RV 011185 Vacuum Pump cycling continuously

[Gyrophoenix]

Yesterday I started hearing a chattering noise from under the dash. Turns out it's coming from the RV 011185 vacuum pump, which creates the vacuum used to move the "doors" which direct the air from the front dash A/C (and non-A/C) air vents.

Turning the mode control dial on the dash does indeed result in the air coming out of the correct vents. But the chattering goes on for 6 seconds, then off for 10 seconds, then on again, off again, etc....

So apparently enough vacuum is being created to move the air diverter, but the vacuum isn't being maintained, and the pump keeps cycling.

I doubt the pump has gone bad. I suspect there's a leak in one of the vacuum hoses.
Which brings me to the most basic question: How many hoses are involved? Only two are connected to the pump. Are there more?

Has anyone experienced this? If so, how difficult is it to track down the leak? Is this something I should leave to an RV service center? I service most everything else on the coach, but a peek at the firewall above the generator is downright scary.

Thank you for your input.

 

[Journey1]

Yes- don't quit now look for broken or (vacuum) leak around those black tubes.

 

[Gyrophoenix]

Yes- don't quit now look for broken or (vacuum) leak around those black tubes.

I'm more concerned about the leak being in one of the hoses attached to the black tubes that come out of the pump. I'd think that's where the leak would most likely be located.

The question also occurred to me that I'm not sure how to detect a leak in a vacuum hose. In a pressurized hose it's easy: You can feel the air blowing out through the hole. How would I detect air being sucked into a hole in the hose?

 

[Jubilee]

Check for vacuum leak with smoke

 

[Gyrophoenix]

Check for vacuum leak with smoke

Have you seen where the pump is located, on the firewall above the generator, surrounded by wires? No way to use smoke.

 

[Jubilee]

Well, maybe you can pressurize the system then

 

[Gary RV_Wizard]

Maybe time for help from an HVAC pro, most likely an automotive a/c shop.

 

[Gyrophoenix]

That's what I'm thinking, Gary. I have a few easy ideas to try first, and if no joy.....

 

[workinonit]

A good HVAC tech is going to want to pressurize the system with a little nitrogen traced with some refrigerant so it can be leak detected. Are all of the connections accessible?

 

[bigb56]

You can buy a Mityvac for a reasonable cost at any parts store or HF and isolate where the leak is by disconnecting and capping different lines and using the Mityvac on different branches. The leak can be in a line, in a vacuum motor, in a one way valve, in a switching mechanism or a crack/seal in the reservoir. Start at the main pump reservoir connection and work from there using the process of elimination for each branch/component.

Also note that many mode selector switches will leak by design when switching between modes, but should hold vacuum once in a mode.

 

[Gyrophoenix]

A good HVAC tech is going to want to pressurize the system with a little nitrogen traced with some refrigerant so it can be leak detected. Are all of the connections accessible?

Not easily. I may have to take it in to be fixed.

 

[RayIN]

Depending on the brand of your dash air conditioning, the blend doors vacuum motors might have a leak somewhere.
Close to the vacuum pump should be a small vacuum storage tank, most are a round ball shape. As they age they can crack. Clean it off and if you see anything that looks like a crack, seal it with thin glue, fingernail polish, etc.
The 1/8" vacuum hose runs from the pump to the storage tank, from the storage tank to the blend doors in coaches of our era. Like in this example..

093-0554SS - Comfort Air GR LLC RV HVAC Parts

Vacuum & Electrical Diagrams for Class-A Motorhomes with Rotary Controls

comfortairgr.com

This video is for engine vacuum, but should also work for the dash controls vacuum system.

 

[Gyrophoenix]

Alright.... new stuff to report!

To reiterate from the original post:
"Turning the mode control dial on the dash results in the air coming out of the correct vents. But the vacuum pump chattering goes on for 6 seconds, then off for 10 seconds, then on again, off again, etc....

So apparently enough vacuum is being created to move the air diverters, but the vacuum isn't being maintained, and the pump keeps cycling."

So... the question is: Is the leak in one of the hoses/connections (which, from a repair time and cost perspective would be a disaster), or has the pump gone bad? The seals do wear out eventually.

After cogitating on the problem, it occurred to me that as there's only one vacuum hose leaving the vacuum pump, which goes directly to the round, black vacuum reservoir, if I isolate the vacuum pump from the vacuum reservoir, that would tell me if the leak is in the reservoir/hose part of the system, or in the pump. In other words, if the hose going to the vacuum reservoir is pinched closed, then if the vacuum pump is OK, it will operate until the proper vacuum in the hose is created, and then shut off. And should not come on again. If the pump is leaking, it will start to cycle again.

(I know you're sitting on the edge of your chair by now.)

The engine was started. The pump started the same cycling observed before. 6 seconds on, then 10 seconds off. The hose pinching clamp was applied and... the cycling changed. It now stayed on for 5 seconds, and stayed off for 45 seconds. Over and over.

I have some thoughts as to what this means, but rather than make a fool of myself, I'd rather hear what those of you with more experience in this area have to say about the results of the experiment. Thank you for your input.

 

[RayIN]

IMO that is a great testing idea/method! For comparison, my 26 yr old MH vacuum pump will run then shut off for 10 minutes or more at a time.
Now you know the pump or short length of hose is the problem.
These small vacuum pumps are rather generic, so any 12V model should work fine for a replacement. I suggest NAPA as a source, because you have a person to work with you and your old pump you remove and take in with you. Incredibly Their prices are not much more than Amazon, but the quality is quite different IMO.

 

[Gyrophoenix]

"Now you know the pump or short length of hose is the problem."
Well, possibly.... but think about this:
Originally the pump went for about 12 seconds between cycling on. With the tube pinched shut, it is going about 45 seconds between cycling on. That means the vacuum was decreasing faster before the tube is pinched shut. That seems to indicate that there's a leak in the tube beyond the pinch point. (There's obviously a leak in the pump as well.) So... Not a good situation.

 

[CharlesinGA]

From the vacuum storage cannister, find the line going to the control valve, do the same thing, from the control valve, you will have to pinch every line at the same time to check the valve. The MityVac pump is the best way to check diaphragm motors, hoses, etc. In years past I have done this several times, to find and repair leaks.

Charles