-2009, 02:49 PM	#3
77gmcjoe What do we get for $10? Join Date: Oct 2004 Location: Beardstown, Illinois Posts: 341	Re: Defroster Duct Doesn't Blow I had trouble with no air blowing out of the defrosters on a couple of different trucks, one time it was a bad vacuum pod and the other was a rotted hose going to a vacuum pod. Unfortunately I had to pull the whole plenum assembly out from under the dash to get to both of the 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 )

09-28-2009, 09:46 AM	#4
75sierraclassic Registered User Join Date: Jun 2002 Location: Kansas City, MO Posts: 186	Re: Defroster Duct Doesn't Blow Thanks for the info guys, found a rotted vacuum line under the hood. She's blowin again

09-28-2009, 10:28 AM	#5
STOCKISH Formerly- 1972SuperCheyenne Join Date: Oct 2008 Location: Wake Forest NC Posts: 5,782	Re: Defroster Duct Doesn't Blow Sweet, glad you got it fixed. Missouri will turn to Misery real quick with no heat. __________________ --Josh My Build Thread:http://67-72chevytrucks.com/vboard/s...d.php?t=799218 A good crossthread is better than Loctite any day. Life is not about what you have, but who you have to share it with.

09-28-2009, 11:28 AM	#6
kelsfine69 18 Till I Die Join Date: Mar 2005 Location: Alberta, Canada Posts: 1,522	Re: Defroster Duct Doesn't Blow I was watching this thread hoping for ideas on mine. Mine all works but air bleeds from the a/c vents when on heat and defrost. Mayby I will wait till the core craters then I will pull it all out. i was hoping for an easy fix to a similer problem __________________ 2011 GMC Acadia 1969 GMC BBC LS7 2007 GMC Crew Duramax 1987 Camaro Iroc 350 Tuned Port Injection 42000Km 1987 Chevrolet Silverado 350 TBI 1987 Chevrolet Silverado 350 TBI(yes 2) Take me drunk, I'm too home...

10-19-2009, 01:05 AM	#7
632bbc Registered User Join Date: Nov 2008 Location: Excelsior Springs MO Posts: 24	Re: Defroster Duct Doesn't Blow this may be a dumb question. . . but where does the vacuum pod get it's vacuum from? I found the line coming out of the cab but that's all. . . my heater is the same way. . no defrost or floor just blows out the vents Last edited by 632bbc; 10-19-2009 at 02:14 AM.

10-20-2009, 12:06 AM	#8
77nomad Registered User Join Date: Sep 2009 Location: Muskegon, MI (the beer tent capitol of the world) Posts: 16	Re: Defroster Duct Doesn't Blow There is a tiny kind of hard plastic vac line under the hood that gets engine vacuum. Mine is teed into carb base.



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