A Workhorse ABS diagnostic problem and a solution

            Hello, all. I recently experienced an unusual diagnostic problem, this time on an ABS system. This was on a Workhorse route truck of an account my employer services. The vehicle runs the Meritor Wabco type D ABS, which in my experience is a reliable system. Usually problems with sensor signals are due to bad sensors or incorrect bearing adjustment. This vehicle had been in numerous times and we had been challenged with a repeated appearance of the light. Codes kept resetting for the rear sensor data improper or the circuits shorted or open. The tone rings were badly corroded (no real surprise there), but changing hub/tone ring assemblies and proper bearing adjustment failed to provide the solution. The sensor voltages were checked by spinning the wheels in turn, and the sensors were providing an adequate signal. On a road test, the left rear sensor had a varying and intermittent rpm signal which never matched that of the front signal, and the right rear sensor always read at 0 rpm. . Complicating the problem was the fact that the truck was only reliably available on Saturdays.

            Finally, the truck came in with the right rear sensor always reading 71 rpm, no matter the operating conditions. When the sensors were reconnected left for right, the right sensor still read 71 rpm. Even with the sensor unplugged, the sensor output 71 rpm with the truck stationary.  Obviously the module had failed, so a new one was ordered for the next weekend.

            When the part arrived, the tech first attempted to eliminate the possibility of a multiple problem by monitoring sensor voltages at the module. The rear of the truck was jacked up one side at a time so it could be run at idle. The green plug to the module was disconnected and voltage was taken across pins 3 and 10, then pins 6 and 7. The voltages were found to be steady and approximately the same side-to-side, with each side arriving at the proper set of pins.

            The module was changed and the codes cleared, but a code immediately reset for a short or open in the rear sensors. The vehicle was driven to check the rear signals against the fronts, but the rear sensors both read at 0 rpm. At least the right side was no longer reading as 71 rpm, so some progress was made. Again the green module connector was unplugged and each sensor circuit was checked. The left side was fine, but both pins for the right side were shorted to ground. However, the resistance was several hundred Kohms, so we obviously had an unusual problem. The whole wiring harness was visually checked, but all looked fine.

            To break the harness down, it was disconnected at the midships connector located in the frame just in front of the modulator. Back at the module the short was now gone, so it looked like the problem was in the back half of the truck. However, ohming the rear portion of the harness found the short was now gone there now, too! The right sensor was unplugged and the harness ohmed from one pin to the other, to eliminate the possibility of a crossed connection. Each wire was insulated from the other.

            When the miships connector was reassembled, the short reappeared, so the problem was clearly at the connector, somehow. The wires for that sensor came from the module as red and black, then changed to brown and black at the midships plug which made sense, as the sensors both have brown and black wires. The red wire from the module connected to the black sensor wire, whereas on the left side, the yellow wire from the module connected to the brown wire to the sensor. (see the image)

ABS wiring for rear speed sensors on Workhorse truck

Eureka! The right side wires were removed from the rear part of the midships plug and their positions exchanged. Everything was buttoned back up and the codes cleared. No code was reset and the vehicle was driven. At 10mph, the light went out and stayed out for a short test drive.

            Apparently the harness was assembled incorrectly in manufacturing. I don’t know why we were sometimes able, at first, to get the light to go away for a short period of time, or why the vehicle could have passed manufacturing inspection without a light being set. The problem must have somehow fused that circuit in the module, causing the indicated rpm to freeze at 71, unless someone somehow applied a voltage to it that did the damage.

    Thanks for viewing   Bruce

What should students learn in a good Truck Mechanic school?

I believe good instruction in Heavy Duty Truck Technology should include four areas of study:

·         Teaching entry level core skills in the technical aspects of repair.

·          Helping give students the tools with which they can continue learning and advancing in the career.

·          Providing the experience of working with others as part of a group.

·          Instilling the habits of working safely and with consideration for the environmental impact of their actions.

Giving students the means to obtain the all-important first job and successfully performing in that job depends on the core skillset the student brings with him or her. Certain basic ideas and capabilities are paramount in efficient repair of any vehicle system. Among these core competencies are: knowledge of electrical circuit design and troubleshooting, brake system function and failure patterns, proper periodic maintenance procedures, and steering, suspension, and driveline function and design. These four areas encompass the bulk of the day-to-day workload in any repair facility, and competence here will give basic skills for job performance. Hand in hand with the knowledge of specific systems is competence in basic tool and equipment use. 

Continued learning is the means to advancement and as most training is now done online, students need to be able to efficiently navigate websites and diagnostic software. Besides formal training, student self-improvement will utilize online resources and these must be used discriminately in order to extract useful information. Diagnostic information is now accessed online, and in the future more of it could be based on social media, real-time type formats. I believe this will be one of the biggest changes that will be seen in the near future, as dealers and the aftermarket move diagnostics completely into the digital realm. The technicians who are ahead of the curve will be in the best position for advancement.

Working in groups is an important skill in any job, but mechanics tend to work as individuals and being helped only on an as-needed basis, not as a function of day-to-day operation. That said, working together is still an important part of working in a shop atmosphere. Wherever we work, a group effort is needed and all employers look for employees who demonstrate an ability to work with other personalities and in various situations. Everyone learns from fellow employees, and those who work best with others learn the most, and the most quickly.

Working safely is extremely important not only to the individual, but for all those he or she works with. The repair industry provides ample opportunities for injury from flying objects, burns from welding and heating, contact with moving parts and equipment, falls from ladders or vehicles, crushing or trapping injuries, chemical inhalation or skin contact, and from lifting, pushing, or pulling. Each year thousands of mechanics are killed or injured, and most of these incidents are completely preventable. Hazards are mitigated by the use of appropriate safety gear, by following proper procedures, and by notifying supervisors of dangerous situations. Students need to realize that workplace safety is largely in the hands of the employee. Safety also extends beyond the shop, to the larger environmental aspects of waste generation and discard product handling. Waste oil and antifreeze, used batteries, and takeoff brake chambers are among the waste items generated in repair, and students must learn to handle each one properly.

The repair industry is on the verge of change. Some changes are evolutionary, as new technologies enter diagnostics and wireless communication is expanded. Other changes will be more drastic, changing the way things are done on a basic level. Students who are prepared for all these aspects will be more employable, will command higher wages, and have more upward mobility. The responsibility of learning will be more focused on individual effort to keep up with change. School needs to be regarded as an essential starting point to a career, an opportunity to get basic but valuable skills for the future. The technician of tomorrow will be as accomplished on a laptop as with a torque wrench, and the best will earn a wage above that traditionally found in the industry.