| Author |
Message |
LABryan
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, June 10, 2005 - 10:28 am: | |
Need help troubleshooting sudden air loss. My bus loses air and suspension drops within 24-48 hours of being parked. I always assumed it was due to slow leaks from airbags or fittings, but was never able to locate any leaks. Last weekend I was sleeping in back after a long drive when awakened by loud hissing and feeling the bus settle. Next morning air was at zero. I needed to reposition the bus in campground so aired up to about 90 and moved the bus a few feet. About 8 hours later, this time while I was outside the bus, the same thing happened. Loud hissing from rear axle area for 10-15 seconds and bus settled to bottom of suspension. I wasn't able to get there fast enough to determine exactly where the sound was coming from. Any ideas on what's wrong? 90 Prevost XL. This bus has liftable tag, which works fine, but it seems to me may be involved. Any thoughts or ideas will be appreciated. Thanks.
Bryan |
Doug Wotring
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, June 10, 2005 - 11:45 am: | |
Get it over a pit and look could be a number of things.
bad leveling valve, bad air hose, bad air bellow etc |
R.J.(Bob) Evans (Bobofthenorth)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, June 10, 2005 - 12:14 pm: | |
My guess would be a QRV or a relay valve but that's all it is - a guess. You need to get it on blocks or over a pit so you can get up close and personal with the leak. One nice thing about a sudden rapid leak - it will likely have blown a clean patch around the leak location which makes it easier to find. |
Buswarrior (Buswarrior)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, June 10, 2005 - 1:43 pm: | |
Hello LABryan.
the quick and simple answer might be:
If the suspension air tank is empty, if the bellows slowly leaked down, the height control valve will eventually move to the position to put more air in the bellows, to pick the height back up. This will have the effect of exhausting the bellows rapidly "backwards" into the empty tanks, lowering the bus to the axle stops.
Unfortunately, it becomes more difficult to play detective without additional gauges. Your instrument panel gauges, in a 1990 Prevost, are intended to show air pressure in the two dual brake system service tanks, and that's it.
Now, as to why the tanks may be empty, perhaps you have a tired or sticking valve somewhere.
By tired, I mean that the internals of some valve have grown dependant on the tank pressure to keep itself properly sealed inside, instead of its own springs and seals.
As tank pressure drops to the critial pressure, this tired valve gives up, its internals moving into positions which by design they shouldn't and allows the remaining air to leave the system, when that part had its own air integrity.
The question will be, which valves might be suspect to behave this way?
Perhaps a pressure protection valve, which is supposed to allow the compressor to build the brake tanks to a functional pressure before allowing the air to go to the suspension and other less important accessories.
When parked, if the brake tanks leak pressure first, the whole system will drop together until the pressure that the PPV will close is reached. At that point, the air pressure in the suspension tanks will be trapped, and the brake tanks will continue to slowly leak down. If the internals of the PPV are only being held tightly in place by the brake tank pressure, eventually, they may move to a position that allows that suspension tank air to follow into the already lower brake tanks.
If the PPV has completely failed, the suspension/accessories and the brakes are directly connected and will build and leak down together.
If the PPV is working properly, either the brakes or the suspension/accessory will retain air pressure, if that part isn't leaking.
At this point, it will depend on what type of suspension height control valves you have. The suspension will go down with the release of air in the tank, if your height control valves are not designed to trap the air in the bellows.
Stock height control valves are supposed to trap the air in the bellows when the coach is at proper ride height. Even with empty tanks, the bellows will stay inflated, if they and the control valves are sound.
If you have some sort of leveling system in your Prevost, which leaves the bellows exposed to tank pressure, down you go.
I hope these ramblings are of some use to your thought processes.
happy coaching!
buswarrior |
Jim Ashworth (Jimnh)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, June 10, 2005 - 4:53 pm: | |
Prevost had a service bulletin that dealt with this phenomenon. Originally, the tag axle air valve depended on accessory air to keep it CLOSED. Any loss of air (like you are experiencing over time) would allow the tag valve to open, rapidly exhausting the air in the tags, making the rear end drop. The changes are made to use accessory air to keep the valve OPEN when the bus is running and when shut off, the valve closes due to lack of air. This locks the air in the tag when shut down. Talk to Prevost about this service bulletin and make the recommended changes.
The changes involve moving air lines on valves, reversing the tag lift valve plumbing on the actuating lever valve beside the driver and installing a relay to actuate the dash lamp. Prevost has a kit available with all the parts needed to do the job and it varies from year to year. You need to do only the changes that apply to your serial number.
Jim |
LABryan
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, June 10, 2005 - 5:41 pm: | |
Thank you, guys for your help. Excellent advice. Jim, I think you have nailed the immediate problem, and Buswarrior, your post helps me understand the problem and gives me additional ideas regarding my slow air loss. Right on!
Bryan |
Jim Ashworth (Jimnh)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, June 10, 2005 - 8:28 pm: | |
Bryan-As a follow-up to the air leaks, the belt tensioner valve mounted on the body over the curb side engine door is famous for leaking. I now have resolved to accept a loss in accessory air pressure due to that valve. Your accessory tank has a gauge on it but that is plumbed in after the suspension protection valve. If you have an extra hole in the dash, you might put an air gauge showing the accessory tank pressure. VERY different than the brake tank gauges, even when running. Suspension changes, air wipers, etc come off the acc tank which goes up and down totally independent of the brake tanks which have check valves on them. Once air goes in a brake tank, it can only go out through the brake treadle. None of the brake tank air is used to supplement the accessory tank air. Even the parking brake is on the acc tank. Soap everything in your search for the air losses. And... good luck!
Jim |
Buswarrior (Buswarrior)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, June 10, 2005 - 9:38 pm: | |
Nice job, Jim!
All together, we will conquer air loss!
happy coaching!
buswarrior |
John Jewett (Jayjay)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, June 10, 2005 - 10:49 pm: | |
LAB, try the soap routine on all air line fittings, or buy SNOOP at an industrial supply house. I'ts a super fine bubble stuff that will show leaks that the soap won't. Kinda' pricey, so I use it when all else (soap) fails. ...JJ |
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