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Vacuum Generator for Dash Controls - Venturi Type

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Back in October I posted a question on the old site about my Dash AC system when the proverbial problem hit my dash AC system where you lose control of the vents and all of the air comes from the defrost vents.

There was a post from a member, Jim M. with a 2002 Knight that I saved to work on the problem later. Here is his post below.


I went through this a few months ago. Yes, the black rectangular box is the vacuum generator. I disconnected the hose from it to the reservoir and confirmed no vacuum from the generator.

In my case—and likely yours—the vacuum generator is a venturi gizmo, as verified by an air pressure line going to it (I know; seems ironic). So the next thing to check would probably be air pressure to the vacuum generator.

I ended up fixing mine by taking it apart and blowing out the venturi; it was plugged. But many folks replace them with vacuum pumps.

Jim M.

2002 Monaco Knight 30PBD

Jim M, or anyone else reading this post, I would love some more specific advice as to what to do as I now have the box and components removed from the firewall.

Do I just use my compressor and blow air down through the venturi from the top? Is there anything else I need to do before reassembling it back to the firewall?

If this doesn't solve the problem I will most likely purchase an electric vacuum generator to put in it's place. However I am very confused as to how THIS vacuum generator operates the air flow direction control when I don't see any vacuum hose going through the firewall into the dash area.

Does anyone know how this all works because I don't!

Thanks so much!

Edited by Dr4Film
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The vacuum line from reservoir will go to a set of vacuum switches/solenoid, likely at your left hand under the shifter pad. From there through the usual vacuum hoses to individual actuators. Inside the black box is a vacuum pressure switch and an air valve to open the pressurized air until enough vacuum is reached. If you have no vacuum at all, disconnect the pressure line and check for air coming out from your tanks, you have to have at least 60psi or more in the tanks. The two wires are ground and positive from your fan switch and control the air valve through vacuum switch. It's been a while since I had black box open but I thought it was pretty simple in there.

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I guess I need to trace the vacuum line from the reservoir as it appeared to head over towards the vacuum generator ONLY but based on your explanation there must be a T someplace between the round black reservoir tank and the vacuum generator.

I will look closer but it's difficult with so much wiring,  hoses and air lines in that area of the firewall.

I appreciate the explanation.

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Hmm, I don't know why you think that there is a T, there is none in my installation. Straight line from the generator to reservoir and an other line from reservoir to the complex system of vacuum solenoids upstairs behind a side cover by my left knee. Notice the output line changes from a rubber hose to a thin nylon line going up. But if you have no vacuum at the reservoir, I would not even dig into the solenoids, these are hard to access and a bit complex to figure out which is which. On an other note, I already replaced 2 of the solenoids over the years. The red line with a push connector at the bottom of the generator is the high pressure line. 


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Thank you SO much for the explanation and the photo. After reading your post I went to my coach to look closer at the entire system. Lo and behold, while working around the firewall area this past summer on a special project I had inadvertently knocked off the tiny tube from the second port of the vacuum reservoir by accident and didn't realize it. So most likely my vacuum generator is operating just fine.

But since I have it all apart I am going to do some cleaning on it anyway, replace the vacuum  hoses and assemble it back together. At least now I understand how this operates better and all of the connections.

Thank you again. You have saved me lots of work.

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To close out this topic I wanted to do a follow-up with the final results.

I went to NAPA and purchased a section of new vacuum line which goes from the reservoir tank to the vacuum generator. I then took my compressor and blew out the ventrui in both directions and reassembled it back onto the firewall. Started the coach up to air up the system and voila, the dash air flow system is back working correctly.

Now to be fair, I believe that the main cause was self-inflicted in that I must have accidentally knocked off the tiny vacuum line that goes from one port on the vacuum reservoir through the firewall into the dash controls. I didn't realize it and frankly that tube is so tiny that I had to go looking for it as it wasn't quickly apparent to me.

Ivan helped a lot in explaining the operation because I didn't even realize that there was a tube missing. Thank you Ivan!

Vacuum Generator.jpg

Vacuum Reservoir.jpg

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  • 1 year later...

My vacuum generator was damaged and I need to find a replacement for my 2005 Dynasty.  It looks exactly like the picture referenced earlier in this post.  It has the high pressure red air-line coming into it and a vacuum hose coming out of the unit connected to the round looking vacuum reservoir.  Does anybody know where I can find a replacement unit?  Could I replace this box with a standard automotive electric vacuum pump and plug the red high pressure air line?



steveg 2005 DDIV

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