ABS Troubleshooting: Is There Still A Problem?

Why are technicians still misdiagnosing ABS?

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."

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