A/C Heat Vacuum Hose Routing

[BillG_69gmc]

I've finally got around to swapping carbs out and with that I promised myself to reestablish the vacuum lines that control the heat/ac vents.

Does anyone have the basic info on what hoses are tied together so that I can spend less time in trial and error.

It's on a 69 GMC with Deluxe system..
Control unit has three ports. Numbered 1, 2, 3. How convenient!

Port 1 is the main vacuum line. No problem here. I put in a new vacuum canister and have that line inside the cab.

Port2 and Port 3 on the controls. One should be for controling the hot water valve to the heater, the other should control the vents for floor, dash and defrost.

The other big question is, the hot water valve that is vacuum controlled. The valve should be inserted in what direction? There is no arrows and I don't know which way the water goes. Althought I suspect it flows from the top of the water pump thru valve, into heater core, then back out.

Another thing is the control cables, which were not hooked up. Which gets connected to which lever?

The valve itself is a plunger type and normally closed with no vacuum. Does the direction of the valve matter? Will water pressure force the valves open if installed one way or will water pressure force valve piston to be harder to open by vacuum if installed the other way? Or do I just over analize this and it dont matter?

Any help, written or pictures would be appreciated.

Thanks

Billg

[jef5150]

elbow that comes out of the valve points forward towards radiator,

[JimKshortstep4x4]

There are two port and three port vacuum switches. I have only seen two port or a three port with the one port plugged.

Anyway here is the diagram for two port. Hope it helps.

Jim

[BillG_69gmc]

Interesting...never thought that one port would be plugged... The images help alot! I am aware that there are also two different water valves... The one I purchased is normally closed, but would be open with vacuum applied. If that works contrary to my needs, I guess I have the wrong valve... it should be right though... I think later year trucks had thier vacuum states changed from one to another.

I did not know the wate valve tied into the vent door valve. The truck had a mechanical replacement valve in place.

Any idea on whether the direction the valve faces matters?

thanks a bunch!

billg

[JimKshortstep4x4]

I am not sure if it would matter but on my 69 El Camino which is setup very similiar to the trucks, the water valve is installed with the vacuum connection closest to the firewall. This makes sense to run it that way for ease of hookup and a shorter vacuum line.

Jim

[Andy4639]

It will take two post, because there are two different hook ups for these trucks. One is for trucks up to 1969 and the other is for 1970 and up. The first is for trucks up to 69. The second is for the 79 and up.

[Andy4639]

This one is for 1970 up trucks.

[BillG_69gmc]

Did the first image show that all 3 ports on the control head were being used? It sorta looked that way, but it was difficult to make out.

I hooked mine up with Port1 Vacuum Source,,, Port 2 Door Diaphram/Outside air... Port 3 Capped.

I have not installed the WATER VALVE yet...it can be tapped into the other vacuum line easily...or if it is the 3 port, it can be added easily.

All in all the control unit functions well, although it seems the OUTSIDE AIR diaphram is sluggish.

thanks!

billg

[Markedz]

Hello all, I am new to the forum but I do have my whits about me. I have a 1970 C20 Longhorn with factory A/C and I am in the process of rebuilding the heating / Air Conditioning system myself. I have some information as well as some questions. In the two images of the manuals above one has a water valve that says "Open" when vacuum applied and the other is "Closed". I believe this does matter when talking about the vacuum head. When the controls are in the heat position the vacuum head is closed (Not allowing for vacuum). This would not work with a a water valve that is "Open" when vacuum applied. My vacuum head only has 2 ports, not three. The summation is: if the controls are in the "Heat position" and the vacuum head is open then you must have a water valve that opens under vacuum. If the vacuum head is closed when in the heat position then you need a water valve that is closed under vacuum. The direction of the flow for the water valve does matter. The intake side of the water valve is the curved tube, the outlet is the straight tube. You can use a garden hose to verify, it will not work if hooked up wrong. Anybody have a part number for a water valve that is closed when vacuum applied?

[Bob_H]

Bringing this back to life with a new but related question.
I have hooked everything up in my 71 factory air GMC. The two port valve opens when the lever is in the full drivers side position - which states "inside" air flow, yet that is when it opens the various flaps to let fresh "outside" air in. Bottom line -it is working opposite of the intended way - as is the entire top lever.
Ideas?
It will come out the "defrost" vents when it is on the drivers side, and the "AC/heater" vents when put to the pass side. This is opposite of the labels on the AC panel.
It seems the two way vacuum valve should be open in all positions except "inside" air - but that is not how its working?
I'm all ears - trying to get it all working properly.

[Andy4639]

Here is a better clearer picture. The cowl vent will close only as good as the seal is and how strong the diagfram is. This stops outside air from coming into the cab. The kick panel side on the passenger side should be open.

[Bob_H]

I fixed the airflow issue- two of the cables were backwards. However, the vacuum is still working opposite. When "inside" is selected,(top lever fully to the drivers side) the vacuum valve allows engine vacuum to all the ports- the fresh air, cowl and heater water control valve. Move it to "outside" or further right and they all close.....
No leaks in my lines or actuators and I have a good 15"-20" at idle. Should work the opposite if I understand correctly, ie vacuum passes through the valve at all times except when it he lever is fully to the drivers side and it is "inside". Am I missing something?
Posted via Mobile Device

[randy.powell]

Subscribed! Getting ready to tackle this myself!

[Andy4639]

The diaframs look the same on these trucks. You need to make sure they work the way they are suppose to when vacume is applied. The outside one should close and inside should open.

[Bob_H]

Ah-ha, I get it now. I was thinking the inside "fresh air" as I thought it was, worked like the drivers side - open = outside air. Not so, the top/cowl one closing means that becomes the "return" route for air in the cabin.
Got it - so in short, working as advertised because I have the cowl vent out right now. That is why I was getting outside air when it opened.
Thanks so much!

[71CHEVYSHORTBED402]

Originally Posted by Andy4639

Here is a better clearer picture. The cowl vent will close only as good as the seal is and how strong the diagfram is. This stops outside air from coming into the cab. The kick panel side on the passenger side should be open.

Yes...........

Old thread but a good one. I'm working on a 1971 with a three port control valve, which I really scratched my head over #3 running to the water valve. After checking my water valve(s) today it's because it doesn't work

#3 opens vacuum when the controller is set to outside defrost (de-ice) which closes the A/C water valves I've used since? doing the opposite what it's supposed to do.

My guess is factory was using up old stock and plugged #3. Can't swear to it, I chucked the old F/W grommet, the hoses were mostly gone, though I'm 99/1 the water valve is correct.

So yeah, the above diagram for the old three port / water valves opens the valve, it's counter-intuitive, at least to me, but maybe that's why they changed them

[Richard]

Originally Posted by 71CHEVYSHORTBED402

Yes...........

Old thread but a good one. I'm working on a 1971 with a three port control valve, which I really scratched my head over #3 running to the water valve. After checking my water valve(s) today it's because it doesn't work

#3 opens vacuum when the controller is set to outside defrost (de-ice) which closes the A/C water valves I've used since? doing the opposite what it's supposed to do.

My guess is factory was using up old stock and plugged #3. Can't swear to it, I chucked the old F/W grommet, the hoses were mostly gone, though I'm 99/1 the water valve is correct.

So yeah, the above diagram for the old three port / water valves opens the valve, it's counter-intuitive, at least to me, but maybe that's why they changed them

As explained in your other thread the three port uses the normally closed water valve. Typically the one with the mounting bracket. All vendors I have seen it seems to be the way the replacement valve are described. 71-72 would have came with the bracket type valve from factory. Of course you could plug the #3 port and plumb like a 2 port but would then use a normally open valve without the bracket.