Understanding maintenance and driver training issues
of anti-lock braking systems

Most vehicles, including school buses, now come standard with anti-lock braking systems. The Department of Transportation has required ABS on air-braked vehicles since 1998 and on hydraulically-braked trucks and buses with a gross vehicle weight rating of over 10,000 pounds since 1999.

While there are different types of ABS for both hydraulic and air brakes, they have the same purpose — to prevent wheels from locking up during hard braking or when braking on wet or slick pavement. Considered an “add on” to normal brakes, ABS does not increase or decrease stopping power, or reduce stopping distance. It may even increase distances on dry pavement.

Under less than optimal conditions, ABS can reduce the stopping distance up to 25 percent, often the difference between a safe stop and a crash. Most important, ABS can help prevent dangerous skids and allow the driver to maintain steering control to avoid obstacles.

But ABS only works when traction is marginal or during panic stops. Otherwise, it has no effect on driving or braking. Should it fail, normal braking is retained. School buses have yellow ABS malfunction lights on the instrument panel that illuminate at start-up for a bulb check and during the check of system components for proper operation, and then go out. On some older systems the lamp may stay on until the vehicle is moving faster than 5 mph. If it stays on or comes on while driving, there is a possible ABS failure. Drivers should be trained to observe the light at startup and report it if it remains or comes on the route.

Regardless of the type of system, ABS wheel speed sensors monitor deceleration rates of the wheels during braking. On “four channel” systems, each wheel has its own speed sensor. With “three channel” systems, there is a sensor on each front wheel and a common sensor for the rear ones, usually mounted on the differential or transmission. It monitors the combined or average speed of both wheels and both are controlled simultaneously. Some school buses on van chassis have only a “single channel” ABS that monitors and simultaneously controls both rear wheels.

One wheel that starts to slow down faster than the others, or is faster than that which is programmed in the ABS module, indicates the wheel is starting to slip and is in danger of losing traction and locking up. The ABS system’s electronic control unit (ECU) responds by momentarily commanding the appropriate modulator valve(s) to reduce hydraulic or air pressure to the brake on the affected wheel or wheels to provide maximum braking without danger of lockup. ABS reacts much faster than a human driver can and only modulates the brake pressure on the wheels that are in danger of locking up.

Training and Maintenance
After two decades of use, ABS has proved to be reliable but should be checked periodically like other brake system components. Cheri Clymer with Thompson Schools Transportation Services in Colorado, in consultation with district maintenance technicians, recommends changing fluid in hydraulic systems at least every 50,000 miles. Others say every one or two years. That’s because brake fluid absorbs moisture over time resulting in internal corrosion in critical, and expensive, components.

While operating in the mountainous areas around Denver, she has experienced no problems.

“It is reassuring to the driver that, in a panic situation, the steering wheel will not lock with ABS,” said Clymer, an instructor of the state’s mountainous driving program. “The ability to steer the vehicle may help the driver avoid a rollover on a curve.”

The required training was sparked by a fatal incident in the Rocky Mountains in the early 1990s when a school bus driver lost control of the vehicle and it flipped end-over-end and rolled one-and-a-half times down an embankment and into a river. Investigators later discover three of the four brakes were so far out of adjustment that the driver was able to operate the bus with the emergency brake engaged. The crash also led to a state requirement for brake retarders on school buses.

Being more complex and computer-based, ABS often require more hours to service as well as special diagnostic tools and software. However, there are self-diagnostic software programs to help technicians. Diagnostic tools are compatible with standard hand-held and computer-based diagnostic tools. Most ABS have a self-diagnostic capability and can use the same connector used to troubleshoot electronic engines. Technicians experienced with modern engines should have no problem with ABS. But that doesn’t mean there are none.

Thomas Built Buses’ Tom Wilkens notes that one trouble area is the wheel sensors that can be affected by road corrosion. For example, the sensor cannot operate properly if the tone ring is encrusted with crude or is missing a tooth due to corrosion. Thus, the need for cleaning the tone rings regularly.

“There are concerns about excessive corrosion on tone rings in ABS systems,” commented Wayne Johnston, the transportation director for Springfield Township, Pa., School District. “Although the problems of corrosion appear in my area of the world to be limited, there are signs that this is hardly the case industry wide. In the northeast, dealers are seeing three- to four-year-old buses with heavily corroded drum and rotor tone rings on both hydraulic and air-braked buses.”

For areas that experience more snow and slush, he said ice may be collecting on the rings and causing some of the blocks to corrode to the point that they create a void and consequently place the ABS system into a non-operational mode.

“As if we did not have enough to worry about, now the solutions used to make our highways safer during inclement weather appear to be accelerating the oxidation of a vital part of the brake system,” Johnston added. “Time will tell, but it is something you must keep and eye on or your drivers will be eyeing up the ABS light in the dash.”

In 2002 and 2003, the National Highway Traffic Safety Administration investigated the premature erosion of uncoated hydraulic disc brake rotors in trucks and buses due to exposure to road salt and other corrosive elements. The problem centered on the ABS tone ring. The corrosion compromised the quality of the ABS wheel signal, which increased vehicle stopping distance in certain cases when proper fixes were not made.

“The best thing they came up with is a magna coating (on the disc) to address the problem,” said David Mohr, vice president of technical and OEM sales at Performance Friction Corporation, which supplies all Navistar vehicles with hydraulic discs that pass signal tests for disc cycles of zero, 45, 90 and 135. “The manufacturers convinced NHTSA that was the best thing to be done. But the problem was really not abated by coating.”

Mohr said the intensive labor required to remove the hubs and discs and then replace them with new discs and new pads is a big problem, especially in larger fleets located in the northern part of the country. In addition to all IC Corporation buses being equipped with Performance Friction brakes, he said First Student also specs them.

“When you lose ABS signal, you lose brake functionality. It’s a real safety issue, especially when children are involved,” he added. “It’s been their No. 1 maintenance problem the last eight years. The big fleets especially, they’re just up in arms. So it was causing a lot of problems for the (school bus) OEMs.”

Johnston said he sees corrosion problems caused by the wicking of sodium chloride solutions used to improve the effectiveness of road salt. Seals are drying out with age, which is causing corrosion to appear on electrical items on the undersides of buses.

While most drivers are familiar with ABS in their personal vehicles, new drivers still need training on it before driving an ABS-equipped school bus, which today includes the majority in service. While there are good videos, such as from as from GM and Bendix, showing how an ABS works, “behind the wheel” training is important. Clymer recommends new driver training include experience with a full four-wheel lockup and then an ABS full brake application to show the difference in the “feel” of each.

“If they are uncomfortable trying the system themselves, I take them out for a ride and they get to see what a passenger goes through during a panic stop with ABS,” said Johnston on his 30-year training practice.

Siuru is a freelance automotive journalist based in Temecula, Calif. He has a master’s degree and doctorate in mechanical engineering. Editor Ryan Gray contributed to this report.