| Not that RCA. |
For those in engineering fields, RCA, root cause analysis, has deep office-politics ramifications: doing an RCA for most below average engineering groups (half of the engineering groups fall below the average!) means somebody will be blamed for things not working at at risk of losing a chunk of their annual bonus. Fingers will get pointed. And people will scurry to avoid the RCA fickle finger of fate. All of this is too bad, because when we learn the root cause of something, it presents the easiest way to fix the problem with the most certainty the same issue won't return. Turns out, the real world is a bit more messy.
What Does the OBD tool say?
For my Freestyle, the traction control and ABS lights have been a part of my dashboard experience since purchase. The car's magic computer reports back to me the following diagnostic codes: C1175, C1297, C1222, C1296, showing my wheel sensors aren't working and another code that seems to be the ABS system saying "I give up".The lights on the dash and codes from the computer are symptoms, but of what? The trouble codes say the sensors aren't working, but it could be that sensors work just fine and there's a wiring fault from the ABS unit to the sensors, or the wheel hub isn't correctly made so that sensor can detect the wheel rotating, or the ABS system itself it not getting power to drive the sensors.
Starting Somewhere
To help frame the problem, here's a crude rendering of an ABS system from an electrical perspective:| ABS system, simplified |
1) Non-functional sensors
2) Failing ABS unit
3) Electrical path to sensors
4) Power or ground connectivity
Since the car is about 11 years old, I figured I'd start with replacing the ABS sensors, as that seemed to be a good place to start as any and would be my idea of fun.
But common sense and too many RCA meetings tell me the sensors not working might be a symptom and not the root cause of the problem. If several sensors had failed, it might be a problem at the point where all of sensor's wires get wrapped into a bundle or pass through a common point, where there's been some wear. I really wasn't sure, but with the sensors approaching a decade old, I figured it might be that they failed one at a time during the prior ownership of the car, and me, being the new owner, now sees several failed at the same time.
Let's get to work!
Here's the basic steps for replacing this (or any) Wheel sensor:
- Gain access to the sensor by removing the wheel
In the Freestyle, the sensors are placed in the wheel kuckles, access is behind the rim, so the tire must be removed. - Unhook sensor
The sensor interfaces to the electrical system via a connector. In the front of the car, these are behind the inner fender liner in the front and in the rear, the connectors are in roughly the middle of the rear axle. - Remove sensor from wheel hub
The sensor part needs to be close to something spinning to detect movement, in this case, there's a hole in the wheel hub for placing the sensor, and sensor reads some data from a magnet in drive shaft or a toothed ring (sometimes called a tone ring) close to the drive shaft. - Replace with new sensor
What comes out, needs to be replaced with something new!
Front Wheels
I have the best pictures for the right (passenger's) side, so lets describe what happened there, the driver's side was pretty much the same scene.| Step | What Happened |
|---|---|
| Removing the wheel is the first step in the process; I still use a breaker bar, no need for air tool to unwind the wheel nuts.nbsp; The sensor is connected via a wire from a connector in the wheel well, which meant removing the fender liner to locate the sensor. The junction is close to the top of the fender, so removing some of the plastic inner fender's faster's doesn't give great access. To get a picture, I removed the entire piece, but guessing that's not entirely necessary, but still probably a good idea. One that side was disconnected, I turned my attention to the other side of the sensor attached to the wheel knuckle. | |
| Not much do to here, right? Just remove one nut (9mm) and unclip the sensor, pull it out of the knuckle and you're done! Well, Not really. The sensor was stuck in the wheel knuckle and didn't respond to liberal applications of penetrating fluid. | |
| Tried removing the sensor by prying on the tab with the fastener hole, figuring it would make a good levering spot. Not much happened, until I broke the exposed plastic part. Removal of the sensor would need more a more drastic approach. But, we're making progress because less of the sensor is there versus when we started. | |
| I then fetched the drill and started with a small bit to make a pilot hole and worked my way up to a larger bit (pictured). The hole now filling with debris, I used a shop vacuum to clear the scrap, as I didn't want it to fall inside knuckle and have the metal affect the sensor. A few more runs with the larger drill and a pick and I was able to remove the remainder of the sensor with a pick. | |
| Thinking optimistically, in case the next owner need to replace the wheel sensors in another 11 years, I put some brake grease on the part before snugging it back into the hole and winding the 9mm bolt back to hand tight. The brake grease isn't metallic, so it shouldn't interfere with the operation of the sensor |
All Back Together
| Look Familiar? |
Bottom line (at least for the front sensors), the problem isn't with the sensor, it's the communcation path to the sensor or maybe the ABS unit that can't read the values from the pins. While I have new sensors, I didn't find the root cause, so the problem remains.
Next post, I'll replace the rear sensor, with the expectation that I'll still have the same problem reading the sensors, but I'll have a much better idea of where to start looking next.
