Tuesday, May 26, 2009

How To Troubleshoot Air Brake System ABS Code .

This ABS system is by Meritor Wabco found on most highway transport vehicles. An ABS light on the dash indicates a problem that has to be repaired since an ABS fault is a fail according to Motor Vehicle Inspection criteria. The ABS system does not operate normally when there is an active code.

The code description reads "Right Front Wheel Sensor - Open"

When looking at the Right Front wheel the sensor looks like it's intact. The first thing to check is to make sure the sensor is pushed all the way into the housing. The sensor reads a tone wheel on the hub and sends a signal to the ABS Control Module.

According to the troubleshooting specs the resistance of the sensor should be between 900 and 2000 ohms. When I checked it out there was no continuity causing an obvious open circuit as indicated by the fault description. The first step is to check the wiring itself for any damage or the possibility of a faulty sensor. Most times exposed wiring to the environment is at fault when troubleshooting a circuit problem.

Further inspection shows a spot on the sensor wiring that has rubbed through on the brake pot assembly. One of 2 wires has been damaged and caused the open circuit condition. I use heat shrink butt splices since they seal very nicely from the environment.

Once the sensor was replaced I moved the bus in a forward direction & the ABS warning light went out. This was an easy fix and it was nice to stumble across the bad wire in an easy to reach location. This conventional school bus is a only a year old but you never know what can happen to wiring and components when operating on a daily basis. I hope this mechanic information has helped you out and maybe you will have it as easy as I did with repairing this failure.


Anonymous said...

Great Site! My father was a school bus mechanic, 19a-trainer, and heavy equipment recovery specialist.

We enjoy viewing your repairs, suggestions, and years of knowledge!

Definetly enjoy your pusher repairs. Spent a lot of my childhood around Thomas ER's!


John said...


Thanks for the comment. I have a lot of content on the ERs so their will be more to come.

Anonymous said...

first student in Ancaster has many of their buses with ABS light on, mechanic says its okay to drive with it like that.

John Whelan said...

ABS codes need to be fixed for the system to operate normally. We have a fleet of buses so there are times when the ABS code won't get repaired immediately but definitely there will be a rejection report or even an "out of service" from the Gov't inspector until the code is fixed.