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.
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.
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.
This vehicle uses a Magnetic Resistive type of wheel speed sensor and is not your normal AC output type. On a typical magnetic sensor, you will find a field coil wrapped around a magnet. This sensor field coil could commonly have a resistance value of about 500-1200 OHMs depending on its design and reads off a metallic trigger wheel. It will output an AC signal whose amplitude depends on the strength of the sensor magnet, resistance of the field coil and the air gap between the magnet and trigger wheel. The only downfall with these types of sensors are that they are more prone to pick up unwanted electrical noise and cannot produce enough AC voltage below 3 MPH to detect vehicle creep.
The Magnetic Resistive type sensor looks similar to a magnetic sensor but its circuitry is different. This sensor also uses two wires, but it is dependent on a reference supply voltage and a reference ground feed to the sensing device that will in turn act on the reference voltage line to toggle it and produce a digital signal. The sensor itself will need to be triggered by magnetic bar segments and the air gap between the sensing device and magnets is crucial for proper operation. This type of sensor is more dependable because it can actually measure vehicle creep because its amplitude is not dependent on vehicle speed and it is less susceptible to electrical interference.
The front hub bearing has a seal on each side of the bearing. Only one side of the bearing has segmented magnets imbedded into the seal along its circumference for the ABS wheel speed sensor to read. It is not uncommon for any shop to put this bearing in backwards because it can be installed in either direction. The segmented magnetic ring side must face the sensor. There is an installation tool (ATE #760130) that you can purchase off of Amazon to insure proper installation that houses a very fine metal powder in an enclosed plastic film that when placed over the proper side of the bearing will show the magnetic segments. My guess was that the bearing could be in backwards.
I had the shop pull apart the right front suspension so I could take a close look at the repairs. This was a point where I had to perform “Sesame Street” tactics that I learned many years ago when I was but a six-year old kid watching my favorite show and singing “One of these things was not like the other.” It is so vital to perform visual inspections as part of your diagnostic routine. I had one vehicle a few months back where a shop installed a used spindle on a vehicle and the wheel speed sensor hole was offset by a half inch because it was the wrong year spindle for the car so at this point I had to keep an open mind and play outside the sand box.
When I started to inspect the right-side hub bearing I could see it was installed properly with the magnetic ring facing toward the sensor (Figure 5) and everything seemed fine. At this point I was scratching my head and decided to take the other side apart to start comparing side to side measurements. As I was looking closely I sat there in awe with what I discovered. If you looked at the Wheel Speed Sensor hole on the left side of the vehicle the hub bearing was protruding three-fourths into the sensor hole diameter (Figure 6). The right-side bearing was only about one-fourth into the hole diameter. I could not believe what I was seeing and I never knew this could even happen.
After taking out the new hub assembly and comparing it to the old hub assembly you could see that the inner part of the new hub bearing was shorter in length (Figure 7). This goes back to the old saying that the parts guy can be your best friend or your worst enemy. It’s so hard to believe that a parts guy could hand you a wrong part and at the same time an installer does not take the time to match a part up. We are in such a rush in today’s automotive world that we just don’t take the time to check everything in our routine repairs. There are no longer guarantees that you are getting a correct part or even a working part. It just ends up putting you down a different path of “denial diagnostics” that can ruin your day and create many unwanted hours of wasted time and not to mention your loss in confidence in your work. My only hopes is that this story hits home with many of you techs out there and that you keep on the watch for this not to happen to you.
