Air Dryer?

Over the last week, I have noticed that both air gauges, primary and secondary, are dropping pressure from operating 125 PSI to 10 PSI, within 4 - 6 hours. I lifted the rear of the coach with the air bags and used the excellent 'Jon supplied' jack stands to support the rear of the coach, then crawled under to take a look. I used the venerable soapy water squirt bottle on the valves below the tanks, with no leaks. I doused numerous other fitting looking for leaks, but no joy.

But I kept hearing what sounds like air escaping in 10 second intervals (on of 1/2 second, then off for 10 seconds). I hear a slight leak (sort of like a squirt of air, then quite for 10 seconds. It sounds like the leak in internal, being it sounds like the air in being taken from the tank connected to the air dryer. I did not check to see if air was leaking from the air dryer discharge, but should have.

I wonder if a pressure relief regulator inside the air dryer is leaking? Very strange.

Has any others had symptoms such as these and what was the fix?

Hector

Hector,

I think our coaches use the WABCO air dryer and here is the link to everything you ever wanted to know about the dryer including the schematic and troubleshooting tips.

Hope it helps.

JDUB

http://www.meritorhvs.com/MeritorHVS_Documents/mm34.pdf

Has anybody changed their air dryer filter? Does it really need to be changed? How tough is it to change. If I'm looking the correct piece of equipment under there it seems that there is a plug missing on the underside . I say this because it looks really clean inside this little hole about an inch in diameter on the underside of what I think houses the air dryer filter. Can you tell I'm pretty new at this. I'm trying to change everything that needs it so I cvan start out with everything serviced.

I have no clue what to recommend for a change interval but I have changed mine every two years. The result is no issues with the air system such as high moisture or oil in the system, and the frame of the bus near where the dryer "spits" is dry and free of oil.

As the element is working it gets a coating of oil on the surface of the dessicant. Once the dessicant is oil coated (its normal to get some oil from the compressor) it will no longer function and water and oil get past it and into your air system. Life as a bus owner really sucks if you do not maintain the air system because water and oil start affecting how things work, and especially the valves in the suspension system and all your aux air supply devices. I know of one owner whose suspension failed to be controlled because he experienced hydraulic lock on the valves controlling the suspension. That is extreme, but is an example of the type of stuff that can happen.

Change the dryer element routinely, and drain the air tanks every 5000 miles and you will have no issues with the air system in the bus as a result of water or oil getting in it.

I suspect your air dryer element is a spin on cartridge which is actually easy to change. I use a large strap wrench and with a lot of grunting I can get it loose and off. They seem to stick so it does take some effort, but once it begins moving it goes fairly easy. If it was installed without some lube on the sealing gasket I would assume you will need to eat your Wheaties that morning because I use a lube on the gasket each time I install one and it still taxes my superior muscular strength to the limit.

If you see a coating of oil on the bus frame where it "spits" your element is well beyond the point at which it should have been changed.

Please remember that before changing the air dryer cartridge, first bleed (by pumping the brake pedal repeatedly) the air brake system completely. Yes, there are check valves that are "supposed" to prevent primary and secondary tank air from coming back to the aux system, but NEVER RELY ON SAFETY DEVICES to protect you. Bleed all the air out first. Use extreme caution when dealing with the underside of the coach and air systems, as again, there are valves that should trap suspension air in the air bags, but NEVER RELY ON SAFETY DEVICES to protect you. Block the coach body up first before getting under it.
The fleet standard for air dryer replacement PM interval is generally every 20-30,000 miles, depending on humidity/climate. As Jon indicated, changing this too early is never a bad thing. Water in your air system, once it happens, is MUCH more costly. Since the new year, we have had two late 90's Prevost XL's with bills in the thousands for valve and component replacements, both Prevost and house auxiliary air, resulting from water damage in the air system. Hope this information helps.

Scott13
The hole on the underside of the dyer is probably the exit port for bleed air. If you look at the assembly diagram for your dryer, it might show that this could be a threaded port for a nipple.

I've changed mine, and it is not all that difficult. There are several different configurations, so each one has a different procedure for changing. Unfortunately, mine is located in an awkard place, but it is a spin-on. Like Jon said, without change intervals, it is hard to know for sure, but I too have changed mine every 2 years on both buses I've owned.

Pictures of the dryer base.

Sorry if this is annoying, I have to keep in practice posting photos as it is so complicated and requires periodic review:p

I replaced mine last July and I will do it again this year. I have no idea as to how long it was before it was replaced. Liberty may have replaced it 2 years ago when I acquired the coach from them. I would have to check the records. I think though that I will just replace it annually.

It is easy & safe . Mine is located at the top front of the opening after removing the passenger side rear fender skirt and requires no under coach efforts..

As suggested elsewhere, bleed the system down first.

Jim, I for one love pictures keep them coming. They great for us simple minded people.

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Originally Posted by HarborBus

Jim, I for one love pictures keep them coming. They great for us simple minded people.

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That's good to hear Elliott, it is often said " a picture is worth a thousand words", I hate to type, so that is my way out:eek:

;)JIM

Keep them coming, Jim. I can take the picture and after that I am dead in the water. Don't know how them off of the camera. Several people have explained it to me but I am lost. I need show and tell

I'd like to supplement the caution Ben just made.

DO NOT UNDER ANY CIRCUMSTANCES GET UNDER THE BUS UNLESS IT IS PROPERLY SUPPORTED UNDER THE CHASSIS SUPPORT POINTS AS SPECIFIED BY PREVOST

Prevost shows where the support points are on their web site and your Prevost owners manual or shop manual should also show where they are. Hydraulic jacks are not fail safe supports.

A word about the suspension system....When the key is off, if the suspension system is functioning properly air is "locked" in the air bags but like all mechanical things the valves that are intended to close off the flow of air from the air bags can leak. Air bags can leak. The associated fittings can leak, and although it is rare, and air bag can suddenly, and without warning, burst. Unless supported your bus can drop, usually slowly, but occasionally very quickly. We lost one POG member because he was under a non-supported coach.

FWIW, regarding the changing of air dryer elements, I have changed mine without dumping system air pressure. If your bus has a properly functioning air system no air will escape. Dumping air in your air system, outside of the suspension will not cause the bus to drop if everything is working OK, but why take a chance? Support the coach.

Aside from the safety point of view the other reason to dump all the air is; now that the bus is properly supported for the cartridge change a body might as well get under it and drain the air tanks.

The only way to do that is when they are empty.




JIM

Just to add to Jon's comments about air bags exploding... We were at Titusville 2 years ago at a mini rally and while visiting with everyone, we heard a loud BANG! Tuga had an air bag explode and his Newell had been sitting 2 or 3 days.

So, it does happen and I can't imagine the underwear cleanup needed if that happened while under the bus!

The pictures are great but now I'm wondering if I have a spin on filter like the picture or the complicated looking thing in my service manual. That one has a canister bolted on the top with the cartridge inside. The Detroit dealer in town suggested I change the whole dryer if it does not have a spin on cartridge. What do you guys think about that? I have access to a shop with a pit, that should make the job simpler shouldn't it? At least the bus can't fall down on me.

I have had two types of dryers. The first was one with 3 or four bolts and a snap ring that allowed the cartridge to drop down, and the second was a spin on type. I'll take the spin on type any day. If yours is not it may be advantageous to swap dryers just for ease of cartridge replacement.

As to your access to a pit, I think you may have a false sense of security and I would urge you to make sure the bus is supported even if it is over a pit. I have a pit and know from experience that a lot of what we do beneath the bus often puts our body or arms or head between pinch points if the bus should come down unexpectedly. When over a pit, it you are reaching over an axle, or across a driveshaft, or are just bending over the edge of the pit to reach up on the chassis, if the bus were to drop suddenly the consquences could be bad. As a standard practice I back in my garage, raise the bus and place the supports under it. That way when I decide to go down into the pit to look at something, or do some work I won't get so involved I forget and place myself in a dangerous position.

Quote:

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Originally Posted by sawdust_128

I replaced mine last July and I will do it again this year. I have no idea as to how long it was before it was replaced. Liberty may have replaced it 2 years ago when I acquired the coach from them. I would have to check the records. I think though that I will just replace it annually.

It is easy & safe . Mine is located at the top front of the opening after removing the passenger side rear fender skirt and requires no under coach efforts..

As suggested elsewhere, bleed the system down first.

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Scott, reference Ed's post above, yours might be the same, but I see it is a few years newer. (nice to have pit access but not necessary) The canister style is very convenient to change.
If yours is different try to change it before you exchange the whole unit, the maintenance change might not be that difficult, just different.
When you get to the task take pictures and post your contribution on the forum. If you don't know how to post pictures take them anyway and some day you will learn how to post or someone else will do it for you.


JIM

The non spin on like I have, the AD-2 will cost you about 238.00 at Prevost for the rebuild. The bag of dessicant and parts come in a kit.

Jim,

Question for you. Are the pictures looking up from the floor or down onto the top of the mounting base? I just need to get oriented this morning.

Thanks

Hector, If that does not prove to be the source of the leak, check the small (1.8") tubing going to the guages on the dash and the brass fittings on the back of the guages. That proved to be the "big leaker" on our 93 CC. New brass fittings and spliced the tubing well back into the dash area where it had been rubbing. We now hold pressure for weeks.

I see the last cartridge I purchased from Prevost (Mar 08) was $61.28. I need to get another one while I doing my annual routine maintenance on the bus.

I did find a Prevost document ( http://www.prevostcar.com/DB/service...ng/Mi04-12.pdf) that said if you have the AD-2 dryer, change it every year. Change all other dryers every 2 years.

I just called my local NAPA that specializes in bus/truck parts, and they have the spin-on cartridge for $29.67.

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Originally Posted by Will Garner

Jim,

Question for you. Are the pictures looking up from the floor or down onto the top of the mounting base? I just need to get oriented this morning.

Thanks

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Will, they are top views, that is looking down.

On an XL you can not get your head in a position to see what the photo shows. You Will, will work blind :p with your arms in the air.

There is a space to get your torso in a workable position, but invariably when you start to work there is some tool or part that is not at hand and you must crawl out to get it. For this job you need the canister some rags a 20" breaker bar w/6"extension and a heavy duty strap wrench to fit, also a little lube for the O-ring and large seal. Be sure to remove the old O-ring. mark the canister with a magic marker (for tightening reference and keep it in the plastic bag until needed.

Schedule Carol to be standing by for tool/part runs.

This job has a three bumps on the head rating :( :rolleyes:


JIM

I believe Will has the cartridge that is held in place with 3 or 4 small bolts and a large snap ring. His is located more conveniently on the passenger side up on the frame in between the tag and drive axles. His cartridge drops down.

Quote:

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Originally Posted by JIM CHALOUPKA

Scott, reference Ed's post above, yours might be the same, but I see it is a few years newer. (nice to have pit access but not necessary) The canister style is very convenient to change.
If yours is different try to change it before you exchange the whole unit, the maintenance change might not be that difficult, just different.
When you get to the task take pictures and post your contribution on the forum. If you don't know how to post pictures take them anyway and some day you will learn how to post or someone else will do it for you.


JIM

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I think my wife has figured out the posting past. I'm going to start with an oil change and a grease job. Then move on to something harder. I have a small window in the door to replace also.