Using a Freightliner XC chassis, consider this a primer to learning your way around a diesel chassis’ air system.
By Martin Sherlock, F166464
September 2005
The compressed air system is relatively new to the typical diesel-pusher motorhome chassis, and coach owners may not understand how it all fits together. This article is merely an introduction to the compressed air systems and is intended to provide readers with a working knowledge of the systems. It has been written using our coach, a Freightliner XC chassis, and its technical manuals (purchased from Freightliner) as guides. There are many variations on other manufacturers’ systems, but most use the same components. Just remember to check out your compressed air systems with experts before attempting any repairs or modifications.
Compressed air is used as a source of stored energy, similar to the battery bank. Compressed air should be treated with the same respect as one would treat gasoline, high voltage, or gunpowder. If released suddenly, compressed air can cause serious bodily injury. It can tear flesh, embed rust or dirt particles into flesh, and cause blindness if released into or near the eyes. Components that are propelled by the sudden release of air pressure can actually kill or maim. Never disconnect any part of the air system until you are sure that the entire system is at atmospheric pressure.
The service brake systems and the parking brake systems are separate systems that use some of the same components; however, they function entirely opposite of each other. The service brakes are applied with air pressure and held off with spring pressure; the parking brakes are held off with air pressure and applied with spring pressure.
The air brake was developed many years ago and remains essentially the same today. The addition of the antilock braking system (ABS) in 1998 to air brake systems prevents the brakes from locking up and causing the tires to skid under heavy braking. An electronic motion sensor inside each brake drum causes air to be vented from the affected brake drum until the wheel begins to rotate again. This action is repeated many times per second until the wheel rotation is maintained.
The air compressor is located on the engine, usually on the same driveshaft as the fuel pump, so the engine must be running for it to function. Clean, filtered, turbocharged “” pressurized “” air from the intake manifold is force-fed into the air compressor. (Dirt and grit in any air system are abrasive and accelerate wear on valves, piping, and other system components.) The air compressor charges the compressed air system whenever the system’s pressure falls below preset parameters. When the maximum pressure is reached, the compressor output is diverted.
The air is then passed through the air dryer, which filters the compressed air again and removes moisture. The air dryer has an automatic purged air valve on the bottom of it to expel the accumulated moisture back into the atmosphere. The purged air valve opens when the air compressor stops pumping and goes into standby, and the air pressure blows the moisture out of the system. The purged air valve also blows moisture out each time the compressor reaches the maximum output pressure and shuts off. A desiccant cartridge in the air dryer removes any moisture and oils in the compressed air. The coalescing filter in the air dryer removes any particles of debris still remaining in the compressed air. Both the desiccant and the filter cartridges need to be changed at regularly scheduled intervals.
The air dryer has a connection on it, similar to a tire valve stem, for charging the air system if the engine is inoperable and the vehicle’s park/emergency brakes need to be released for towing. Releasing the emergency brakes can also be accomplished by manually turning the long screw inside the dual air brake chamber until the brakes are off the drums, but this is a slow and difficult task; compressed air is an easier option.
From the air dryer the compressed air is sent to the supply/primary air tank, usually located between the rails over the front axle. The automatic moisture ejector valve on the bottom of the tank identifies the supply/primary tank. This tank has two compartments in it and is protected from overpressurization by a relief valve set at 150 psi. The air enters the supply or wet tank where remaining moisture and oil are removed. The air is quite hot at this point, and the tank serves as a place to cool the air after being compressed. As the air cools, any moisture will drop out of the air or condense in the tank for ejection from the system by the automatic moisture dump valve.
The compressed air is then routed into the primary side for use by the air system. The primary side of the supply/primary tank is the source of air for the rear brake system, which is discussed below. Once the pressure in this tank drops to about 65 psi, a pressure protection valve in the outlet line closes, reserving the remaining air pressure for exclusive use by the rear brakes. Under normal pressure, the air will pass through piping from the wet and primary tanks into the secondary tank. The primary and secondary tanks both have manual drain valves with cable lanyards attached. These valves allow you to drain moisture from the tanks or completely depressurize the system to perform maintenance work on the system.
The secondary tank is divided into three compartments, and compressed air enters only the secondary compartment. The other two compartments serve as reservoirs for the two front air bags of the air suspension system.
The secondary tank supplies compressed air to the front brakes, the entire air-ride suspension system, the exhaust retarder operating cylinder, and the customer air manifold. A pressure protection valve shuts off the air to the customer service manifold, air suspension manifolds, and the exhaust retarder when the secondary tank pressure falls below 65 psi. This feature reserves the remaining air pressure for the front service brakes whenever pressure falls below minimum prescribed limits.
On chassis equipped with air bags, there may be another separate air tank that is divided into two compartments. It is located between the frame rails over the rear axle and serves as a reservoir for the two rear air bags.
Air Brake Operation
As you are traveling down the road, when you want to stop or slow down, you depress the service brake treadle. As this brake treadle is depressed, it positions two small spool valves inside the treadle valve, usually located under the coach floor. These valves are proportional flow valves and regulate the air pressure depending on how far they are depressed. This regulated, pressurized air goes to the relay valve (near the rear axle) for the rear brakes and the quick-release valve (near the front axle) for the front brakes. This small quantity of regulated air pressure passes through the quick-release valve and flows into the front service brake operator air chambers to apply the front service brakes. The diaphragm (piece 4) retains the air pressure, and the piston rod assembly (piece 10) moves downward, applying the brake shoe against the brake drum. The length of the piston rod assembly limits the travel. Under normal operation, it will not bottom out. The more you depress the brake treadle, the higher the air pressure in the brake chamber becomes, and the tighter the brake shoe is pressed against the brake drum.
The regulated air pressure signal, which is sent to the relay valve near the rear axle, positions a spool valve in the relay valve to allow a larger quantity of compressed air to flow to the rear brake operator air chambers. The relay valve spool is repositioned to shut off the air pressure when the brake operator air chamber reaches the same air pressure that the brake treadle has requested by the amount of its depression. The rear relay valve and the rear treadle spool valve use air from the primary tank only. The front treadle spool valve uses air from the secondary tank unless it is depressurized (in which case it uses air from the primary tank). The air pressure applied to the chambers will be at the same pressure that is supplied by the brake treadle. The rear relay valve opens and closes, depending on the position of the brake treadle, either pressurizing the brake chamber or venting air from the brake chamber. When the brake treadle is released, the air is vented, which vents the air off the brake chambers; next, the return springs reposition the piston rod assembly upward, and the brakes are then released.
Each of the brake operator chambers consists of a rubber diaphragm attached to a rod and the outside edge of the metal housing. Air pressure moves the diaphragm and the rod inward, which presses the brake shoes against the brake drum via a cam. The front brakes use air directly from the treadle valve into the brake operator chambers, while the rear brakes use a relay valve that ports compressed air to the brake operator chambers directly from the primary tank. This setup allows the four service brakes to be applied and released simultaneously. Once the air pressure is released, the return spring on the non-pressurized side of the diaphragm returns the air chamber to the normal “off” position, thereby releasing the brake shoes from the brake drums.
The rear service brake chamber contains two separate chambers and three springs. The service brake chamber is the one closer to the axle, and the other chamber contains a very large spring that is compressed to some extent all the time. Do not attempt to disassemble this chamber without first compressing the spring using the bolt in the middle of it.
Parking Brake Operation
The parking/emergency brake system is controlled by the yellow valve handle located close to the driver’s seat. Pulling out the valve vents the air pressure off the emergency brake chamber of the rear brakes via a relay valve. Without air pressure in this air chamber, the very large spring is allowed to apply the brakes on both rear wheels. When the parking/emergency brake valve is depressed, air from the primary tank flows to the relay valve and applies full air pressure to both parking/emergency brake chambers. This air pressure overcomes the spring pressure, compressing it fully and releasing the brakes. If an air line or other component ruptures while you are driving, air escapes from the system, including the parking/emergency brake chamber. About the time the low air pressure alarm goes off and the light on the dash illuminates, the rear brakes start to be applied, and you can’t stop them. The only answer is to get to the side of the road as soon as possible, because you are going to come to a gradual stop.
Air Suspension Operation
The air suspension system operates automatically. If it is equipped with a driver-controlled suspension exhaust valve, the air can be vented off the air bags when the coach is parked, allowing the coach to be lowered to the axle stops. When this valve is placed in the auto position and the engine is started, air from the secondary tank fills the air bags and raises the coach to its normal ride height. Each air bag is connected via a hose to a separate compartment in either the secondary air tank or the rear air tank. This arrangement effectively increases the volume of the air bags for a smoother ride.
There are three height control valves: one for each air bag on the front axle and one for both air bags on the rear axle or one in the front and two on the rear axle. Each height control valve is mounted on a chassis rail and connected to its axle’s air bag(s). If one axle is too high or too low, the height control valve adds or vents air in its appropriate air bag(s). As the coach is traveling down the road, the axles are in a constant state of movement from the bumps, cracks, holes, or objects that the tires pass over. The pressure in the air bags prevents the axles from moving up too far, and the shock absorbers dampen movement in either direction. Axle movement affects the ride height control valves, which are constantly venting or pressurizing the air-ride suspension system to maintain an even ride height. The air line is relatively small, but even on a smooth road you still manage to use up compressed air. Observe the pressure gauges on the dashboard and you will see the pressure drop until the air compressor cuts in and builds it back up. Some coach air suspension systems provide you with the ability to lower the coach approximately 4 inches prior to extending the leveling jacks. This is accomplished with the auto/dump lever positioned to the “dump” position, which vents all the air from the air bags and corresponding volume tanks, lowering the coach onto its axles. The air bags cannot be vented if the park/emergency brakes are not applied. This feature was installed so that the air bags could not accidentally be vented while you are driving.
Miscellaneous Systems
The exhaust brake is also operated by air pressure. Air pressure closes the butterfly valve in the exhaust pipe when the exhaust brake switch on the throttle is in the closed position. Spring pressure opens the butterfly valve when air pressure is vented off the cylinder.
The coach manufacturer normally mounts the customer manifold on the front of the firewall. The air horns usually are connected to it, but that still leaves a number of ports for the customer to use. One such use is a quick disconnect on one port for an air hose to inflate tires, toys, etc. This allows you to start the engine and then check and correct tire pressure before you drive the vehicle. (Remember, all tire pressures are listed as cold inflation pressure, and tire temperature will increase even with a short drive at low speeds.)
Control of the air horns is through a solenoid valve that opens when you press the horn button, sending compressed air to the horns. This solenoid valve can be made inoperable with a switch on the dash. Because of the high noise levels, most major metropolitan areas do not allow the use of air horns.
Air pressure is monitored through a gauge mounted on the dashboard. This gauge has two pointers; the green one indicates the pressure in the primary tank, and the red one indicates the pressure in the secondary tank. Under the coach you will find that the air lines are also color-coded. Green is for the rear service brakes, red is for the front service brakes, black is for the park/emergency brake system, yellow is for the park/emergency brake control, and blue is for the air suspension system.
Maintenance Tests
The following steps can be performed to test the proper operation of the air system and to verify that the system is not leaking.
Air Governor Test
The air governor test will verify that the air governor is maintaining the proper pressure in air tanks.
1. Start the engine and build up the air pressure until the compressor governor cuts out. This will be evident when the pressure gauge needles stop rising.
2. Depress the service brake treadle slowly and then release. By observing the gauge, note that for each depression of the brake treadle, the service air pressure decreases.
3. Continue depressing and releasing the brake treadle until the governor cuts in and the air pressure begins to increase. When this occurs, the air pressure in the tank as indicated on the gauge should not be less than 85 psi.
4. Let the air pressure build up until the governor cuts out as indicated by a steady air pressure. The air pressure in the tank should not exceed 125 – 130 psi.
5. These listed air pressures are federal standards and the actual cut-out and cut-in pressures may be different on your coach.
Static Brake Test
The static brake test will verify that the vehicle air system that is normally charged as you travel is not leaking.
1. Block the tires.
2. Run the engine until normal air pressure is achieved.
3. Release the parking brake and note the pressure on the gauge.
4. If the coach is equipped with an air-ride system, do not move around in the coach or leave the coach during this phase of testing.
5. The air pressure in the tanks should not decrease by more than 2 psi in one minute or 60 psi within 30 minutes.
Applied Brake Test
The applied brake test will verify that the vehicle’s air system, which normally is charged as the service brakes are applied, is not leaking.
1. Depress the service brake treadle and keep it fully depressed for the duration of this test.
2. After the air pressure has stabilized, observe the pressure gauge.
3. The air loss should not exceed 3 psi in one minute.
4. Any movement of the service brake treadle will vent or pressurize the service brake chambers and consume air pressure, causing a failure of this test.
Low Air Warning Device Test
The low air warning device test will verify that the low air pressure alarm goes off as designed.
1. Turn the ignition on (key switch) but do not start the engine.
2. Observe the air pressure gauge and pump the service brake treadle until you can see the low air pressure warning light come on and/or hear the warning buzzer.
3. The low air pressure warning should come on at about 60 psi.
Emergency Brake System Test
The emergency brake system test will verify that the vehicle emergency brake system functions as designed.
1. Turn off the ignition and then reduce the air pressure by depressing and releasing the service brake treadle until the emergency (parking) brake handle pops up.
2. By observing the pressure gauge, you’ll see that this should occur when the tank pressure falls below 45 psi.
3. Start the engine and immediately place the transmission in gear. Try to move the coach slowly against the emergency brake with low air pressure. Do not exceed 1,000 rpm.
4. At this pressure, the emergency brakes should be applied by the large springs in the rear brake chambers.
5. As soon as the engine is started, the air pressure will start to build up, but it will not be routed to the emergency brake chamber because the handle has popped out.
6. Caution should be used to not apply too much pressure against the brakes and drivetrain.
Emergency Brake Test
The emergency brake test will verify that the vehicle emergency brakes can be applied as the vehicle is moving. When applying the emergency brakes on a moving coach, the driver and all passengers should have seat belts fastened, and the vehicle speed should not exceed 5 to 7 mph.
1. Allow the air pressure to build up until it cuts out.
2. Put the coach in gear, release the parking brake, and allow the coach to move forward slowly.
3. Apply the emergency (parking) brake by pulling up on the button.
4. Do not operate the service brake treadle.
5. The vehicle should come to a complete and sudden stop.
Check Service Brakes
The check service brake test will verify that the service brakes are adjusted so they do not cause a pull in either direction when applied.
1. Set the coach in motion slowly and apply the service brake treadle firmly to determine if the steering wheel pulls to the left or right.