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

Lurayfarmer · 08-07-2015, 10:13 PM

My '72 GMC has what appears to be a replacement heater control valve in the engine compartment. However, the vacuum fitting is not connected to anything, and I can't find any information on this in the service manual. Anyone know what it connects to?

**davischevy** · 08-07-2015, 11:39 PM

Should be a small vacuum hose coming through the fire wall to a "T" then to the vacuum valve, passenger side cowl and the heater valve.

Lurayfarmer · 08-07-2015, 11:44 PM

Thanks for the reply. Unfortunately the previous owner or owners did some fairly sloppy repairs. Lots of wiring and vacuum line errors. In this case, there's no vacuum line in the engine compartment that is unused. The reservoir is connected OK and there's a line to the outside air valve that works. I suspect the heater valve T's into something under the dash, as you say, just need to know what.

**davischevy** · 08-08-2015, 04:10 PM

Quote:

Originally Posted by Lurayfarmer

Thanks for the reply. Unfortunately the previous owner or owners did some fairly sloppy repairs. Lots of wiring and vacuum line errors. In this case, there's no vacuum line in the engine compartment that is unused. The reservoir is connected OK and there's a line to the outside air valve that works. I suspect the heater valve T's into something under the dash, as you say, just need to know what.

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**davischevy** · 08-08-2015, 04:14 PM

There should be a tee in that vacuum hose that goes to the air valve. One leg of that T will go to the heater valve.I can't figure out how to send a pic on this phone so I will send you a picture later
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**davischevy** · 08-09-2015, 01:37 PM

Here's a pic. Also check vacuum. You should have 15+lbs.

moon · 08-09-2015, 02:07 PM

This helped me:

Lurayfarmer · 08-09-2015, 03:36 PM

Wow, that's great! Just what I needed to straighten it out. Can I ask where that diagram comes from?
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**davischevy** · 08-09-2015, 04:10 PM

Note, the vacuum reservoir failed on my truck. I had too little vacuum to close the heater valve.

08-07-2015, 10:13 PM	#1
Lurayfarmer Registered User Join Date: Mar 2014 Location: Luray, VA Posts: 73	Heater Control Valve My '72 GMC has what appears to be a replacement heater control valve in the engine compartment. However, the vacuum fitting is not connected to anything, and I can't find any information on this in the service manual. Anyone know what it connects to? __________________ Bill Shenandoah Valley of Va 1972 GMC 2500 Sierra Grande 402, Custom Camper Package

08-07-2015, 11:39 PM	#2
davischevy Senior Member Join Date: Sep 2008 Location: Harrison, Arkansas Posts: 9,860	Re: Heater Control Valve Should be a small vacuum hose coming through the fire wall to a "T" then to the vacuum valve, passenger side cowl and the heater valve. __________________ other Larry Build thread, Arkansas K10 https://67-72chevytrucks.com/vboard/...=755797&page=5 The ability to speak several languages is an asset, but the ability to keep your mouth shut in any language is priceless.

08-07-2015, 11:44 PM	#3
Lurayfarmer Registered User Join Date: Mar 2014 Location: Luray, VA Posts: 73	Re: Heater Control Valve Thanks for the reply. Unfortunately the previous owner or owners did some fairly sloppy repairs. Lots of wiring and vacuum line errors. In this case, there's no vacuum line in the engine compartment that is unused. The reservoir is connected OK and there's a line to the outside air valve that works. I suspect the heater valve T's into something under the dash, as you say, just need to know what. __________________ Bill Shenandoah Valley of Va 1972 GMC 2500 Sierra Grande 402, Custom Camper Package

08-08-2015, 04:14 PM	#5
davischevy Senior Member Join Date: Sep 2008 Location: Harrison, Arkansas Posts: 9,860	Re: Heater Control Valve There should be a tee in that vacuum hose that goes to the air valve. One leg of that T will go to the heater valve.I can't figure out how to send a pic on this phone so I will send you a picture later Posted via Mobile Device

08-09-2015, 01:37 PM	#6
davischevy Senior Member Join Date: Sep 2008 Location: Harrison, Arkansas Posts: 9,860	Re: Heater Control Valve Here's a pic. Also check vacuum. You should have 15+lbs. Attached Images __________________ other Larry Build thread, Arkansas K10 https://67-72chevytrucks.com/vboard/...=755797&page=5 The ability to speak several languages is an asset, but the ability to keep your mouth shut in any language is priceless.

08-09-2015, 02:07 PM	#7
moon Registered User Join Date: Nov 2007 Location: Los Angeles, CA Posts: 84	Re: Heater Control Valve This helped me: __________________ 1970 CST long bed Fleetside

08-09-2015, 03:36 PM	#8
Lurayfarmer Registered User Join Date: Mar 2014 Location: Luray, VA Posts: 73	Wow, that's great! Just what I needed to straighten it out. Can I ask where that diagram comes from? Posted via Mobile Device

08-09-2015, 04:10 PM	#9
davischevy Senior Member Join Date: Sep 2008 Location: Harrison, Arkansas Posts: 9,860	Re: Heater Control Valve Note, the vacuum reservoir failed on my truck. I had too little vacuum to close the heater valve. __________________ other Larry Build thread, Arkansas K10 https://67-72chevytrucks.com/vboard/...=755797&page=5 The ability to speak several languages is an asset, but the ability to keep your mouth shut in any language is priceless.



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