Tag Axle Modification

[MyPrevost]

I know Prevost has issued a procedure for modification of the pneumatic control for the tag axle to prevent the tag axle from releasing upon loss of aux. air. I have searched the Prevostcar web site to no avail.

Does anybody know where to get a copy of this modification?

Hector

[MyPrevost]

Hector, we haven’t gotten any in a long time but whenever Prevost had a modification they deemed worthwhile they would issue service bulletins that defined a problem, and then the fix or improvement. The details were explicit and showed step by step what had to be done along with a complete parts list.

If you want to follow up on it you may wish to contact their engineering in Canada to see if you can get a copy unless they are posted on-line.

[MyPrevost]

Thanks Jon.

Hector

[MyPrevost]

I checked with that when I owned my CC. As I remember it was a costly upgrade, as it required replumbing a large part of the suspension system.

General opinion it was not worth the cost.

Ray

[MyPrevost]

Not sure I agree with that Ray.

When the aux pressure drops to some value, like 35 PSI, it dumps air and the bus goes into the leans.

It might be cheaper to do the mod rather than spend megabucks trying to get rid of the aux system leaks on some coaches. Later model coaches will not lean if the aux system goes to zero as long as the 5 port Norgrens are good and there are no air bag or fitting leaks.

[MyPrevost]

I can only comment on my experience. With my CC, when the tag did dump (which in my case wasn’t often), it wouldn’t lean, by that time is was pretty much just “settling down”. I don’t ever recall a time that this caused a lean, as wouldn’t that generally indicate you had pressure on one set of bags and not on another?

So, in my case, it was simply the coach squatted down to the ground.

As I remember the cost to do this retrofit was several thousands of Lewbucks. Personally I didn’t find it worth the expense. I did have problems with leans, but I did track them down through normal methods.

Of course, everyone’s coaches are different, and they may want to move forward with this. Just it ain’t cheap, as it was more than a simple fix (as it was explained to me by PVML)

Ray

[MyPrevost]

After looking through the Prevostcar website several times to no avail. I call Jim Malcomn, Prevost Texas Region Service Manager, he pointed me in the right direction and ‘like magic’, we found it.

The link below goes to the pdf file with the modification information.

http://www.dreamchasers.us/pog/Sb96-14B.pdf

Another bit of information I will pass along. The device that in mounted on the inlet air line to the aux air filter/blowdown bowl (mine is gray color) located in the lower cabinet, below the drivers seat is not a regulator. It is a pressure protection valve that will not pass air unless the aux air tank (inlet) is above 75 psi. Its function is to maintain at least 75 psi in the aux air tank for braking. If you had a failure in a aux air component, the pressure protection valve would close once the aux air tank was depressured to 75 psi. Imagine that!

 

Hector

[MyPrevost]

Wow! Now I know why it cost so much!

[MyPrevost]

Ray,

By leans I mean the rear squats and now the back end of the bus is downhill.

Hector,

That valve is why if your air systems are down to some low value, you can start the engine and begin airing up the systems, your driver’s seat (aux air) won’t rise until you get to 75 PSI in the brake systems. At that point the brake and aux air systems fill simultanelously.

[MyPrevost]

Hector,

I’m confused! Are you saying that the brakes are affected by a reduction in Aux air pressure? How can this be possible when we know the brake tanks, primary and secondary (front & rear) are independent of the Aux tank?

[MyPrevost]

Hector,

I’m confused! Are you saying that the brakes are affected by a reduction in Aux air pressure? How can this be possible when we know the brake tanks, primary and secondary (front & rear) are independent of the Aux tank?

truk4u,

The brakes are not directly affected by a reduction of the aux air pressure. Since the coach normally uses the primary air system to apply air to the brake chambers. Our coaches have three air systems, Primary, Secondary and Auxiliary.The primary air system is isolated by a check valve from the sec air tank. So the primary air tank will not bleed down through the sec tank under normal conditions. Air can only travel from the sec tank to the Primary tank.

The aux air tank (system) is supplied by the sec air tank (system). They are tied together at the aux air cluster located under the drivers seat, lower compartment (XLV). That is how the aux system is supplied. I am using Prevost drawing #D140196 as a reference. I have spent considerable time with the pneumatic drawing trying to learn how the suspension system works. It is a very busy drawing, but has mucho information.

Since I am troubleshooting an air leak (imagine that) where my primary and secondary tanks leak down overnight, I have been studying these pneumatic drawing lately. (I have found several bad check valves in the air tanks and air dryer in my system.)

If you drain your secondary air tank and the primary air tank pressure drops, your ‘double shuttle check valve’ , which seperates the primary and secondary systems is leaking.

I can see another POG Rally seminar brewing concerning these different air systems. Jon, Dale are you guys listening. I can help with the multimedia presentation.

Hector

[MyPrevost]

Hector,

We are reaching for the stars here. I agree that a knowledge of our systems, and espcially the braking systems is very important (that was my first seminar) but when we get into check valves and relay valves the number of owners that need to know that can be counted on the fingers of one hand.

What is critical is telling owners how to interpret gauges to know when they have a problem, and where in the system that problem might be. The area you are addressing right now is a good one for those who will literally be doing their own diagnosis and repairs (I can name names), but few others will benefit.

What I think will be important is giving an overview of a typical system and how the components come into play. For example the foot pedal doesn’t just open a valve and send air to the brake chamber. It is like a switch that tells the relay valve to send air. The only thing that makes that important is in knowing there are components under our bus that play a critical role such as relay valves, quick release valves, check valves, etc. Perhaps that seminar can be targetted to the serious gear heads who want to know when a relay valve is not functioing properly. I just hate seeing us split the group into two groups. I think seminars should be of common interest.

We got deep into the suspension valving, but that being available to all helps protect everyone from a mechanic who is supposed to fix the leans replacing valves on the LH side to cure a lean to the RH side. I am not sure I ever heard of anyone having to replace protection valves or relay valves.

[MyPrevost]

Hector,

I understand the system, I don’t understand your statement, “Its function is to maintain at least 75 psi in the aux air tank for braking.”

If we were sitting at a Rally sipping a beer, this would be a lot easier!
Tom Chilcote

[MyPrevost]

The way our air systems are set up, we can have a catastrophic failure anywhere in the system (within reason) and we can maintain some marginal pressure in our braking system.

Bruce and the Air Force One folks have proven that. When he literally broke a line he lost pressure up to a point and at that point the protection valves did their thing and he was able to travel with no loss of braking, and without his brakes locking up.

There are some conditions such as a failed emergency brake diaphragm that will disable a coach, but an aux system leak will not, nor will some system failures.

Truk is right. This topic, if it is chosen as one worthy of a seminar needs to be done face to face. If there is sufficient general interest, the ideal time and place is under the bus that is put up in the air at the Oklahoma City rally. That way every valve and device can be pointed to, seen, and its purpose explained. I can envision one of the techs at the facility following the air from the engine driven compressor all the way to the slack adjuster, and experts like Orren providing information on the nuances of the various devices. I would hope that folks with long term experience can put some of this in perspective by explaining the frequency of certain types of failures.

If the seminar is strictly braking system(s) it can be easily digested, but I still think the most critical part will be explaining how to interpret what the gauges are saying and how to react to them.

[MyPrevost]

Hector,

I understand the system, I don’t understand your statement, “Its function is to maintain at least 75 psi in the aux air tank for braking.”

If we were sitting at a Rally sipping a beer, this would be a lot easier!

truk4u,

Correction, “Its function is maintain as least 75 psi in the secondary air tank for braking”. Whew!

Good catch! Too many tanks!

Hector

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