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Old 03-07-2016, 07:25 PM   #1
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HELP-Air leak at front of '94 Holiday Rambler Imperial

I have searched the forums and web DILIGENTLY for an answer to this - hoping someone out there will be able to help out.

I have a '94 HR Imperial w/Cummins C8.3 and my air tanks have suddenly started leaking down after only a few minutes. I bought this used rig almost 10 years ago @102K miles and it now has 157K on it. It has been quite reliable and trustworthy during that time.

I have developed a significant air leak .. first there was the TOTAL air leak, which I finally traced to a squirrel (I think) having eaten about a 3 foot section of one of the brake line hoses between the front and rear of the coach (no detail needed about that .. I think). After replacing the hose .. and while making sure there we no air leaks in the hose fittings I replaced, I discovered a NOISE from the front of the coach - driver's side. I finally traced it down to this 'valve'? Our '94 HR Motorhome - the one on the right (driver's side) at the end of the air line train .. with the 3 rubber hoses coming off of it. As soon as I shut down the engine, I can hear it leaking, and I don't recall ever hearing that leak before. Originally, the leak would stop if I tapped on the valve, but now the leak continues until the tanks are empty. The air tanks drop from 120psi or so down to nothing in about 10 minutes. Pretty sure this leak was not there before recently. I used to maintain pressure for at least 24 hours.

I have contacted quite a number of RV places... Spartan Chassis, RVChassisParts.com, Camping World, Lazy Daze, Monaco, Allied Recreation Group and a couple others.. as well as FORD Motor Company with no good results.
That part is a FORD part - Part# D8HZ19C747A (but has a different number on it (D7HT-19C747-AB - which I was told was a 'design' number which ref'd the part number). It is a Venturi Vacuum Generator apparently used to provide vacuum for the HVAC controls on the dash.
Spartan says they "know NOTHING" since the builder (HR) added that part. Of course there is NO ONE around who remembers anything about 'build' on a '94 HR coach.. at least none that I can find.

I need to get 2 answers if possible.
1). Does air "leak" out of this part ALL THE TIME? (I'm guessing I might have just never noticed it (possibly it had a 'muffler' on it that has broken off?) and air FLOW is required to generate the flow that creates the vacuum.) There are 3 hoses attached to it. One of them carries vacuum (presumably to the HVAC controls), and one is the 'exhaust?' and is where the air leaks out - it is open to the air. I haven't yet tracked down where the 3rd hose goes to. There is also the main air pressure input that is hard line. It is possible that there is another leak some where that I have not yet discovered.
2). Since this part is no longer made and there is no subsequent part number according to Ford... is there any alternative beyond capping off the line and installing a vacuum pump of some description to drive the HVAC controls?

I believe that his was used in heavy duty Ford Trucks in the mid 90's, but can't prove that. It seems like this would have occurred quite a number of time in early to mid 90's motorhomes.

Anyone ever seen a device like this .. or have one on their coach? Seems all (or at least MOST) of the old timers that might know anything about this have passed on into retirement... or just passed.
HELP!
Steve
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Old 03-07-2016, 08:51 PM   #2
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My knowledge about this is limited, but I think the part you're looking at is the vacuum generator. It is not supposed to leak when the ignition is turned off, I think this indicates that the part is defective and needs to be replaced. I'm not sure that you need the exact part, you need a vacuum generator to manage the vacuum dash controls.

I hope someone with a little more wisdom will step in here.

Jim
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Old 03-07-2016, 09:23 PM   #3
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HELP-Air leak at front of '94 Holiday Rambler Imperial

Could the fourth line be the on/off feedback control for the vacuum generator?
A vacuum generator requires a constant flow of pressurized air to operate. Presumably there is no need for this vacuum when the ignition is turned off. A valve should turn the flow of supply air off. It sounds like the valve has failed and the device happily generates vacuum until the supply of compressed air runs out.
Is the problem just that the air leaks out when turned off, or are there any operational symptoms (e.g. HVAC not working correctly)?
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Old 03-07-2016, 10:20 PM   #4
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Agreed .. all of those issues are true. If the valve is NOT supposed to leak when the engine is off, then I am right.. it has probably NOT been leaking all of these years.

As TwelveVolt says, possibly the forth line IS the 'switch'. As Papa_jim says, it's NOT supposed to leak when the engine is turned off...

SO.. let's say it's defective and needs to be replaced. *BUT* there is no replacement.

What do I do about it?
Surely, I am not the ONLY person that has experienced this situation??
Steve
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Old 03-07-2016, 10:36 PM   #5
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SteveD, take it off and go to your local NAPA store, and have them look and find something that will work. They will have one, it may not have the same fittings but something that you can make work! It is not supposed to leak air at any time, just supply your dash with vacuum! You can check your whole air system for leaks with a spray bottle full of a water/soap solution, and your ear! For now I would remove it, cap it off, find a replacement. Make sure the air system is drained completely before you remove it! After you cap it off, start and build your air, and check the system for leaks, if it still loses air pressure! Hope this helps, and you will be able to safely use your MH, till you find the right serviceable part! You just will not have the vacuum to change your dash controls with it disconnected! Rail!
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Old 03-11-2016, 06:07 PM   #6
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Thumbs up Air/vacuum leak - Solved!

OK...just to get the info out there...

I found NO ONE that had ever heard of any device that generates vacuum at the front of motorhomes at any of the commercial places I contacted (Frightliner, Monaco, NAPA, Fleetpride, Camping World, RV World, Spartan .. and several others). I finally contacted an aquaintance that owns a motorhome very similar to mine and turns out he has the same valve. He confirmed that it IS the part that generates vacuum for the HVAC and that it should NOT "leak" continuously. . (Thankfully he is smarter than me ).

The device has 4 ports. The main airline IN .. and the 'vent' which allows air to vent to the atmosphere when it is generating vacuum. ( This is where my "leak" came from.) Another port is the vacuum output, which generally goes to a vacuum reservoir (as mine did) .. and the last port (the one I couldn't figure out the purpose of) is FEEDBACK from the reservoir. When the vacuum reaches a certain level, that last port "tells" the Vacuum Generator that more vacuum is NOT needed and the flow to generate vacuum is turned off.. and the "leak" stops.

What it comes down to is that I had a vacuum LEAK - not a problem with the vacuum generator. I was able to track the problem down to a damaged vacuum hose going to the 'max performance' door that closes (or SHOULD CLOSE) when you go to MAX AIR setting on the HVAC control. Since that line was open, the vacuum reservoir could never achieve the 'standard' level of vacuum and the vacuum generator was always attempting to generate more vacuum... causing my "leak" of high pressure air. Repairing the line stopped the vacuum loss and the flow of "vacuum producing air".. and my system was restored.

Fixing that vacuum leak brought me back to an air system that would 'hold up' for about 2 hours, but my wife kept telling me she could still hear air leaking. Another 3 hours of searching finally revealed yet another "squirrel bitten" hose up over the top of the fuel tank... hidden like 2 FEET away from the end of the tank and JUST out of reach of anything more than a finger - which allowed me to at least confirm which hose was leaking.

After another trip to the "store", I purchased 100 additional feet of air brake hose and decided to Use the bad hose to pull TWO new lines into the RV from front to back. One to fix the known problem and a 2nd as a spare unless there was still another leak somewhere that we couldn't hear over the noise of the known leak.

2 1/2 hours later (and a number of 'rock bruises' from my gravel parking pad), I had both of the new lines installed and one of them terminated and connected. Providing shop air to the rig at the normal "air out" location ... (so that I didn't have to run the engine needlessly) proved that the air leaks were finally FIXED. And now my air stays up for a "long" time. Not sure how long yet, but at least 5 hours. MORE than enough to stop and fill up or have dinner.

I hope this info helps someone else out along the way.
PS... I HATE SQUIRRELS!!
PPS. I have added a few boxes of mothballs in various places under the rig. Hopefully, this will at least make the location above the fuel tank less inviting for them.
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Old 05-03-2016, 01:14 AM   #7
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The venturi vacuum source

Yup it had my AC man stumped. Was almost ready to buy a electric vacuum pump. I instead removed what I thought was a part out of place and experimented with compressed air on it's 4 different ports. I then remembered that I have a vacuum gauge that I bought at a Kmart about 20 years ago that still works and put it on a tee in line with the part that I now was convinced was the machine that was suppose to be producing vacuum. The blue print from Spartan Chassis helped. Well after putting everything back the way I found it and without breaking anything and running the engine again I saw that vacuum build up on the gauge. The AC man said his gauge wasn't showing that when I had it at his shop. He was the man who first thought about putting a electric vac pump on the MH.
Well all is good now and after doing some more inspection I found one of the vacuum motor mounts had been bent and that was causing the door to stay open in the plenum allowing my defroster to keep a blowing even when I ask it not to by the panel control head selection. Also the so called leak at this fitting (the venturi vacuum generator) is not a leak it is the operating exhaust of this generator. But not to worry that air is only bleed out of the brake system after the regulator determines the pressure is high enough to run the brakes. The regulator is a combination pop off valve and regulator in one. I would hope it has a fail safe design that would shut off all air in fail mode instead of venting it all out of the brake system.
So not all hissing in a air pressure piping system is a leak. In this case the hissing is the sign of a functional pop off regulator. When this thing works it seems to work very well.
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Old 05-03-2016, 09:34 PM   #8
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Quote:
Originally Posted by donaldduster View Post
The blue print from Spartan Chassis helped.
Donald, I would LOVE to see any info you have from Spartan about rigs from this era. I have been able to scrape up pretty much NOTHING. Certainly nothing about the air lines. Could you send to me at
Steve [at] WedgeRacing [dot] com, please ?
Steve
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Old 05-04-2016, 06:56 PM   #9
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Print showing the vacuum generator, via venturi effect

This print is property of Spartan Chassis and is only posted for the betterment of the owners of there chassis.
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Old 05-04-2016, 10:27 PM   #10
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Steve, one thing you might want to check out - it looks to me like the valve your vacuum generator is attached to is a pressure protection valve, that valve should close at around 65psi or so - so that an issue such as this doesn't rob all of your air for the brakes etc.
Kurt.
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Old 05-05-2016, 12:05 AM   #11
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Also the so called leak at this fitting (the venturi vacuum generator) is not a leak it is the operating exhaust of this generator. But not to worry that air is only bleed out of the brake system after the regulator determines the pressure is high enough to run the brakes. The regulator is a combination pop off valve and regulator in one. I would hope it has a fail safe design that would shut off all air in fail mode instead of venting it all out of the brake system.
I should note that the pop off valve or combination regulator/dump valve or "whatever" is a completely separate part from the vacuum generator. It is up stream in the pressure piping from the vacuum generator. I do not see it on the chassis blue print detail. The vacuum generator is totally inadequate in service or design to take the place of the "whatever" part. The safety of the brake system I have not studied but it does need air pressure to work unlike an "AIR BRAKE" brake system that uses steel springs to apply the brakes and air pressure to pull the brakes off and allow travel. True "Air Brakes" are actually steel spring brakes. The brakes we are talking about here are hydraulic brakes with air pressure boosters ( instead of vacuum boosters) These brakes are applied by your foot and the helping force of the air pressure your engine mounted pump makes. They are not intrinsically fail safe in the way rail road train brakes are or semi truck brakes are. But they do have safety features like a true "Air Brake" as a parking brake, engine brakes in most cases, lock up converters on the transmissions that allow it to use the engine better as a braking force, and the most important safety is gauges for brake system air pressure and a warning horn for low air pressure in the brake system to allow for a immediate stop while enough pressure remains in system. None the less the system has a max pressure it is designed to hold and once that is reached a pop off value at the compressor will let it loose and all the extra pressure being vented at this vacuum generator is just doing same thing as the other pop off value does. The compressor is over capacity on purpose in order to shorten the time it take to build up at start up and to handle continued brake use on hills. While cruising most of the compressed air being produced is just dumped over board as surplus. Some RV owners I am sure have added more pressure vessels just to feel safer by storing more air for a hill that comes up that they then did not get off the gas for sooner and end up ridding the brakes the whole way down.
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Old 05-06-2016, 09:23 AM   #12
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Donald,
Thanks for the image. It's not for MY vehicle, but hopefully will provide something useful for me. My Rig *IS* full air brakes - just like the road trucks.
Kurt - thanks for the note on the shutoff; valve - mine is definitely NOT working, since the vacuum leak would drain every bit of air from my rig in pretty short order. I'll check out that valve and see if I can get a replacement somewhere. Alternatively, I might just install the 12 volt solenoid valve that I bought to 'resolve' the problem before I figured out the vacuum leak.

Steve
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Old 05-06-2016, 06:03 PM   #13
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Fits Ford Diesel HVAC 4WD Electric Vacuum Pump New | eBay

Put this in parallel with the old vacuum generator.
That is what I did when I replaced mine.
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