Heater problems! What ya think it is? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

sam's 72 · 01-19-2003, 11:37 PM

On my 72' I have the heater valve under the hood. The one that works off of vacuum. Well, here lately, when I turn my heater on, it will not blow hot air. So, I pull over take the hose off the valve, the valve opens up and the heater starts working. Why is this? I mean, if it take a vacuum to open the valve, then why is it when I disconnect the hose from the vacuum valve, it starts working? My main question is this, what is bad, the vacuum valve under the hood, or the vacuum switch, at the ac/ heater controls? Can any body shed some light on this for me.
Thanks,
Sam

sam's 72 · 01-20-2003, 03:26 AM

com on guys! Some has the answer to this, as much knolege that is on this board. help me out here before I go replceing everything
Sam

marcuspboldt · 01-20-2003, 03:40 AM

I'm wondering have tried to pull a vacum on the valve, by unhooking the hose from the vacum source and seeing if you can pull vacum on it with your mouth. If you just keep sucking air the valve's diaphram is shot .
From the way you describe the way that you get your heat to work I kinda suspect that the valve is the culprit.

hmm,,,Mouth,Sucking,Diaphram I hope Liz nor Shawna read this or the'll give me hell

sam's 72 · 01-20-2003, 03:48 AM

What is strange to me about this, is when I turn on the heater, it just blows cold air. Then while it is still running, I get out open the hood, feel the heater hoses, they are just warm, then I keep one hand on the hose, then disconnect the vacuum hose, and I feel the flow of water, and the hose gets so hot I got to move my hand, then it starts blowing hot air, so I know the valve is opening at this point, but what I dont understand, is if it takes vacuum to open the valve, then why does it work when I disconnect the vac hose from the valve. This problem just started two days ago, been working like a charm until then
Sam

magoo72 · 01-20-2003, 07:38 AM

maybe it takes vacuum to close the valve

**GMCPaul** · 01-20-2003, 08:06 AM

The problem sounds as though it is at your in dash controls, the top lever has a small pin that sticks up and engages the valve that switches the water valve vacuum from different positions depending upon if heat or A/C is desired, if this lever is broken valve does not get moved, its possible for pin to break or get out of track on the slot it goes into also, and theres always the possibility that you have the vacuum lines reversed on the heater control vacuum control valve and your water valve is open when you have the A/C on and closed on heat.
GMCPaul

Andy4639 · 01-20-2003, 11:59 AM

The next time under the hood have a small peice of paper with you when the vacuum line is off and see if it sucks up to the hose, that will tell you if you have vacuum or not. Hope this helps.

Check all your connections also, the line could have come lose are rubbed on something and is cut, you never know until you look.

Tynee · 01-21-2003, 11:44 AM

My manual shows the water valve closed with vacuum applied. I know that other diagrams have been posted that showed closed when vacuum applied. I'm with GMC paul on this, though. I'm guessin that if you pull your controls, you'll find the problem. I know that's where my prob is.

Tynee · 01-21-2003, 02:16 PM

I meant that I had seen diagrams posted that showed open when vacuum applied...

01-19-2003, 11:37 PM	#1
sam's 72 Senior Member Join Date: Nov 2001 Location: Pasadena, Texas, USA Posts: 1,005	Heater problems! What ya think it is? On my 72' I have the heater valve under the hood. The one that works off of vacuum. Well, here lately, when I turn my heater on, it will not blow hot air. So, I pull over take the hose off the valve, the valve opens up and the heater starts working. Why is this? I mean, if it take a vacuum to open the valve, then why is it when I disconnect the hose from the vacuum valve, it starts working? My main question is this, what is bad, the vacuum valve under the hood, or the vacuum switch, at the ac/ heater controls? Can any body shed some light on this for me. Thanks, Sam __________________ 72 LWB chevy truck. Pasadena, Texas

01-20-2003, 03:26 AM	#2
sam's 72 Senior Member Join Date: Nov 2001 Location: Pasadena, Texas, USA Posts: 1,005	com on guys! Some has the answer to this, as much knolege that is on this board. help me out here before I go replceing everything Sam __________________ 72 LWB chevy truck. Pasadena, Texas

01-20-2003, 03:48 AM	#4
sam's 72 Senior Member Join Date: Nov 2001 Location: Pasadena, Texas, USA Posts: 1,005	What is strange to me about this, is when I turn on the heater, it just blows cold air. Then while it is still running, I get out open the hood, feel the heater hoses, they are just warm, then I keep one hand on the hose, then disconnect the vacuum hose, and I feel the flow of water, and the hose gets so hot I got to move my hand, then it starts blowing hot air, so I know the valve is opening at this point, but what I dont understand, is if it takes vacuum to open the valve, then why does it work when I disconnect the vac hose from the valve. This problem just started two days ago, been working like a charm until then Sam __________________ 72 LWB chevy truck. Pasadena, Texas

01-20-2003, 08:06 AM	#6
GMCPaul Senior Member Join Date: Mar 2000 Location: Rockville,In. USA Posts: 3,718	The problem sounds as though it is at your in dash controls, the top lever has a small pin that sticks up and engages the valve that switches the water valve vacuum from different positions depending upon if heat or A/C is desired, if this lever is broken valve does not get moved, its possible for pin to break or get out of track on the slot it goes into also, and theres always the possibility that you have the vacuum lines reversed on the heater control vacuum control valve and your water valve is open when you have the A/C on and closed on heat. GMCPaul __________________ Secure Online Catalog for-GMCPauls Truck Parts Email- GMCPAULS@GMAIL.COM

01-20-2003, 11:59 AM	#7
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Valve OPEN/CLOSED The next time under the hood have a small peice of paper with you when the vacuum line is off and see if it sucks up to the hose, that will tell you if you have vacuum or not. Hope this helps. Check all your connections also, the line could have come lose are rubbed on something and is cut, you never know until you look. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

01-20-2003, 03:40 AM	#3
marcuspboldt Registered Abuser Join Date: Nov 2002 Location: Ft.Worth Texas Posts: 1,001	I'm wondering have tried to pull a vacum on the valve, by unhooking the hose from the vacum source and seeing if you can pull vacum on it with your mouth. If you just keep sucking air the valve's diaphram is shot . From the way you describe the way that you get your heat to work I kinda suspect that the valve is the culprit. hmm,,,Mouth,Sucking,Diaphram I hope Liz nor Shawna read this or the'll give me hell

01-20-2003, 07:38 AM	#5
magoo72 Registered User Join Date: Jan 2003 Location: minden la Posts: 114	maybe it takes vacuum to close the valve

01-21-2003, 11:44 AM	#8
Tynee I miss this truck. Join Date: Dec 2001 Location: Lexington, KY U.S.A. Posts: 2,863	My manual shows the water valve closed with vacuum applied. I know that other diagrams have been posted that showed closed when vacuum applied. I'm with GMC paul on this, though. I'm guessin that if you pull your controls, you'll find the problem. I know that's where my prob is. __________________ '07 GMC Acadia SLT. Sweet ride. '08 Crew Cab Z-71 short bed. Really like this truck A LOT.

01-21-2003, 02:16 PM	#9
Tynee I miss this truck. Join Date: Dec 2001 Location: Lexington, KY U.S.A. Posts: 2,863	I meant that I had seen diagrams posted that showed open when vacuum applied... __________________ '07 GMC Acadia SLT. Sweet ride. '08 Crew Cab Z-71 short bed. Really like this truck A LOT.



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