Heater dilemma - The 1947 - Present Chevrolet & GMC Truck Message Board Network

53Rick · 07-24-2009, 12:52 AM

I'm wondering if any of you have experienced this problem. When my heater is off and even when I should be just getting outside ventilation, I get hot air coming from the heater through the interior heat/ac ducts on my '72 GMC. It has factory installed AC. I'm wondering if it is a relay or if for some reason a gate isn't closing. What is the most likely cause? It is hard to drive the truck in the summer with my wife in the cab as it emits hot air and it isn't coming from me (or her). Seems like there is a malfunction somewhere in the heating system where it isn't closing a gate or something. If I close the outside vent, it gets pretty warm in the cab. I don't know much about heating and AC systems. The lever arms on the control panel seem to be working okay. What should I be checking? If a relay of some sort, where would I find it on or in the heater? Thanks.

fixit-p · 07-24-2009, 01:03 AM

My 69 is the same way there should be a valve on one of the hot water hoses to cut the hot water mine is vacuum controled. Also the main flap which diverts either cold ac air or hot heater core air has seperated for the lever and shaft. I can flip the lever but the flap doesnt move I'll snap a pic in the morning.

JimKshortstep4x4 · 07-24-2009, 07:33 AM

The water shutoff valve is probably not working. It is vacuum controlled so a lack of vacuum to the system, vacuum switch or the valve itself can be at fault.

Here is a pic of the shutoff valve, (located on the inner fender). the valve should close with vacuum applied. Also, a diagram of the vacuum routing is attached for reference.

JIm

chevy72pu · 07-24-2009, 10:16 AM

Quote:

Originally Posted by JimKshortstep4x4

The water shutoff valve is probably not working. It is vacuum controlled so a lack of vacuum to the system, vacuum switch or the valve itself can be at fault.

Here is a pic of the shutoff valve, (located on the inner fender). the valve should close with vacuum applied. Also, a diagram of the vacuum routing is attached for reference.

JIm

I have the same problem. How do you do a vacuum check on these to see if they are opening/closing properly? I have ordered a new valve for the heater line to install next week.

mclairmo · 07-24-2009, 12:23 PM

Use a Mityvac to test a component that relies on vacuum for its operation.

**Stocker** · 07-24-2009, 01:18 PM

You can also simply check the temp of the hoses on each side of the vacuum valve by hand. With engine warmed up & running, the hose coming from the engine may feel warm, but shouldn't be very hot (because with the valve closed, hot water will not be circulating). And the hose from the valve to the firewall should definitely not be hot -- if it is, the valve is open.

JimKshortstep4x4 · 07-24-2009, 07:12 PM

chevy72pu,

If you have a 72 as your board name is then the above vacuum schematic is applicable. The older trucks, (70? and back) have a water shut off valve that opens when vacuum is applied.

You can check to see if vacuum is at the valve by having the engine running with the AC on max by pulling the hose off of the valve. You should be able to hear the vacuum leak with the hose off.

Another test would be to run a separate hose to a vauum source on the engine with it running and hook it to the valve.

Jim

chevy72pu · 07-24-2009, 09:43 PM

Thanks for all the info. It is a 72, and I already saved to diagram for future reference. Didn't mean to hijack the thread.

Sammy · 07-24-2009, 09:54 PM

Then really all you would need is a shut-off valve inline on the upper hose. Years ago folks used a prestone flush valve where you could attach a water hose to flush the heater core.
Always learning something around here.

cdowns · 07-25-2009, 07:21 AM

a pair of vise grips to pinch a hose shut is a common way to solve the problem quick and dirty then take the vise grips off around the end of october

**Stocker** · 07-25-2009, 10:35 AM

^^

Red Green would love it!!

07-24-2009, 12:52 AM	#1
53Rick Registered User Join Date: Jul 2004 Location: Silverton OR Posts: 20	Heater dilemma I'm wondering if any of you have experienced this problem. When my heater is off and even when I should be just getting outside ventilation, I get hot air coming from the heater through the interior heat/ac ducts on my '72 GMC. It has factory installed AC. I'm wondering if it is a relay or if for some reason a gate isn't closing. What is the most likely cause? It is hard to drive the truck in the summer with my wife in the cab as it emits hot air and it isn't coming from me (or her). Seems like there is a malfunction somewhere in the heating system where it isn't closing a gate or something. If I close the outside vent, it gets pretty warm in the cab. I don't know much about heating and AC systems. The lever arms on the control panel seem to be working okay. What should I be checking? If a relay of some sort, where would I find it on or in the heater? Thanks.

07-24-2009, 07:33 AM	#3
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	Re: Heater dilemma The water shutoff valve is probably not working. It is vacuum controlled so a lack of vacuum to the system, vacuum switch or the valve itself can be at fault. Here is a pic of the shutoff valve, (located on the inner fender). the valve should close with vacuum applied. Also, a diagram of the vacuum routing is attached for reference. JIm Attached Images __________________ 1971 Shortbox step side 4x4, 350 sbc, 3:07 rear end 1965 Impala SS 400 sbc, Muncie rock crusher 1966 Impala SS 396 bbc, TH 400 1969 El Camino, 350 sbc, TH 350 1971 Snowplow built on a Blazer frame 1972 GMC Short bed, stepside, TH 400, 427 BBC

07-24-2009, 12:23 PM	#5
mclairmo Registered User Join Date: Jan 2006 Location: Take Off Posts: 1,908	Re: Heater dilemma Use a Mityvac to test a component that relies on vacuum for its operation.

07-24-2009, 01:18 PM	#6
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,593	Re: Heater dilemma You can also simply check the temp of the hoses on each side of the vacuum valve by hand. With engine warmed up & running, the hose coming from the engine may feel warm, but shouldn't be very hot (because with the valve closed, hot water will not be circulating). And the hose from the valve to the firewall should definitely not be hot -- if it is, the valve is open. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay Last edited by Stocker; 07-24-2009 at 01:18 PM.

07-24-2009, 07:12 PM	#7
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	Re: Heater dilemma chevy72pu, If you have a 72 as your board name is then the above vacuum schematic is applicable. The older trucks, (70? and back) have a water shut off valve that opens when vacuum is applied. You can check to see if vacuum is at the valve by having the engine running with the AC on max by pulling the hose off of the valve. You should be able to hear the vacuum leak with the hose off. Another test would be to run a separate hose to a vauum source on the engine with it running and hook it to the valve. Jim __________________ 1971 Shortbox step side 4x4, 350 sbc, 3:07 rear end 1965 Impala SS 400 sbc, Muncie rock crusher 1966 Impala SS 396 bbc, TH 400 1969 El Camino, 350 sbc, TH 350 1971 Snowplow built on a Blazer frame 1972 GMC Short bed, stepside, TH 400, 427 BBC

07-24-2009, 09:43 PM	#8
chevy72pu Registered User Join Date: Jan 2009 Location: Sandersville, GA Posts: 298	Re: Heater dilemma Thanks for all the info. It is a 72, and I already saved to diagram for future reference. Didn't mean to hijack the thread.

07-24-2009, 09:54 PM	#9
Sammy Registered User Join Date: Apr 2000 Location: Bonne Terre, Mo Posts: 3,005	Re: Heater dilemma Then really all you would need is a shut-off valve inline on the upper hose. Years ago folks used a prestone flush valve where you could attach a water hose to flush the heater core. Always learning something around here. __________________ '69 Chevy Long Fleet & '71 GMC Short Step & Project "ODD BALL" "You can wash a pig, soak it in most expensive perfume. In the end it is still just a pig." ODD BALL build thread http://67-72chevytrucks.com/vboard/s...d.php?t=478629"

07-25-2009, 07:21 AM	#10
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: Heater dilemma a pair of vise grips to pinch a hose shut is a common way to solve the problem quick and dirty then take the vise grips off around the end of october __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

07-25-2009, 10:35 AM	#11
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,593	Re: Heater dilemma ^^ Red Green would love it!! __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay



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