Another HVAC Question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

mikemileski · 11-19-2010, 02:50 PM

My truck is a '72 GMC 4x4 with factory AC. Right now the vacuum hose that leads to the heater control valve has vacuum to it all the time, regardless of whether the control lever is moved to the hot or cold position. The heater valve that's on there now is the type that's open when no vacuum is applied, which means it's closed continuously for me because my system is applying vacuum to it at all times. Does this mean that my vacuum switch is malfunctioning? I have a two port system and the vacuum hoses are plumbed exactly as the diagram for the two port system shows in the service manual. If the control head vacuum switch is bad, is it a separate piece or would I need to buy the entire assembly (and, if so, who makes something that's a higher quality than some of the cheapo replacements I've read about)? Also, is there anybody in Tucson that has some knowledge and experience with this?

Mike Mileski
Tucson, AZ

sporty1200xlc · 11-19-2010, 08:59 PM

Interesting I have the same problem. Just havent really checked into it yet. Maybe you will get it. Good luck.

nyncompute · 11-19-2010, 09:47 PM

I've had to replace the vacuum switch behind the control head myself. A couple of screws hold it on. I'm sure if you suspect a damaged one (mine had an audible leak) someone on the board will sell you one. I now keep a spare myself. Hope that addresses part of your question.

mikemileski · 11-20-2010, 06:31 PM

No audible or obvious leaks, just full-time vacuum to the heater valve.

Mike Mileski
Tucson, AZ

ChevLoRay · 11-20-2010, 06:36 PM

FWIW, when I was working at a Sears auto center, a customer came in with a heater problem, in his FoMoCo vehicle. Upon doing some checking, there was coolant in the vacuum lines to the control ass'y.

I don't recall that any GM product I've ever seen had that problem.

We didn't work on it, so I don't know what the problem was.

glord · 01-04-2017, 01:45 AM

Hello Mike, did you ever solve your issue. I have exactly the same problem and came to the same conclusion that it may be the Vacuum Control switch. However the entire heater control unit appears to have been recently replaced.

Thank you

Andy4639 · 01-04-2017, 01:04 PM

1 thing some over look.
The vacuum port on the valves are different from model to model. They could have bought a vacuum port for a different model that closes it when vacuum is applied instead of opening it.

Just like the kick panel on the truck is opposite of the water valve.
You cant mix and match them up.

glord · 01-04-2017, 09:09 PM

Quote:

Originally Posted by Andy4639

1 thing some over look.
The vacuum port on the valves are different from model to model. They could have bought a vacuum port for a different model that closes it when vacuum is applied instead of opening it.

Just like the kick panel on the truck is opposite of the water valve.
You cant mix and match them up.

Thank you Mike. Understood in terms of the heater bypass valve. However my understanding is the system should only be under vacuum when air conditioner is on.
Posted via Mobile Device

hdff · 01-04-2017, 10:31 PM

Quote:

Originally Posted by glord

Thank you Mike. Understood in terms of the heater bypass valve. However my understanding is the system should only be under vacuum when air conditioner is on.
Posted via Mobile Device

FWIW this thread is 6 years old and mikemileski has not signed on since 2013, although andy4639's info is correct

glord · 01-04-2017, 11:15 PM

Quote:

Originally Posted by hdff

FWIW this thread is 6 years old and mikemileski has not signed on since 2013

Posted via Mobile Device

11-19-2010, 02:50 PM	#1
mikemileski Registered User Join Date: Jul 2009 Location: Tucson, AZ Posts: 66	Another HVAC Question My truck is a '72 GMC 4x4 with factory AC. Right now the vacuum hose that leads to the heater control valve has vacuum to it all the time, regardless of whether the control lever is moved to the hot or cold position. The heater valve that's on there now is the type that's open when no vacuum is applied, which means it's closed continuously for me because my system is applying vacuum to it at all times. Does this mean that my vacuum switch is malfunctioning? I have a two port system and the vacuum hoses are plumbed exactly as the diagram for the two port system shows in the service manual. If the control head vacuum switch is bad, is it a separate piece or would I need to buy the entire assembly (and, if so, who makes something that's a higher quality than some of the cheapo replacements I've read about)? Also, is there anybody in Tucson that has some knowledge and experience with this? Mike Mileski Tucson, AZ

11-19-2010, 08:59 PM	#2
sporty1200xlc Registered User Join Date: Sep 2010 Location: Richmond IN Posts: 50	Re: Another HVAC Question Interesting I have the same problem. Just havent really checked into it yet. Maybe you will get it. Good luck.

11-19-2010, 09:47 PM	#3
nyncompute Registered User Join Date: Nov 2004 Location: Southern Nevada Posts: 532	Re: Another HVAC Question I've had to replace the vacuum switch behind the control head myself. A couple of screws hold it on. I'm sure if you suspect a damaged one (mine had an audible leak) someone on the board will sell you one. I now keep a spare myself. Hope that addresses part of your question. __________________ 1972 K-10 SWB Cheyenne

11-20-2010, 06:31 PM	#4
mikemileski Registered User Join Date: Jul 2009 Location: Tucson, AZ Posts: 66	Re: Another HVAC Question No audible or obvious leaks, just full-time vacuum to the heater valve. Mike Mileski Tucson, AZ

11-20-2010, 06:36 PM	#5
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: Another HVAC Question FWIW, when I was working at a Sears auto center, a customer came in with a heater problem, in his FoMoCo vehicle. Upon doing some checking, there was coolant in the vacuum lines to the control ass'y. I don't recall that any GM product I've ever seen had that problem. We didn't work on it, so I don't know what the problem was. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

01-04-2017, 01:45 AM	#6
glord Registered User Join Date: Jun 2016 Location: Carmel Valley, CA Posts: 37	Re: Another HVAC Question Hello Mike, did you ever solve your issue. I have exactly the same problem and came to the same conclusion that it may be the Vacuum Control switch. However the entire heater control unit appears to have been recently replaced. Thank you

01-04-2017, 01:04 PM	#7
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,945	Re: Another HVAC Question 1 thing some over look. The vacuum port on the valves are different from model to model. They could have bought a vacuum port for a different model that closes it when vacuum is applied instead of opening it. Just like the kick panel on the truck is opposite of the water valve. You cant mix and match them up. __________________ 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.



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.