ABS Troubleshooting: Is There Still A Problem?

How long should it reasonably take for fleet maintenance technicians to master "new" technology? At what point should you expect your technicians to be able to properly troubleshoot and repair a system that has been mandated on heavy duty tractors since 1997, and trailers since 1989?

According to engineers at brake component suppliers MeritorWABCO and Bendix Commercial Vehicle Systems, heavy truck and trailer anti-lock brake systems (ABS) are still commonly misdiagnosed, even though they have been standard issue for nearly a decade.

"The biggest issue that we see with someone troubleshooting ABS systems is that the mechanics tend to automatically replace the component rather than performing a proper troubleshooting," says Tom Weed, staff engineer, ABS Engineering Group, Bendix Commercial Vehicle Systems. "For example, if there's a sensor diagnostic trouble code, we'll see technicians who just automatically replace the sensor, then when that doesn't correct the issue, (they) replace the ECU, rather than follow a disciplined troubleshooting approach. For example, a sensor fault can be caused by wiring or issues at the wheel end, and not just the sensor or the ECU.

"This is not isolated to Bendix," he adds. "We all analyze warranty return parts, and the number one thing we see is 'no defect found.'"

"Some of the common problems are the inability (of technicians) to use a volt-/ohmmeter, and basically the lack of knowledge of how ABS works," says Matt Williams, technical support manager for MeritorWABCO. "The basic part of troubleshooting is knowing how the system operates, and what we find through our call center is that unfamiliarity with the system—lack of familiarity with when an ABS event occurs and why it occurs—will tend to lead the technician in the wrong direction."


What don't technicians understand about ABS? As an add-on to the common heavy truck foundation brake, ABS does not alter or affect the basic pneumatic/mechanical system that technicians have been successfully repairing and maintaining for decades. What it adds is a toothed exciter ring at each wheel end, a sensor to read the speed of each exciter ring, an electronic control unit (ECU) that receives and analyzes the wheel speed signals from each sensor and then regulates braking pressure at each wheel, modulators in the brake lines to modulate braking pressure to any wheel end that is about to lock up, and warning lights in the cab and on the trailer to notify the driver of a fault in the ABS system.

It's true that early systems did pose challenges to technicians, because the ABS sensors at the wheel ends couldn't distinguish between an ABS-related fault and a fault that only appeared to be ABS-related (but was actually caused by an unrelated wheel end condition).

"In the early days of ABS, systems were a little more sensitive to wheel bearing adjustment and things of that nature, which would typically cause a sensor diagnostic trouble code," Weed confirms. "Today's systems are much more robust in regards to issues like that. Of course, if you get a real issue with the wheel end—really bad wobble, or a wide air gap—the system is still designed to set a trouble code."


John Reid, manager, Field Service & Warranty Group for Bendix, points out that technicians don't always stay up to speed on improvements to the components. For example, while early ABS systems required that a technician use a feeler gauge to create a proper gap between the exciter ring and the wheel speed sensor, some technicians aren't aware that that has changed.

"We just had a complaint the other day that we went out and looked at. The technician was thinking that he had to gap the wheel speed sensor," Reid explains. "Today's sensors aren't like that; you adjust the sensor today by just pushing the sensor all the way in until it hits the exciter ring, and it sets itself. This technician was pulling the sensor out and actually building in an air gap between the speed sensor and the exciter ring."

Reid points out that the information on how to properly set the sensor was readily available to the technician in Bendix's product documentation, training CDs and videos.


The experts agree that as brake systems become more complex, technicians will need to develop their diagnostic skills to keep up.

Today, heavy truck ABS systems are commonly combined with traction control systems and stability control systems, which add more sensors and more ECU computing power to the mix. Bendix, for example, in adding both roll stability and yaw stability to its ABS 6 product, installs a steering angle sensor to the steering column, a yaw rate and lateral acceleration package, pressure sensors, to keep track of what the driver is doing with the brakes, and an ECU with more power.

"That requires technicians to acquire a little more knowledge, but if you can troubleshoot an ABS system, you can troubleshoot a stability control system," says Weed. "But there are a couple additional components added, so mechanics need to stay up on the latest."

ABS is also well-suited for telematics capabilities, which allow a properly-equipped truck to use wireless communications to relay logistics, vehicle performance and maintenance information in real time to a computer at the main office or the maintenance shop.

A further twist could be in store in 2007 heavy-duty trucks, as higher underhood temperatures could affect air compressors.

According to Reid, "As we raise underhood temperatures, the operating temperature of the compressor goes up, and a compressor has a tendency then to pass oil past the rings. That oil gets into the air brake system, and the oil can attack rubber components inside the valve."

"We have a very good cross-functional team that is analyzing this issue," says Weed. "They are working directly with our customers, conducting tests to understand what, if any, effects we're going to see in our products from this higher operating temperature."


According to MeritorWABCO's Williams, the primary reasons technicians don't do thorough ABS troubleshooting are:

  • Tendency to take the path of least resistance
  • Lack of familiarity with the system
  • Lack of training

Add to that list the very real possibility that technicians don't have adequate tools available in the shop: "I still go into shops where I will ask a technician to go get a voltmeter, and they get a blank stare on their face," says Reid. "Yes, you still find some technicians that don't have the proper equipment."

The challenge for fleet maintenance managers, then, is to get technicians to go through the proper diagnostic steps, and not simply change parts and hope for the best.

Today, technicians can diagnose ABS faults by reading the system's internal blink codes, by hooking a hand-held diagnostic tool to the ABS data port, or by using PC-based software. Some tools are multi-mode, such as Nexiq's Brake-Link, a hand-held scan tool for trailer ABS that also interfaces with OEM diagnostic software on a shop PC.

"I think there will probably always be a need for all three types of diagnostic devices," says Bendix's Weed. "As electronic modules on commercial vehicles become more complex—not just for ABS but also, for instance, for an engine or transmission—I think that there's a good chance that you'll see more of a move towards PC-based diagnostics, because that provides your highest level of diagnostic support. That would be PC-based software that you would procure from the software provider and load it onto the laptop and use that to troubleshoot the system.

"The diagnostic software will guide the technicians to inspect the wheel end, and tell them what to look for," he adds. "In the case of an open or a shorted wheel speed sensor, the diagnostic software will actually show a picture of the connector and identify the terminals, and direct the mechanic to connect an ohmmeter, for example, to take a resistance check, and (tell him) what he should get. It's a pretty powerful tool, no doubt about it."


"I think PC-based software is the wave of the future," echoes MeritorWABCO's Williams. "There's more capability for wireless interfacing and things like that, so I definitely think that PC-based diagnostics is the way to go.

"We don't provide our own PC-based software," he adds, "but it is available through a vendor (MeritorWABCO TOOLBOX software from SPX). There are a couple of manufacturers out there that have a wi-fi system, so you have a laptop and a diagnostic interface plugged into the port, and they can literally walk around the truck with that laptop, and still communicate wirelessly through that diagnostic interface."

"You're going to need a computer with software to be able to get in and troubleshoot and diagnose systems correctly," says Bendix's Reid. "We have what we call a Remote Diagnostic Unit (RDU), which is a little unit that plugs into the diagnostic connector of the vehicle, and LEDs light up to tell you where the faults are. It gives you a general idea where the fault is. It doesn't really dig down and tell you whether you would have a shorted wheel speed sensor, or an open wheel speed sensor; it just tells you there's an issue with the wheel speed sensor.

"When you get into PC diagnostics, you could actually diagnose that, yes, the wheel speed sensor is open, or shorted," he explains. "Or you could connect your PC to the vehicle and drive down the road and actually look at the wheel speed signals and see what they look like."


"All the ABS suppliers do a good job of providing information," says Weed. "There are 1-800 numbers, there's maintenance information out there over the internet. I know Bendix offers a very good air brake school. I would certainly encourage fleets to take advantage of that.

"I would also emphasize the requirement for technicians to follow a very disciplined, very methodical approach to troubleshooting an ABS system," he says, "and not just automatically start replacing components."

"I think fleets are going to have to invest in their technicians and send them to training," Reid says. "As systems become more complex, the technicians are going to have to diagnose problems and not just change parts. I just had a situation the other day where the technician had changed three electronic controllers and hadn't done anything else. That becomes very expensive to the fleet."

For dates and locations of Bendix Air Brake and Foundation Brake Training and MeritorWABCO Stability Control Demos, see the Brake Supplement.