All the Stops

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Living with anti-lock brakes has generally been easier than people once feared. After the debacle of the mid-1970s when ABS was first mandated in the U.S. but didn’t perform well, it’s no wonder there was a slow and reluctant response to its rebirth a dozen or so years ago. It took another set of legislative demands to get ABS on all the trucks and trailers we buy now.

Despite broader experience with ABS, it remains misunderstood and perhaps even under-appreciated. Maybe most of all by drivers, whose bacon has likely been saved by ABS more often than they or their bosses know.

No mechanical or electronic system is trouble-free, and ABS is no exception, but the task of getting the maintenance department on side with these newish brake systems might well be easiest of all. That’s due to the glory of simple diagnostics and warning lights that alert the driver — and subsequently the shop, one hopes — to problems. There’s no interpreting to do, as with most other truck malfunctions, because the ABS electronic control unit (ECU) monitors things continuously and holds the anti-lock system’s recent history in memory. Mechanics simply plug in the appropriate diagnostic tool and read the screen.


Any ABS problems that arise typically involve either the toothed wheel that’s installed near the hub and turns with it, or the electronic sensor that constantly monitors the wheel’s speed and passes it on to the ECU. With information from all sensor-equipped wheel positions, the ECU notes differences in wheel speeds, thus sensing impending lockup, and can instantly activate a valve that cuts air to wheels slowing down too fast. It then modulates pressure to those wheels to prevent lockup. The driver can maintain control of the vehicle because braking effort is equalized at the truck’s various corners and all wheels keep rolling.

While the wheel-speed sensors themselves aren’t likely to go south on you, they can get knocked around and wires can come adrift. Corrosion can also set into the connections. The most likely problem you’ll find is that the toothed wheel and the sensor aren’t properly aligned, so the information sent to the ECU is inaccurate. They can get knocked by anyone working underneath the truck near the hubs, so the first order of business is to ensure that everyone likely to get in there knows what to avoid.

Look at RP 1209, section VIII, in the Technology and Maintenance Council’s “Recommended Maintenance Practices Manual” to read more on caring for ABS sensors. You’ll see that preventive maintenance on anti-lock systems is not difficult (and nor are repairs), but it does involve more than reading a diagnostic screen. Changing a drive-axle sensor, for example, means removing the wheel and drum. In fact, this is a prime example of the modern shop where white-coated technician meets coveralled mechanic, and may well be the same person.

One point from the driver’s perspective: it may be easy to temporarily dismiss the need to repair a defective sensor or some other apparently minor ABS fault. The brakes will still work, after all. But the driver who is used to having functional ABS will have adjusted his driving style to suit and may even have raised his level of risk tolerance, comforted by the knowledge that his anti-lock brakes increase his margin of safety. It could be a dangerous situation if his trusted ABS doesn’t work as expected.


The basic truck braking system is a well proven but complex collection of hardware with all manner of things that can go wrong. Think about it: braking starts when the driver tromps on a valve that sends air pressure to a bunch of other valves, and that pressure eventually makes it to each wheel to rotate a lever, turn a cam, and thus press linings against the rotating drum to make friction and slow those 80,000 or 140,000 pounds. But all of that takes a while, and there’s almost no chance that the air pressure — which travels through a total of about 300 feet of air lines on a standard tractor trailer — can get to each of its 18 destinations at exactly the same moment.

Even assuming there are no leaks in the 150-plus fittings along those 300 feet, and assuming a proper supply of clean and dry compressed air, the challenge of achieving balanced braking effort at all wheels is huge. In most eyes it represents a pipe dream, even with a liberal use of booster valves and air dryers and the like, not to mention constant maintenance. Yet without that balance, any braking system is imperfect.

There isn’t a maintenance man on earth who hasn’t scratched his head over that one, bemoaning his failure to master the black art of brake-system plumbing.

ABS helps even things out, but the next technological step will render that consternation obsolete: the electronic braking system, or EBS. Already, it’s an option on some Freightliner trucks, though only as a redundant system over a conventional foundation brake setup, and it’s only a matter of time before it’s common here. In Europe, there are more than 100,000 heavy trucks rolling around with EBS. The Mercedes-Benz Actros mid-range truck, for example, has had EBS as standard since its launch five years ago.

With EBS, braking force is still applied at the wheel as now, using compressed air on the brake chambers. But the actuating signal is carried electrically, not by air pressure, from the electronic foot control to solenoid-operated relay valves at the wheels. It arrives instantaneously at all wheel positions, and in theory you have perfect braking balance. Likewise, when the driver lifts off the brake pedal, air dumps much faster from the brake chambers for a much faster release.

As you might expect, faster application means shorter stopping distances, as well as a better “feel” at the brake pedal, even though the pressure a driver’s foot applies to the brake pedal doesn’t actually dictate the extent of the force with which shoes hit the drums. Instead, the EBS signal sets a level of deceleration. The wheel and speed sensors and the ECU work together to apply the brakes for the desired braking performance, and braking is independent of the loaded condition of the truck.

With EBS, you’ll also see less brake drag and fade pre-warning. You’ll certainly see enhanced brake diagnostics. Add a few more sensors to the brake system and EBS can read brake temperatures, brake torque, or brake stroke, among others. This allows for overcoming variations in lining friction values, poor stroke adjustment, brake pulls, and so on.

By the time we have EBS we’ll also be using disc brakes more routinely, possibly by legislative command, and our basic braking systems will have made a quantum leap over what we have now. But it goes even further, because as long as we have all this electronic capability at the wheel ends, we can actually brake each wheel position independently and instantly, and thus control the stability of the rig, which includes preventing rollovers as much as jackknife accidents. Lesser features, though still useful, will include things like automatic hill holding. This will likely be called electronic stability control, or ESC, by the way.

Attractive as all that may seem, the advent of EBS and ESC and other new high-tech solutions is bound to bring out the skeptic in truck operators. As the birth of electronic engines did, and ABS too, of course. This time, though, the advantage of better brake performance will be weighed against the fear of trusting a bunch of sensors and a small computer to pull 80,000 pounds or more down to a stop.


An invaluable source of brake information comes from the “big red book,” the Technology and Maintenance Council’s “Recommended Maintenance Practices” manual. It costs a bomb, but there’s nothing else like it. Call the TMC at 703/838-1763. TMC also offers a manual and a video entitled “Technician Guidelines for Antilock Braking Systems” for $49.95 US and $59 US respectively.

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Rolf Lockwood is editor emeritus of Today's Trucking and a regular contributor to

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