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A visual inspection reveals that 'one of these things is not like the other'

Thursday, February 1, 2018 - 09:00
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I was called to a shop with a complaint of an ABS light on. The vehicle was a 2017 Subaru Impreza with a 2.0L engine (Figure 1). It was involved in an extensive front-end collision and experienced severe damage to the right front suspension. The shop had replaced a lot of suspension parts including a right front wheel speed sensor. They cleared all onboard control modules of error codes in memory after all the repairs were completed, but the ABS codes still remained in memory.  

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Figure 1
Figure 2

When I arrived at the shop I noticed that the vehicle had multiple warning lights illuminated on the instrument dash panel (Figure 2). There was a light for the antilock brakes, stability control, collision avoidance, lane keep safe and the eye sight control systems. It is almost hard to imagine that all of these operating systems were experiencing problems at the same time. They all shared the same network so it was possible that one problem could be resonating a fault in one controller that had an adverse effect on the operation of the other controllers within the network. Many of these controllers today share input sensors among each other on a CAN bus network rather than wiring one sensor input to various controllers.  

Time to dive in 

It is always best to go into every controller and record codes stored as ”current” or ”past” codes. Then do an overview to see if these codes all point to one area of concern. If there is a problem in one controller that is on a shared network, you can almost be guaranteed that other controllers will record complaint codes redirecting your attention to the specific controller having an issue. Do not get in the habit of doing a vehicle scan of all controllers at once because you may not be guaranteed to pull ALL codes from every control module. Some scan tools may pull “present” codes but may not pull “past” codes and this could defeat the purpose of you putting together a complete game plan of attack. It is vital to compile as much information as needed to narrow down the suspect in your diagnostic routine. 

Figure 3

In this particular case, I performed a quick vehicle scan just get a basic overview of what was going on but knowing that if needed I would have to go to individual modules for a deeper dig. The engine and transmission modules on the network were pointing fingers towards the ABS control module for a fault in vehicle speed error by setting a code P0500. The ABS module stored codes C0022 for front right ABS sensor signal fault and a code C1424 for ECM abnormal (Figure 3). The right front wheel speed sensor had some kind of operational issues. At this point I had to verify the problem and decide whether it was s mechanical issue or an electrical issue. 

Most ABS systems will perform a static and dynamic test of the operating system to alert the driver of any issues. On startup there will be an integrity check just like there would be in the engine management system for component tests. This could require either one or even two-three key cycles. So I simply cleared all the codes and turned off the ignition switch for one minute. I then proceeded to start the vehicle and let it run for one minute. I did this three times to insure a three-cycle event and the warning lights did not come on at all. This is a quick procedure that can be done from the driver seat and holds great value to limit a lot of wasted time by guaranteeing that your problem is not electrical but more mechanical. Understand that the ABS module is sending reference voltage/reference ground into all of the ABS Wheel Speed sensors and checking all solenoid/relay/ lamp circuits for a driver threshold status while it validates the system during key on operation. If there were any electrical issues a light would have been on. Now it was time to go ahead with a dynamic test. 

Narrowing down the possibilities 

I went to the ABS data PIDs and selected the front and rear Wheel Speed Sensors and proceeded to drive the vehicle about 10 MPH (Figure 4). The right front wheel did not show any wheel speed signal at all and as I continued to drive the warning lights all came back on. This was definitely a mechanical condition. The culprits here on my list as I was driving back to the shop were the ABS sensor was not seated in the mounting hole properly, the tone ring was bad or the sensing device within the new sensor was bad. 

Figure 4
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