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Prevost Aux air reads zero
Started up my coach after sitting all summer in storage (no power hookup, just trickle charger on chassis batteries), which required replaceing chassis batteries(another story). Let it idle for about 20 minutes while checking out systems. Everything going OK, then I hear a “Pop” followed by a “Whoosh” that lasted about a minute. Shut engine off, took a look around, coach still up and even, not listing in any direction. Started it back up, no alams, but Acc pressure guage on dash reads zero (see pic). Front and rear guages show 115 psi per normal. The Marathon auxillary system which has a separate compressor working fine – runs toilet, bed lift etc.
As far as I know, the Prevost Aux runs the slide – not sure what else. What I don’t know is, is it safe to drive? Can’t tell from my manuals if Aux system feeds air brakes or not. Any ideas what “popped?” any help appreciated.
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18 Replies
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Mike, Aux air plays an important role in the operation of the coach. Just because we do not attach the same level of need to the aux system as we do the braking systems that does not mean you do not need the aux air.
For example, your suspension needs aux air. Your belt tensioners. if your have a retarder it needs aux air. Your seat, door lock, and other vital systems besides the slides need aux air.
I haven’t looked at your pneumatic diagram (you can easily by going to the Prevost publications page and entering your coach information) but my first guess is your aux air gauge is on the manifold in the steer bay that has a solenoid valve to admit or close off air. Tap on it, or verify it is getting power and if that remains closed either replace it, or temporarily by-pass it.
I think it is the horizontal manifold located on the rear (RH) bulkhead in the steer compartment.
Report back.
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Your Marathon’s aux air is isolated from your Prevost Aux air as long as the yellow valve in the steer compartment is closed. So your problem is with the Prevost side that controls your suspension, slide and belt bellows in the engine compartment. If the actual Prevost aux air is at zero, you can’t go anywhere. To verify you actually have zero aux air, open the Norgren filter in the steer compartment to see if you have any air. If you have no air, you must have a big time leak, compressor failure or component failure.
Try this to determine if you have a leak. With the bus off, no noise, open the yellow valve, your aux compressor will come on and you should be able to find the source if it’s an air leak. That yellow valve in the open position merges the Marathon aux with the Prevost aux.
Go go to the forum topic, Video’s of Interest and view my viedio’s on the Marathon system if you’re not certain of how it all works.
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In your steering bay you will find a air filter probably under the mfg logo of SMC or Norgren.
Over time your AUX air has lost the seal to fill this system. Finding the hose underneath the drivers bay floor put your finger on the tube with the engine running you will hear a pop.That will be the valve that seals this system to build pressure.
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AL & Tom,
Is there Marathon Air and Prevost AUx air? I was under the impression they were the same at first but now after chasing some marathon house leaks in Texas this week I don’t think they are the same? The aux air gauge on the dash is for Prevost Aux? not Marathon right?
Bill
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All Marathon guys,
There’s a post dated 5-27-15 in here that starts with Liberty air systems and moves on to Marathon air system.
It explains how the Marathon air system works and isolates Marathon Aux air and Prevost Aux air.
Bill, Prevost Aux air dash gauge has nothing to do with your Marathon Aux air.
Later
bv
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Just so it is clear, Mike probably has full aux air pressure on the Prevost side. But he cannot use the bus because the manifold that has the aux pressure gauge air line on it is likely not getting air to it. I doubt if it was a leak because his gauges tell me it is not leaking anywhere. If his aux system were leaking so bad his aux system was really at zero pressure his brakes would not have 120 PSI and the noise of the leak would be so loud he would hear it.
The aux pressure gauge manifold has a solenoid valve that only opens with the key on. If that valve does not open he not only shows zero pressure, the other 5 air lines on that manifold also do not have air pressure. That is very easy to check by just starting the coach with the brakes fully charged, and pull one of the hoses from the manifold.
The second Prevost manifold likely has full pressure but the coach is still not driveable because each of the two manifolds has some critical functions. This issue has nothing to do with the Marathon aux (house) system, and the previous thread was more focussed on moisture trapping.
If I am wrong, may the fleas of 1000 camels infest my crotch for misleading everyone.
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Hi,
Each air tank is isolated from the distriubutiion line (the one from the compressor) by a check valve at each tank. The Aux Air tank has a regulator to not allow air to enter the aux system until main pressures are above 90
PSI. If the 2 main tanks have air either the aux system has a huge leak, suspension, etc or the inlet regulator is stuck. Most likely it is a leak in the suspension system. Most likely an air line has to the air bags has failed or an air bag has failed.Sorry Jon about the fleas from 1000 camels… ROFL
Mikee
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The aux pressure gauge manifold has a solenoid valve that only opens with the key on. If that valve does not open he not only shows zero pressure, the other 5 air lines on that manifold also do not have air pressure. That is very easy to check by just starting the coach with the brakes fully charged, and pull one of the hoses from the manifold.
Jon – My Prevost aux air gauge works without the key on, but the brake tank gauges have to have the key on.
Al – His problem is the Prevost side, not the Marathon aux.
The loud pop followed by the whoosh points toward a major air leak somewhere.
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Since I have not taken delivery of my coach, I was going to keep my comments to myself. However, people who know me, know that keeping my comments to myself is as likely as having a politician tell the truth, so here goes…
Rereading the original post, Mike says he heard a “POP” and a minute long “WHOOSH”. To me, a pop does not sound like a relay sticking or a check valve sticking. It sounds like something ruptured (hose, air bag, etc.) The minute long whoosh sounds like a bleed out. Not having the experience with my coach, as yet, I yield to the experienced owners to tell me what size leak would bleed the entire Aux system in a minute.
My first reaction, on hearing the pop, would be to try and establish if it came from the front or back, the right side or left side, or possibly the engine compartment (belt tensioner) system. To me, it sounds like some hose may have popped loose, but that would not make much of a pop sound, rather just the whoosh bleed down, so I would lean toward some hose, or bag rupturing. Coach leaning? Air bag. Mike says not leaning??? Again I yield to the experts.
When I get to the seminar this weekend, I will give Jon three pennies. That, along with my two cents here, will give him a nickle to buy flea repellant.
3-F
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Mike,
Questions are asked all the time on air systems. You’ll be able to sort this out from the suggestions posted.
Mentioned above is the yellow valve on the Marathon system. Pictured is the correct position per Marathon Leon of this valve located in the drivers bay on your right.
AL
The lower picture is also used and discussed while owning a bus. Good to have this in your tool box.
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If there was a large leak in the aux system as everyone seems to focus on, the brakes would lose air until below the set pressure of the protection valve. Further the sound of escaping air would be so great Stevie Wonder would see the leak.
We need the owner of the bus with the aux pressure at zero to participate. Right now we are speculating.
Tom has a bus that is 180 degrees opposed to mine with respect to gauges because my brake pressures show all the time and my aux with the key on only. So the owner of the problem really needs to be here.
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Do you have it figured out? If not does your coach still level up. Where were you when you heard pop ans swish. Just a outside thought but it sounds like POP = hose came out of fitting then swish was a tank draining. If you were in a area where there was a big temp swing or cold not unusal for push fittings.
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Jon,
If the aux leak developed after airing up the brake tanks, a complete loss of aux air should have no impact on the brake tanks.
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Sincere thanks to everyone who offerred help. My bus is in storage 30 miles from where I am staying so I had to get back out this AM to check things out. I learned a lot just from reading the posts.
Thanks to Boaleral for the pic and the suggestion. Attached is photo that shows where seal failed, driver’s bay near “Prevost Air” and the valve several alluded to that connects Marathon aux air to Prevost aux air. Valve was in correct position (closed).
Thanks again, all. I am a novice when it comes to pneumatics. Does someone know the name/part number of this seal and where to find one? Is a special tool required to crimp the metal part of the seal to the plastic tube?
Thanks again in advance.
Mike
Mike
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Either cut the hose off the old fitting and replace with another push in fitting or a compression fitting , you will need to take the old brass out and replace it with what ever you want, you should be able to go to a truck shop and get one , take old brass out take it with you you will need a wrench to fit the brass nut and some thread seal
That one is no good any more throw it out it’s garbage
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