[Andre]

Marathon has it.

[gary]

I don’t know if the Prevost folks are monitoring the site yet, but I am not aware of any in my experience that did not have a tag lift on the tanbdem axle coaches.

Since the tag is not driven I suspect it is necessary to be able to lift it to add weight to the drive for traction and lifting the tag does make a slight difference in the turn radius also.

[Archived]

The 40 footers will lift the tag a few inches, the 45’s will not. It sort of unloads the tag without it coming off the ground,

[Archived]

Tom: my 98 45 XLE will clear ground 2 inches. more if I shorten chains. one reason not to carry spare. Bob

[Bob]

My 45 will lift the tag off the ground. Barely. Given enough time.

[needtocamp]

Jon,

My tag axle on my 45 XLV will lift, but it will definitely take a few minutes. Once I engage the lift tag axle feature, it willl dump the air instantaneously, the rear end will drop a few inches, then if you wait a few minutes, it will re level and lift the tag axle. But don’t get in a hurry.

Hector

[AL]

So If your tag doesn’t come off the ground you need to shorten the chains?

[Rick & Denise]

“So If your tag doesn’t come off the ground you need to shorten the chains?”

anyone know if this is the case?

[Greg]

Don’t know exact answer. The two I have experience with do about two inches… Am sort of currious my self now. Bob

[Archived]

I don’t know this for sure, but I suspect the tag lift chains are sized so the tag axle can operate through the full range of motion. If the chains are shortened the raising of the bus via the air bags is likely to be compromised because the chains will start lifting the tag axle before the rear air bags are fully extended. I am sure many if not all of us have hit dips in the road while driving that have fully compressed our air bags and then fully extended them. Those highway whoop dee doos get our attention, and I would be concerned about what would happen if a tag axle wheel was jerked off the ground as the suspension extended which is what can be expected to happen if chains are shortened.

I have a serious problem with we owners who try to modify our coaches to address what we perceive as a problem, instead of fixing whatever problem we might have. Our coaches undoubtedly have to meet very stringent DOT standards because they are built for use in public transportation. The engineers must devise systems and methods that are not only safe and robust, but which perform as intended over many miles and years.

[Bob]

A little off topic but not much… My 95 XL tag will vent and drop the back of the bus a few minutes after I kill the engine if I do not dump the tag myself. The Prevost books that came with the coach describes this as normal. I notice that later models do not do this. Does any one know when the change was made and can mine be “fixed” so they will not vent? There are times when it would be easier to level the coach if the tag would stay in the drive postition…

DKO

[Larry]

DKO,

Prevost originally had a system logic that requiired aux air pressure to be over a certain value to lock air into the tag axle air bags. If it fell below that pressure (somewhere around 30 to 35 PSI) the tag axle Norgen valves would shift to lock air in the drive axle air bags only, while dumping tag axle air bag air. That placed an addition 10,000 pounds or so on the drive axle air bags and the rear of the bus dropped.

That is what is likely happening with your coach and I suspect the tag axle loss of air to the air bags corresponds to when your aux air system bleeds down.

Later model coaches had a change in system logic and the aux pressure could drop to zero and the air bags would all have their air locked in unless the suspension system had a leak.

Prevost has a service bulletin which changes the system operating logic, but the quoted price as I recall is around $1800. The service bulletin is very detailed and I presume an owner could change the system for the cost of parts.

[Archived]

That is interesting. Thanks for the information. I never imagined it would be that expensve to “fix.”

DKO

[Hector]

DKO,

That also confirms you have a pretty bad aux air leak.

My 94 Liberty would dump the tag at 45 lbs.

[Bryce]

I ditto what Tom says about leaks.

The dumping of the tag axle air is one of the events that directed so much of my attention to the suspension system. I own 1995(1994 chassis) that has the same pneumatic logic that you are describing. It took some study to sort it out, but somewhere in the 1996 chassis year range, a Prevost change was done to the pneumatic logic that eliminated this issue. The newer coaches do not have this issue, however, they can still have auz air leaks.

My view is when the aux air pressure drops, this event happens. The aux air supplies air to the tag axle lift switch in the front of the coach and when this pressure drops, the tag axle logic releases and the tag axle air bags vent. I looked into the purchase of a Prevost supplied pneumatic kit that would rearrange the logic such that this particular event does not happen, but it was cheaper and easier to manage the leaks in the aux air system.

If the tag axle is releasing a few minutes after you turn the engine off, you have a significant air leak in your aux air system. With a little knowledge and patience, you can manage these air leaks such that a small electric aux air compressor will maintain the system pressure when the coach is shutdown.

If you study the “How Does Your Prevost Suspension System Work” presentation, it in the articles, it will give you a good understanding of how the suspension system is interconnected.

Unfortunately, their is no easy fix for this issue, but it is manageable.

Good Luck,

Good Luck,

Hector