Water Valve question

[gman]

This probably sounds ridiculous, but what does the "water valve" do? What is its purpose? I see it has a vacuum fitting on it but mine is not hooked up. The vacuum or the hoses. The lower heater hose goes directly to the intake. After reading a recent post by 6772king, GMC Paul explains his vacuum connections. His diagram shows the valve "open with vacuum applied". My 72 manual shows "closed with vacuum applied". I'm uncertain what this is doing and its purpose. Maybe this is part of my problem where my temp gauge on dash barely moves out of the cold range. Maybe also the thermostat is bad but one of the housing bolt heads is broke and I haven't got a bolt extractor yet. Just recently purchased my 72. Any explanation as to what the valve does and how it works is appreciated. Thanks!

[da-burb]

The main purpose is to shut off coolent flow to the heater core so you get maximum cooling. This wouldn't have anything to do with your temp gauge. That is probably a thermostat that is stuck open or a failed temp sending unit/temp gauge. It would cause minimal heat in your cab if the valve was always closed.

[Tynee]

I'm currently having issues with that valve. Its called a "Heater Control Vavle." I hate to disagree with more experienced members, but my valve closes with vacuum applied. I just bypassed the thing, too and I don't think that any of the problems I have can be blamed on that. I would have left the heater hoses running through the valve if I could figure out why I always have vacuum to that valve, but that's for another time. I'm guessing that the vacuum to that valve is controled by one of the 3 busted levers on my Heat/Ac control.

I would definitely change the Therm for the cold temp gauge. (Actually sounds more like there isn't one there at all.

[gman]

da-burb,

Thanks for reply. Then, does that mean that come summer, my inside cab temps will be roasting me to death? Hell, I was planning on getting the AC working but it sounds like that would be useless unless I use the valve for what it was designed. Bypassing the water valve will allow coolant to the core all the time so the cab will always be hot, correct? What do you think, da-burb?

[da-burb]

Well not any hotter than the rest of us non-A/C truck owners. We don't have a shut off valve in-line on our heater hose. It is possible that air will always pass through the heater core on an A/C truck. I am not real familiar with the A/C system. If that is the case, then yes you would have heat all summer long without the valve installed.

Tynee, on one of your broken levers is a small valve switch. It will have 3 or 4 small hose connections. 1 goes to the heater valve and 1 goes to the fresh air door on the passenger side below the glove box. The third comes from a vacuum source on the engine. That switch is either broken or it is switched "on" or connected wrong.

And I might be forgetting one thing or another. The last time I worked on one of these was last winter, it has been a long time and I don't remember everything. The best thing you can do is get a factory service manual and a rebuilders manual. They are worth every penny.

[Capitol68]

well I have a non ac cab and I do have a water shut off on the heater core line. I defiantly shut that puppy off in the summer, I dont need any more heat in the cab during the summer mths,
and in the winter even in the upper 20's my heater will roast me.
One thing I do is stand a clip board on edge to block air from going under the seat puts much more heat to the cab.I dont have a radio console yet

[Tynee]

Burb, Thanks for the input. I've got the factory manual, but by-passing the valve was quicker than doin' the research and trouble shooting for the winter anyway. Time is precious when you aren't ready for finals yet.

[lofly'a]

heres a tip for getting the most of your cooling/heating system in some emergency cases. the control valve is there to stop flow thru the heater core (as stated in previous posts) during warm weather when not needed, but can serve as emergency engine cooling if you are having trouble on a temporary basis like towing or hauling during hot weather, and will lower engine temp if you run it to get to a service point. if you need a cheap control valve, you can install a manual cut-off in that location to stop excess heat during sweltering weather. this will inhibit defrost capabilities if the valve is not open though.
most manufacturers found out that they could cut down on radiator size if they circulate coolant thru the core at all times(instant mini radiator helper), and dumped the control valve years ago (i think around 86 or 87), so now then the thermostat on the dash is turned form cool to hot, a damper dumps all the heated core air to the outside. on a side note this also benfits the consumer by not letting coolant sit stagnant in the core all summer to degrade it, and they last longer.

[gman]

Thanks da-burb and everyone else. I'll be looking at my Assembly Manual this evening. Just got it from Chevy Duty two days ago.

[Andy4639]

You said in your post that it is closed with vacuum applied then what does it do if you take the vacuum loose? If it opens then you either have a bad valve are the hose is hooked up wrong. Both are easly fixes. If it's the valve just replace, if it's the hose see how it's hooked up with the manual and check hook up. You also said that your handles on the control panel were broke, check and make sure it is operating the vacuum valve when you move the lever. This is the small valve on the dash unit.

[Tynee]

OK,
After further investigation of my factory manual, The valve is functioning properly. It is supposed to close when vaccum is applied. I checked the levers and the cables attatched to them. In the position that there should be no vaccum applied to this valve, I'm still getting vaccum. I'm thinkin' bad vaccum control switch. Sound resonable to any one else? Sorry G-man, I didn't mean to steal your post.

[chewed 71'er]

I think one of the reasons for the shut-off valve is the heater core is located INSIDE the cab on AC trucks. Non AC trucks have thier heater core in the engine compartment. For AC, air is diverted through the evaporater, but a 180-190 degree heater core in the cab is going to radiate heat. As mentioned, check your vacuum control valve, and you might check the proper routing of the vacuum lines using your manual.(my 2cents)

[Tynee]

Good point chewed. Thanks

[gman]

No problem, tynee. I'm learning from your questions too. What chewed says sounds logical to me too. I kinda figured their would be some heat in the cab when bypassing the water valve. But thanks to everyone for there replies. I'll be checking mine, and I've been planning on redoing all my vacuum lines and hoses anyway. I'll be busting out with the vacuum gauge too to make sure all the components using vacuum are working. Thanks!