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

haysonj · 10-17-2008, 08:29 AM

I turned on my heater for the first time yesterday and no heat. The hose out of the water pump is hot but the one returning to intake is not. My question is can the heater valve be installed backwards or do I have a bad valve. There is vacuum at the valve when I turn the controls on. Also this is a new valve,heater core and hoses. Anyone know about this before I start tearing into it. Thanks, John

ChevLoRay · 10-17-2008, 09:36 AM

My original was leaking (several years ago) so I replaced it with one from NAPA that has a screw-type valve that has to be opened and closed manually. Solved the working or not conundrum for me. However, it started leaking from the threaded stem.....

If the switch is sending vacuum to the vacuum chamber, then it should open the valve. It could be that the chamber is bad. There are hand-held vacuum pumps designed to test such components, so getting your hands on one would tell you if you are working with a good, or bad, valve.

Maybe the valve itself, not the vacuum chamber, is sticking and just won't open.

But, from what I remember, it should be hard to get the valve in wrong. When it mounts to the fender well, if it mounts like mine did, it's kinda obvious as to which end goes where, when you put the hoses on. The end closest to the firewall got the hose that went to the heater core. The other side got the end that went to the water pump outlet. I know that sounds simple, but it's all I got....brain damage runs in my family.

JimKshortstep4x4 · 10-17-2008, 06:51 PM

There are two different water valves for our trucks. One shuts the water off when vacuum is applied and the other turns the water on when vacuum is applied. You will want to check your valve to see if it opens or closes when vacuum is applied.

Attached for reference is the vacuum diagram for your year truck. The diagram shows vacuum as it would look for the AC operation. For heat the vacuum would be cut off to the heater valve so that it would be open when the heater is on.

Jim

Longhorn Man · 10-17-2008, 06:58 PM

That valve is only actuated when the A/C is turned on. Check the hose temp on both sides of the valve to see if it is stuck closed, or if it is something else.
If it is hot on both sides, you have bigger problems, there is a door inside the heater box that is probably not working.

haysonj · 10-18-2008, 07:19 AM

Thanks for all the input I found the problem. I thought the feed came from the water pump but on mine that is the return. When I put the FI on there was a sensor that went in the manifold outlet so I moved that hose to the radiator thus eliminating the flow to the heater core. I put a tee at the manifold and moved the hose back now I've got all kinds of heat. Thanks again for all the info.

C20 Blues · 11-15-2008, 10:49 PM

First I want to say Hi,

I have been away from the boards and just started working on my truck again.

Most if not all the parts are missing for the Heater/AC units vacuum lines and wires are cut also. So I have no idea where anything needs to go. Can anyone tell me if the Vacuum Diagram in this thread will work for 70 C20. And if so what is the Vacuum Source and Control Head Vaccum Switch locations?

Thanks for any and all help.

Aarons72s · 11-16-2008, 09:53 AM

The diagaram above will work for you c20 blues. But I would use the later heater valve like on the 71-72 that mounts to the passenger inner fender instead of on the intake like yours. The original vacum source was a vacum t in the intake manifold right behind the carburator. But I usually just look for a full timevacum source off the carb. It is just easier. As for the location of control head vacum switch, it is on the a/c control panel inside your cab. It is controled by the defroster/ inside and outside lever on the control panel.
Here is a quick tip for you. The original system only shuts off the water with the a/c runing but if you would like to shut it off during the summer just pull the vacum line off the heater valve and plug the factory line. Then put a t in after the vacum tank and run a line straight to it. That way it cuts the water flow all summer long with or without the a/c runing.

C20 Blues · 11-16-2008, 10:04 AM

Aaron,

Looks like as you were posting a reply. I Started a new thread. Thanks for the info. It's going to start getting cold here in the next few months so I want to get the heater up and running ASAP.

Thanks again.

10-17-2008, 08:29 AM	#1
haysonj Registered User Join Date: Dec 2007 Location: Santa Maria, Ca. Posts: 1,423	Heater Valve on AC Truck I turned on my heater for the first time yesterday and no heat. The hose out of the water pump is hot but the one returning to intake is not. My question is can the heater valve be installed backwards or do I have a bad valve. There is vacuum at the valve when I turn the controls on. Also this is a new valve,heater core and hoses. Anyone know about this before I start tearing into it. Thanks, John __________________ 69 Chevy 4x4 383 Power

10-17-2008, 09:36 AM	#2
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: Heater Valve on AC Truck My original was leaking (several years ago) so I replaced it with one from NAPA that has a screw-type valve that has to be opened and closed manually. Solved the working or not conundrum for me. However, it started leaking from the threaded stem..... If the switch is sending vacuum to the vacuum chamber, then it should open the valve. It could be that the chamber is bad. There are hand-held vacuum pumps designed to test such components, so getting your hands on one would tell you if you are working with a good, or bad, valve. Maybe the valve itself, not the vacuum chamber, is sticking and just won't open. But, from what I remember, it should be hard to get the valve in wrong. When it mounts to the fender well, if it mounts like mine did, it's kinda obvious as to which end goes where, when you put the hoses on. The end closest to the firewall got the hose that went to the heater core. The other side got the end that went to the water pump outlet. I know that sounds simple, but it's all I got....brain damage runs in my family. __________________ 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.

10-17-2008, 06:51 PM	#3
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	Re: Heater Valve on AC Truck There are two different water valves for our trucks. One shuts the water off when vacuum is applied and the other turns the water on when vacuum is applied. You will want to check your valve to see if it opens or closes when vacuum is applied. Attached for reference is the vacuum diagram for your year truck. The diagram shows vacuum as it would look for the AC operation. For heat the vacuum would be cut off to the heater valve so that it would be open when the heater is on. 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 Last edited by JimKshortstep4x4; 10-17-2008 at 06:58 PM. Reason: added wording

10-17-2008, 06:58 PM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Heater Valve on AC Truck That valve is only actuated when the A/C is turned on. Check the hose temp on both sides of the valve to see if it is stuck closed, or if it is something else. If it is hot on both sides, you have bigger problems, there is a door inside the heater box that is probably not working.

10-18-2008, 07:19 AM	#5
haysonj Registered User Join Date: Dec 2007 Location: Santa Maria, Ca. Posts: 1,423	Re: Heater Valve on AC Truck Thanks for all the input I found the problem. I thought the feed came from the water pump but on mine that is the return. When I put the FI on there was a sensor that went in the manifold outlet so I moved that hose to the radiator thus eliminating the flow to the heater core. I put a tee at the manifold and moved the hose back now I've got all kinds of heat. Thanks again for all the info. __________________ 69 Chevy 4x4 383 Power

11-15-2008, 10:49 PM	#6
C20 Blues Registered User Join Date: Jan 2007 Location: Hesperia,Ca Posts: 7	Re: Heater Valve on AC Truck First I want to say Hi, I have been away from the boards and just started working on my truck again. Most if not all the parts are missing for the Heater/AC units vacuum lines and wires are cut also. So I have no idea where anything needs to go. Can anyone tell me if the Vacuum Diagram in this thread will work for 70 C20. And if so what is the Vacuum Source and Control Head Vaccum Switch locations? Thanks for any and all help.

11-16-2008, 09:53 AM	#7
Aarons72s Registered User Join Date: Apr 2006 Location: oklahoma Posts: 10,670	Re: Heater Valve on AC Truck The diagaram above will work for you c20 blues. But I would use the later heater valve like on the 71-72 that mounts to the passenger inner fender instead of on the intake like yours. The original vacum source was a vacum t in the intake manifold right behind the carburator. But I usually just look for a full timevacum source off the carb. It is just easier. As for the location of control head vacum switch, it is on the a/c control panel inside your cab. It is controled by the defroster/ inside and outside lever on the control panel. Here is a quick tip for you. The original system only shuts off the water with the a/c runing but if you would like to shut it off during the summer just pull the vacum line off the heater valve and plug the factory line. Then put a t in after the vacum tank and run a line straight to it. That way it cuts the water flow all summer long with or without the a/c runing. Last edited by Aarons72s; 11-16-2008 at 04:28 PM.

11-16-2008, 10:04 AM	#8
C20 Blues Registered User Join Date: Jan 2007 Location: Hesperia,Ca Posts: 7	Re: Heater Valve on AC Truck Aaron, Looks like as you were posting a reply. I Started a new thread. Thanks for the info. It's going to start getting cold here in the next few months so I want to get the heater up and running ASAP. Thanks again.



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