HVAC vacuum diagram

Tiny
SYSADM
  • MEMBER
  • 1995 GMC VANDURA
  • 250,000 MILES
The van listed above is a C3500. I need the diagram for the HVAC dash control vacuum valve for color/path vacuum lines. Color/numbers for each vacuum line coming from the control valve that is pictured. I believe port #8 on the valve is the source. And I believe port #6 is the MAX setting.

But I am having no luck once installed. If I jump the source to the blue vacuum line, my main flap opens, which would mean MAX, but when installed, nothing works. This is a brand new vacuum valve, and I believe someone has moved the vacuum lines around.

In other words, when the slide is moved to MAX on the dash, what path should the vacuum take? - Source vacuum to what color/number vacuum line?

Thank you.
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Thursday, May 28th, 2020 AT 4:23 PM

12 Replies

Tiny
DANNY L
  • EXPERT
Hello, I'm Danny.

I found this vacuum line guide for your van. I've attached the pictures below. Hope this helps and thanks for using 2CarPros.
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Thursday, May 28th, 2020 AT 5:32 PM
Tiny
SYSADM
  • MEMBER
Thank you Danny for the speedy reply. Once I get back out to the shop I will use the diagrams you sent and hopefully figure this out now. Thanks again. Two thumbs up for this website and help. Thank you.
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Thursday, May 28th, 2020 AT 7:25 PM
Tiny
DANNY L
  • EXPERT
You're welcome!
Let us know what you find out. Thanks again for using 2CarPros.

Danny-
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Thursday, May 28th, 2020 AT 8:00 PM
Tiny
SYSADM
  • MEMBER
Okay. Followed diagrams you sent, everything looks to be correct. When I plug the rubber manifold into the back of the vacuum valve, I hear "hissing". This is a brand new vacuum valve (spent a lot of money for it, hard to find NOS).

So then I blocked every port on the back of the vacuum valve and only plugged in the Source line, still "hissing" sound. I then just plugged in Source and MAX (Blue) in the back of the vacuum valve, still "hissing" and no go.

If I bypass the vacuum valve completely and run a straight line from Source to MAX the vent opens as expected. I am thinking of manually leaving it this way so I at least have MAX A/C in the truck? I know I will not have Def, or NORM, and I might not have heat, but when Winter comes I can manually hook those up and still completely by pass the vacuum valve?

I guess I have a "bad" new vacuum valve? Can the inner rubber grommet in the vacuum valve be lubricated to prevent "leaking" air? Is the rubber just shot? Is there a MacGyver trick to getting the vacuum valve to work again? If not, I can just run the system with this manual setup.

I have included pictures showing the setup. Thanks again.
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Friday, May 29th, 2020 AT 4:08 PM
Tiny
DANNY L
  • EXPERT
Hello again.

The hissing sound usually does denote a leak. Was new vacuum line used? Thanks again for using 2CarPros.

Danny-
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Friday, May 29th, 2020 AT 8:41 PM
Tiny
SYSADM
  • MEMBER
None of the lines have been replaced, because when I bypass the vacuum valve none of the lines leak, and no hissing. Keeping the rubber manifold disconnected from the vacuum valve and running new vacuum lines directly into the vacuum valve to the rubber manifold; as pictured, vacuum valve hisses.

Also, blocking every port on the back of the vacuum valve except the source, vacuum valve still hisses. But plugging straight into the source port and Into the Blue port on the rubber manifold, no hissing. As pictured.
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Friday, May 29th, 2020 AT 10:10 PM
Tiny
ASEMASTER6371
  • EXPERT
Good morning,

I am an old-timer who worked on many of these. That rotary control valve was a common failure for leaking vacuum. There were a lot of times that I got new valves and they leaked as well.

I am afraid you may need to look for a used one with a good valve. I looked for a new control head and they are not available as well.

I would go to www. Car-part. Com and enter the information and your zip code and see if there are any locals around you that may have a good one. I found 11 pages of them available.

Roy
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Sunday, May 31st, 2020 AT 6:10 AM
Tiny
HMAC300
  • EXPERT
To me it sounds like damaged vacuum valve hard telling if it is actually good if it's NOS as it's already 25 years old when you put it in. Try Doormanproducts. Com they may have it as they supplied a lot of stuff OEM. The reason i'm thinking this is because you bypass it and everything or at least Max works as you haven't said anything about trying other areas. The rubber connector could also be cracked causing the leak.
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Sunday, May 31st, 2020 AT 6:16 AM
Tiny
DANNY L
  • EXPERT
Thank you guys. I'm still thinking it's a leak as well.
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Sunday, May 31st, 2020 AT 6:24 AM
Tiny
STEVE W.
  • EXPERT
Inside of these units is nothing special, there are the plastic halves and the moving vacuum control. It is sealed using o-rings that set into pockets molded into the plastic. I have taken them apart and repaired them a few times by using new o-rings. The old ones get a flat spot on them and then leak or the ring breaks. New ones setting on the shelf can have the same issue. I take them apart, install new o-rings and apply some silicone grease so the valve moves freely but seals. Do not use any other type of grease as petroleum based lubes will damage the seals and the plastic. The 1995 Vandura conversion I had leaked when I first got it and I took it apart the same way. If you look on it, one side has a spring that holds the parts together with a plastic pin that has two tabs that come through and snap in place on the other side.
If you gently squeeze those tabs together you can release the parts. Then you can see what the issue is.
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Sunday, May 31st, 2020 AT 5:41 PM
Tiny
SYSADM
  • MEMBER
Thank you all for the input. I have taken apart my old original one, no o-ring, just a rubber pre-molded rotary valve with channels molded in it. It would appear that over time these rubber molded parts shrink and no longer create the vacuum seal needed. That is why I asked if anybody has repaired these. Even NOS will have the same issue, as I have experienced.

These particular 9-port vacuum valves are almost impossible to find new, no third party makes them anymore. Yes, I can find them used, but at this point and time in this project I simply bypassed the vacuum valve and went source to MAX. This seems to fine for me now. The vent works, the heat works, the fan works, etc. I just won't have windshield DEF.

I own a 3D printer, and I have printed rubber parts before. I would love to re-create that internal rotary valve and print it out of rubber like the original. But the channels are amazingly designed, and the mold that made these must have been a great piece of tooling. The other idea I had was a dash mounted assembly, just move the SOURCE to whatever port is needed, manually. But I have way to many projects to even think about this in detail.

Well, thanks again everybody, I took the truck out for a test drive keeping the SOURCE and MAX directly hooked up, bypassing the vacuum valve completely. Worked like a charm. After replacing all the A/C components, I could almost see my breath in the cab.
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Sunday, May 31st, 2020 AT 7:58 PM
Tiny
DANNY L
  • EXPERT
Thanks again for using 2CarPros. We hope this helped.

Danny-
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Sunday, May 31st, 2020 AT 8:48 PM

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