PREVOST, The Ultimate Class

Understanding your Prevost Coach Air System

Of the many systems buried deep within the Prevost coach is the air system. It provides power to the most essential functions of your coach: Brakes, suspension and a wide variety of other operations including windshield wipers, the belt tensioner, stepwell cover and the Prevost level-low system. Every Prevost coach owner should have at least a working knowledge of the basic functions of the air system to be able to better enjoy travel and life in the Prevost coach.

Here’s an overview of the basic operations and functions of the Prevost air system which will help owners get a working knowledge of the air system. There are over 200 components in the Prevost coach chassis, with more than 500 individual connections. It’s a very complicated system.

On the Series 60 engine, the air compressor, driven by the engine, is located on the passenger side of the coach. On 8V92 engines, the compressor is located in the same location, or on the front of the engine and it is cam-driven. The air compressor has a governor, which turns the compressor on or off to maintain between 90 and 120 psi. More about this later. From the air compressor, a single line feeds the air dryer.

The air dryer’s job is to remove water from the compressed air. Here, on a Series 60 coach, the air dryer is located just in front of the drive axle, connected by the visible green hose. The air dryer is the device which ‘blows off’ when your coach is ready to run.

From the air dryer, compressed air goes to a single tank which is split into two parts: The ‘wet side’ allows water to settle to the bottom of the tank. The ‘dry side’ is filled with air which is free of water..

From this point, the compressed air feeds into two other tanks: The auxiliary tank and the brake tank. The auxiliary tank maintains an air supply to windshield wipers, the belt tensioner, air dumps, pneumatic doors, the flush and to air bags.

The brake tank is for the air brakes on the Prevost coach. It will draw both from the feed from the air dryer or the auxiliary tank, in order to supply sufficient air pressure for the operation of the brakes on the coach.

You may monitor the air pressure in these two tanks from the pilot’s seat. Usually, the red needle will monitor the brake tank and the white needle will monitor the auxiliary braking system. However, some convertors will refer to these two levels as the ‘front’ or ‘back’ tank gauges. Check your operator’s manual for the specifics for your conversion.

It is essential that you understand the implications of how your gauges work: What they are telling you...if there is any problem and what you should do about it. Over the road, the air system will provide you and your passengers with the finest ride of any coach; the pilot should be completely familiar with understanding the operation o f the air system to be able to make the decision between normal operations and any problems along the road. One of the places you will see air system connections easily is underneath the pilots seat in the lower street-side compartment. Here, you will find a typical installation of an air manifold system. It is an aluminum block, with a single feed and multiple hoses, which in turn supply air to various operations of the coach. This may look differently, depending upon the year of your coach and any possible modifications to the coach. It may also be a trouble-shooting place for the do-it-yourselfer, as a connection which may have worked loose will be rather loud, easy to repair

Should you walk outside you view your wonderful coach and see a definite lean: ...and you didn’t leave it that way, then perhaps you might wish to know the following:

First of all, there are three air leveling valves on a Prevost coach, two in the rear, one in the front. This is how the coach maintains it level both going down the road and when you are parked and wish to level the coach. The Prevost level-low system is on the pilot’s left hand side, to the rear of the transmission controls.

If the front is down that probably means that there is a leak between the right height control valve and the air bags in the front of the coach.

If the coach has one side down it may be the same as a ‘front down’ problem, or a leak on the low side height control valve.

If the coach is evenly low your coach could be experiencing a leak just about anywhere in the system.

Remember, every coach has air leaks, in a certain way of looking at things. Just think about the complexity of the system, even from an overview:

The above graphics (may be difficult to read on varying computers) are the ‘simple’ version of the air system schematic.

Reminder: The governor on the air compressor is set to turn off and on between 90 and 125 psi. It will normally go off and on as needed. Don’t be concerned about watching the pressure go down to 90 and up to 125. Braking will bring down the air pressure by itself.

In very Prevost coach, the ‘front tank is the auxiliary tank and the rear air tank is the brake tank. The brake tank will always draw from the auxiliary tank when needed. Here’s the most important part of all:

Should your gauges read air pressure building above 125 psi that means the governor on the air compressor has failed. The air compressor will continue to pressurize your entire coach system without stopping. The only remedy is to tap on the brakes to relieve the air pressure build up and safely pull over to the side of the road and seek assistance. Most Prevost coach owners don’t carry the tools and parts necessary to diagnose and replace the air compressor governor.

Should your gauges consistently run below 90 psi, that means you are experiencing a serious air leak in either one location or in a number of locations in the coach. As air pressure begins to go below 90 psi, the air brakes in the coach will progressively engage. At 60 psi, the air brakes in the coach will definitely engage and it will be very obvious to the pilot that air pressure is very low.

Despite the complexities of the design and manufacture of the Prevost air system, it is a marvelous system and relatively free of problems if they are addressed on a timely basis. As any system which is complex, seek out qualified assistance for maintenance and have repairs made when discovered, don’t let them pile up.