Be Aware II

[phorner]

Being new to bus ownership and maintenance, I wanted to be sure that I had a clear understanding of how to properly and thoroughly drain the air reservoir tanks on our 2000 Liberty.

So, while discussing this with Jon and Roger at the Florida Mini POG Rally, they graciously offered to loan me a couple of jacks and give me instruction, while I crawled under the bus after lunch.

I started the bus, brought it to its maximum height and then shut it down. At this time Roger and Jon showed me the correct position for the rear support jacks and they placed them into position. I then managed to crawl under the bus and drain the air tanks near the drive and tag axles completely. I the extricated myself from beneath the bus. At this point we were all under the impression that the air was locked into the air bags and the bus would not move when the support jacks were removed.

However, when the left side jack was lowered, the bus dropped! This was completely unexpected and still unexplained.

Never, never, never place yourself under a bus that is only supported by the air bags!!!

Although we all thought that it was perfectly safe to drain the air from the tanks, and that the bus would still be supported by the air bags, we were wrong.

Always support the bus at the appropriate support points whenever you place yourself under the bus.

A very important lesson was learned here.

And, I discovered that I have new Koni shocks.....

[garyde]

Hi Paul. When you turned the ignition key to the off position, what was the setting on your air switches. Were they in the up position, drive positon, or low position? Was the switch off or on at the time you turned the ignition off.

[phorner]

Gary,

When I shut the engine down, the 3 "level" switches were in the "up" position and the "mode" switch was in the "manual" position.

[truk4u]

Paul,

What was Jon's take on this since he was there?

[dalej]

Paul, this will now lead you to inspect and either clean or replace some of your check valves that keeps air from coming back into the wet tank and brake tanks.

It never stops...this maintenance thing.

[dreamchasers]

Very interesting post. Jon and I have been chatting about the suspension system and the Prevost control process. If you are able to attend the Tennessee Rally, the mysteries of the suspension system will become clearer. Developing the presentation that Jon and I will use has been a great learning experience for me.

While I am still wearing my "Newbie" cap, I will hold back offering any advice or comments. Again, if possible attend the Tennessee Rally in April. I think Jon and I will have a great presentation to offer on the workings of the suspension system.

After studying the suspension system, I definitely agree with your statement of "not getting under the bus unless it is supported by jacks", particularly if you are going to do anything with the air system.

Hector

[Jon Wehrenberg]

Gary may be on the right track. I asked Paul to check for power at any of the leveling system solenoids.

On my vintage, key off removes power from all leveling system solenoids and the five port valve should return to the normally closed position thus locking air in the air bags.

In this instance, with the switch positions Paul described it appears that since the switches were in the "up" position, and the control was in the "manual" position it acted like the 5 port valves were held open, and thus the air bags were effectively going to hold the aux air system pressure. When he bled the air from the tanks, he was also bleeding it from the air bags.

My 87 coach switches and valves would function with the key off but I was under the impression that was changed because my 97 requires the key on for them to function.

What is critical here is as Paul stated previously....never ever get under your coach unless it is either supported, or in the lowest position so it cannot drop.

[Johnny64]

I may be wrong but when you lifted the bus with the jacks your leveling valves let the air out of the air bag.
John

[Jon Wehrenberg]

The bus was raised by the air bags. The jacks were placed under the support points only to hold it in that position.

[Jon Wehrenberg]

I finally got a chance to look at the pneumatic diagrams on the Prevost site regarding this issue.

First, I want to emphasize that Prevost and the converters have made many changes to the leveling systems on our coaches over the years. I doubt that anyone.....anyone at all.... can say they understand the suspension system specific to any coach. So rule number one must be to follow Paul's advice and the advice given on this forum in the past to never work under a coach unless it is fully and safely supported.

Specific to the issue of Paul's coach, and others of a similar vintage it is important for all to understand that draining the air tanks under the coach also drains air from the air bags. There are far too many pneumatic diagrams relating to the various coach serial numbers to make blanket statements so it is imperative for anyone working on the suspension system to support the coach, and to recognize that because one coach does not drop when air is depleted from the tanks, that another one will not drop.

My coach tanks can be drained to zero pressure and the coach will not drop. Paul's coach will drop under the same conditions. We are a few years apart in our serial numbers, but our systems are completely different.

To further increase the complexity of this situation is the fact that two coaches built by Prevost exactly alike may vary because the converters may have made modifications to the pneumatic systems or the controls.

Hector will be doing a power point presentation on a leveling system in Sevierville, and I can tell you that it is very clear and easy to understand the system he has selected. For those who have a different system or vintage coach it will not reflect how your system works, but it will enable you to understand how to trace the pneumatics for your particular coach and understand the operating principles. Plan to attend Hectors seminar.

[truk4u]

Good stuff Jon, it's interesting that my 99 chassis is the same as Pauls, but mine doesn't drop when the air tanks are drained. Just goes to show you that the converters do modify the Prevost systems as you pointed out.

[Jon Wehrenberg]

Tom,

Don't be so sure. I would want to review the drawings applicable for both your serial numbers, and then look at both coaches before generalizing. I am of the opinion (which may be all wrong) that the Liberty changes were valving only and the air tanks and their relationship to the air bags was unaffected. I don't know if yours is pure Prevost or if it has the HWH on top of the Prevost system, but I wouldn't be willing to make a bet on this without some research.

Not all tanks are tied to the air bag side of the system (between the ride height valve and the air bags), so I presume some of the tanks are there solely to provide additional volume to the air bag side of the leveling system, and had those air tank valves remained untouched Pauls coach would not have lost air bag air.

[dalej]

For you guys that are always wanting to improve your coach....remember just because it's that way now, doesn't mean it has to stay that way!

[truk4u]

Wet tank, both brake tanks and the aux tank drained while on the lift at Prevost and the bags never moved. The two small tanks outboard by the rear wheels were not drained as they would would have deflated the bags according to the Tech Dude. I was there, watched the tanks being opened. I have the HWH along with the Prevost Level Low.

Further, the entire driver rear side was being held up by just the tag axle and bag, as the duals were removed for a wheel seal and the air tanks were empteeeeeee.

[dalej]

Tom, on ours the air stays in the bag until you turn the key on and the leveling valve is in a position to dump air or you dump air yourself with the level low. I can dump all the tanks with out the air bags loosing air.

[dalej]

I installed my new Leveling valves today. These replaced the level low manual valves that I had. I put in a 4 way leveling so I can move the left or right side without the front being raised to keep from twisting the body,

[Jon Wehrenberg]

I just want to make sure my previous explaination about why Paul's air bags lost air is understood.

On coach systems prior to Paul's and Tom's there were no air bag air tanks. On earlier vintage coaches their air system has the wet, primary, secondary and aux tanks, but none tied directly to the air bags themselves.

On Tom's and Paul's and XLII coaches it appears that each air bag has a dedicated air tank that is plumbed to the air bag apparently to increase the volume of air available at the air bag. As long as these tanks are not bled when draining moisture from the air system the coach will not drop or change position.

All other tanks in the bus air system can be purged and the bus will remain. I did not trace all pneumatic paths, but it also appears that the rest of the valving arrangement is the same as our vintage coaches with the exception of whatever changes have been made by converters.