Trailer ABS light on solution

      Hello all, and welcome to this post on an ABS problem and a solution. I will use this opportunity not only to talk about ABS problems in general, but also about the diagnostic process for any system.
      Once again I inherited a problem that previous efforts had failed to solve. This time the vehicle was a fairly new Great Dane dry van with the ABS light on. The story was that everything that could be replaced pertaining to the system had been changed out, without success. At least two shops had worked on the problem. Curiously, the customer said that the light only came on when the trailer was hooked to a Volvo tractor.
      This type of problem, where lots of diagnostic work has been done without success, is often a good opportunity to hook a new customer and gain status for the shop that takes care of the problem. Since lots of potential problems have already been eliminated, they can also be surprisingly easy to solve. The customer statement that the light only came on with a Volvo tractor was suspicious, but I thought maybe there was something about the ground path on the Volvo that was different that the other tractor it was hooked to.
      As always, I started with just a quick visual inspection before I hooked up to the trailer. It looked to be in very good condition, and I was happy to see a very simple Meritor Wabco system with wheel sensors on the front axle. The module on top of the service relay, the modulators, and the sensors looked new. One of our customers parks his Volvo tractor on our lot and we got permission to use it. I hooked up and the ABS light was indeed on, so I pulled flash codes with the tractor by turning the ignition switch on-off-on. A 4 code was set, which is the code for wheel sensor YE 1, the curbside sensor.

      Absolutely the first thing that must be checked with a sensor code on any ABS system is the wheel bearing clearance, or lash. When I jacked the wheel up I found the curbside wheel did have excessive play, and it took over 2 full turns on the adjustment nut to seat the bearings. I always set truck wheel bearings with a torque wrench at 35 foot pounds with both dual wheels and tires on, after the initial seat/back off process. If I need to turn the nut to align the lock I can safely tighten it some more, but never loosen from that setting. If that sounds like too tight an adjustment, keep in mind that you are lifting both wheels and tires, as well as the hub and drum, with that bearing nut. It is actually pretty hard to get the bearings too tight with all that weight on them. I couldn't get at the sensor to reset it, so took the wheels and drum off and tapped it in against the tone ring. The tone ring looked to be in good condition, but the sensor gap was at least 1/8 inch, so I was pretty confident I found the problem. The Wabco system seems to be especially sensitive to bearing clearance, and is one of the most common issues when a light comes on. The bearings on the other side were not as loose, but I went through the same procedure there. 

      I pulled the trailer out of the shop and the light went out, then the repair survived a test drive around a city block, so it was pronounced fit to return to the customer. As for the problem being only evident with a Volvo tractor, I don’t know what happened. Maybe the not-Volvo tractor they tried had no power at the trailer accessory pin. Maybe it was right after the sensor change and the air gap was good for that test. Or maybe it’s just one of those things.

      This experience illustrates what I call law #1 of diagnostics: make sure the simple stuff is right before moving to more complicated/expensive possibilities. I can’t say that this trailer didn't have other problems that required the parts to fix, but to miss the basic check of bearing clearance cost the customer money and time. I am sure that most of the shop rip-off stories have their roots in this way: a customer takes his or her vehicle in to be repaired, the tech misses something simple, and lots of time and money is spent without fixing the problem. Both the shop and the customer get frustrated and the vehicle goes to another shop, where the problem is found to be simple, easy, and cheap. The customer feels the first shop tried to rip him off by selling parts and service he didn't need.

       No shop will stay in business very long by blatantly ripping customers off, and I don’t think that happens very often. It is too easy to uncover a rip-off, especially when it would be as easy to investigate as in the scenario I just outlined. However, a shop can get a bad reputation after just one of these mistaken diagnosis incidents. Nobody can do everything perfectly every time, and early in my career especially, I made my share of mistakes. We as mechanics can’t be so arrogant that we don’t bounce problems off other techs or to look for help with difficulties, and we all need to keep law #1 in mind while looking at a problem. Novice techs need close supervision to make sure they don’t get off track and that they follow efficient procedures.

      Thanks for looking at this post, and check back soon for a post on a Sprinter no start problem.

Bruce