The Leans....Episode II

[phorner]

Jon,

I would very much like to be in attendance at your next "air" seminar. So far, everything looks pretty good on my bus, but you can never be too educated as to the why and how things work.

Thanks for all your efforts in undertaking this project!!

[Just Plain Jeff]

Zen and the Art of Detecting Leans?

(Oh no, here he goes again)

Having lived with this problem for many years in various coaches, the discussions with techs, rallies, here on this forum, etc., it's clear that the air system of the coach is very complicated and I am not sure that we have a complete fix on how every coach's air system operates in a precise way.

There are year-to-year differences between the delay and non-delayed systems, perhaps even some VIN number coaches, converter tap-offs for sliding steps, toilets, doors, dump valves, what have you.

And, we also have the differences between coaches moving and sitting. Then there are differences between those sitting while running and those with the ignition switch off.

Adding to all of this is that from what I can figure out, the Prevost system is basically a closed system; that is, when air is released to change the position of the coach relative to the ground, it does not escape from the system, it goes somewhere else.

To further complicate this issue, the tag on coaches I have seen, uses a brake chamber to help regulate the tag. Tag action on say, a 92 40-footer is quite different than that of say, a 98 XLV. The 92 will lift that sucker right off the ground and the XLV will ease off the tag somewhat, shifting the weight rather than a lift and drop movement.

Most of us have probably seen an old banger of a seated coach bus going down the road at about the 2 o'clock position, wondering why someone didn't maintain the air system.

Not to create a silly argument here, but in pursuit of The Grail, it might be good to stick with a particular coach in a particular mode so that we can come together and understand how that one might work, let's say in a parked mode with the engine off. Since some of the leaks in the components of an air system cannot be detected by soap and bubbles, maybe that would be the first thing to take a look at and understand.

In a dynamic system, which is what the Prevost air system is, with multiple variables changing in parallel, defining each part of the puzzle first is one way of figuring out how to describe subsequent events and how they relate to that one snapshot that we can understand.

So that's 2 cents worth and you can keep the change if you want. I'll stop writing right now.

[Jon Wehrenberg]

Tom,

When in transit the aux air for the suspension is going through the ride height valves into (or out of) the air bags.

I have not watched gauges on a coach equipped with instant reacting ride height valves, but I have extensively on the delay reaction valve. On what we consider smooth level roads with the delay valves there is very little cycling of the compressor due to the filling and exhausing of air in the bags. As we get side winds, change lanes (the crown tips us), or have undualtions you would begin to see air consumption.

I suspect but do not know for sure that the instant reacting ride height valves dump air and refill constantly, even when we are not aware of changes in the road, the winds or the crown.

I was doing some homework tonight and I have to get under the bus and compare my actual system against the pneumatic diagrams Prevost has on line. I thnk the ones I am looking at on line are for seated coaches without level low. If you want to play maybe we can set up a day to work in Casa del Grease Pit.

[Jon Wehrenberg]

JPJ,

This will require some investigation, but I suspect while the systems may in fact vary from converter to converter and from model and year to model and year, I think there are going to be some generalities that we might be able to make that will help almost all of the POG attendees. (Obviously I have a Liberty to use as my learning tool, but I know guys. And these guys can look at their CC, and Marathon, and Royale, etc. and we can all compare notes.)

I say that generalization because when some converters create their own variations, they appear to not alter the valving, as much as they create an additional electric system that piggy backs on top of the Prevost provided leveling controls. When the coach is in the "automatic" or self leveling mode that switch position just takes the manual controls from the Prevost circuitry and the automatic circuitry becomes the control of the valves via the solenoids.

The more we dig into this, the better our understanding of the variations will be.

[lewpopp]

this would be an ideal place and time to announce that Mr. Paul Horner was giving tours of the compartment under the drivers area. He is so very proud of his newly installed pressure guage.

I told him it should have been tilted for an easier read. This was said after I stood in line for a while to see his installation. What a smile he had on his face. I told him that Jon would be very proud of him. No light though, he won't check it at night no matter how much you want him to.

[GDeen]

Thanks Joe and Jon for the detailed answers to my questions...I have a lot to learn!

[Jon Wehrenberg]

I've got it.

I have been crawling around under the bus, looking at schematics, studying parts lists and diagrams and generally trying to get a handle on this whole topic we call the leans.

I'm done. At the spring rally we can review the miniature single air spring suspension system, but instead of looking at the entire topic as a big puzzle we can break it down into specific valves and their individual functions so it will make pin pointing problems less intimidating.

The deeper I get into this (starting with building a mockup) the simpler to uderstand it becomes. I will not mislead anybody into thinking if the coach leans to the left it is a simple matter of replacing the single valve responsible for keeping the left side air bags inflated. There could be any number of reasons why the coach is leaning, but at least you will be able to understand exactly which components could affect the left side, and how to verify they are the culprits or they are not the problem.

I have written a 14 page article. I am going to let some folks review it this weekend, and if they understand what I am outlining I will see if we can get Jim to post it.

When we get done with this thing called Prevost we should be able to understand the whole thing.

[Kevin Erion]

Jon,
I can't wait, I used the bus this weekend and when I went to lower the front, nothing! It would air up or down left and right back and up the front but not down.
I just got home and did a little more playing with the Prevost rotary switch and I found that if I go down with one of the rears and then try the front down, it works. If I try to go down in the front after going up on any of the 3 positions, nothing.
I will play with it a little more in the morning, any ideas?
Kevin
I will post a story about my engine driven air compressor tomorrow.

[Jon Wehrenberg]

Kevin,

I can give you some clues, but remember...I am doing my looking on a Liberty, reading diagrams from an unaltered Prevost system, and looking at Prevost pneumatic diagrams on line that do not match anything else.

But if you are still interested I will tell you where to look.

The second solenoid down (in the five valve assembly in the steer compartment) controls the front air control valve. The five port and the three port valves in the steer compartment control the blockage of flow into and out of the front air bags (the five port valve) and the adjustment of the height of the front air bags (the three port valve below it).

I would make certain the solenoid valve is opening when you command it to via the switch on your panel. If it is, then make sure the five port valve is functioning, since even if the three port valve is not funtioning, the five port should shift the spool and allow the ride height valve to come into play.

I think you have a generic system, and I think the above is valid.

My guess is the bus has not had the system exercised and once you get things moving the problem will go away.

[Jon Wehrenberg]

Kevin, I'm not sure it is necessary to check this, but I want to make sure this is understood.

The third solenoid valve down drives the opposite side of the five port valve and it is that action that puts the ride height valves into play.