# 69 GTO w/AC Hot water valve theory?



## Jstreet (Mar 9, 2009)

The 1969 GTO came with a vacuum-activated Hot water valve that mounts to the AC box in on the firewall. I have two questions as mine is missing this valve. Is the theory behind the valve to shut off heated water with vacuum from the heater core when the AC is in use? Or, is it to stop the flow of water to the core until the engine has reached operating temperature? From what I've read and can deduce, it would seem that the first is the case, as the thermostatic vacuum switch sensor has no hook-up for this valve. Thanks,

J


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## GTOsarge (Oct 9, 2007)

My thoughts are it's a heat control valve. Do you have heat in side the car now? If the heater core went bad sometimes people would loop the hoses and isolate the core instead of replacing it.


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## Too Many Projects (Nov 15, 2008)

You are correct in your evaluation that the valve was only used in the A/C mode to stop hot water from entering the core and making the A/C work harder to cool it. In A/C mode, the air is drawn from the outside so the system only has to cool the ambient air to the desired temp.


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## Jstreet (Mar 9, 2009)

Great. Thanks. So by this theory, the valve remains open until the AC is switched on. Then, vacuum is applied and the valve closes. So, if I hook my line to the valve and apply vacuum to the other line I should see it activate when the AC lever is in place. But, that would work either way. How can I know I'm not reversing the lines? (I could crawl under the dash I suppose)

There were two lines comming out of the firewall from that area. One is for full manifold vac and the other is for this valve, correct? If I'm reverse-testing the lines and applying vacuum to the valve line while plugging the other, I should get vac while in the AC mode. Switching to heat would void the system and the vacuum would go to 0. I'm guessing if I was on the manifold vac line with the other plugged and switched from heat to AC, I'd see a slight drop but not a full loss. Assuming the other vacuum actuated valves are good of course. I guess I'll have to test those individually to be sure. Looks like I'll be crawling under the dash anyway.


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## Too Many Projects (Nov 15, 2008)

Applying the vacuum to the valve hose may well work in reverse also. I just looked at a control panel I have for a 70 Chevelle and the hose for supply has a larger internal size. Look at your 2 hoses and see if 1 is larger than the other. That should be the supply hose from the intake. The hose for the heater valve has a very small opening.


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## Jstreet (Mar 9, 2009)

Both are ragged out and seem to be the same size, so that won't work. One does have a red stipe on it. the other is solid black. (of course it could have been replaced at some time) I have the Zazarine/Roberts Restoration guide(1985 version). It's not been much help with deciphering the hoses. Although, all the pics of the hot water valve (black and white pics) show a stripe on the line going to the hot water valve. Does anyone know the color codes for the vac lines?

I tried the vacuum gauge as I had suggested and I could get no vacuum lock on either of the lines in the AC mode. (very discouraging)


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## Jstreet (Mar 9, 2009)

OK, from a diagram I found in the restoration guide, it appears the vacuum hose comming out of the grommet on the firewall closest to the driver side is for the hot water valve. This one also has the red stripe and does appear slightly smaller. Although upon vacuum testing, it will not hold vac with the other line plugged and the control in the AC mode. Looks like I will have to test lines and actuators under the dash.


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## Too Many Projects (Nov 15, 2008)

Jstreet said:


> OK, from a diagram I found in the restoration guide, it appears the vacuum hose comming out of the grommet on the firewall closest to the driver side is for the hot water valve. This one also has the red stripe and does appear slightly smaller. Although upon vacuum testing, it will not hold vac with the other line plugged and the control in the AC mode. Looks like I will have to test lines and actuators under the dash.


The control I have here is from a 70 Chevelle and the valve hose has a Red stripe. The supply hose is Black With a wide White Stripe. You probably won't get a vacuum lock in A/C as there are 2-3 more vacuum operated doors doing different things in the A/C mode. Try putting the control on heat and see if the solid Black hose gets a lock. The Red stripe may still bypass thru the control to the other doors. At some point, you most likely will be chasing the rest of the system, as it should hold a vacuum and other hoses/actuators may be leaking.


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## Jstreet (Mar 9, 2009)

Too Many Projects said:


> The control I have here is from a 70 Chevelle and the valve hose has a Red stripe. The supply hose is Black With a wide White Stripe. You probably won't get a vacuum lock in A/C as there are 2-3 more vacuum operated doors doing different things in the A/C mode. Try putting the control on heat and see if the solid Black hose gets a lock. The Red stripe may still bypass thru the control to the other doors. At some point, you most likely will be chasing the rest of the system, as it should hold a vacuum and other hoses/actuators may be leaking.


Thanks Mitch, at least I know which hose is for the valve now. I'll try it on heat and chase down other leaks under the dash.


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