A customer comes into your shop. They say that their ABS light is on and it usually comes on after one or two minutes of driving. You scan the vehicle and there are codes for a wheel speed sensor that say either intermittent or erratic. These can vary depending on the manufacturer and the type of code. What do these codes and that customer symptom have to do with the wheel bearing? Well, the reality of it is on most modern vehicles, the encoder, tone ring, or reluctor is part of the wheel bearing hub unit or part of the inboard seal on most wheel bearings on late model vehicles.
When this is there, well, when the bearing has in play, run out, or other issues, that tone ring can become unseated and it changes the air gap for the sensor. When the air gap changes for the sensor, this changes the signal or wave form coming back to the hydraulic control unit for the ABS. If it cannot get a consistent signal, it’s going to set that code.
Does it really matter? It’s just a land one wheel? Yes, because those systems actually check the four wheels, and if it sees behavior that is erratic or not normal, it sets that code. So how do you test to see if it actually is the hub unit? If you’re dealing with an active wheel speed sensor, it means that this seal actually has magnets that alter polarity going back and forth, back and forth, so it’s able to sense those magnets going by the wheel speed sensor head. When this happens, it registers the speed of the wheel or a degree of movement.
If you’re dealing with an older style passive wheel speed sensor, in those cases, you’re actually dealing with AC voltage being generated by a magnetic wheel speed sensor and window pans inside of something around the axle or part of the hub itself. In those cases, that sensor does not become active until the vehicle reaches five to seven miles per hour. But active is what you’re going to be dealing with on a lot of late model vehicles.
How do you test this? Well, you can use a scope. You can also use your scan tool to look at the different speeds and compare all four wheel speeds to see if something drops out when you’re making a corner or a turn, and this will confirm that there might be an issue with the hub unit. After that, perform a visual inspection. Take the wheel and make sure that there’s no in play. Then if possible, and you may have to remove the axle, look at the hub unit on the vehicle when it’s attached to the knuckle. If you see large pieces of debris stuck to it, try cleaning it and then going for a test drive. But if this seal is broken out of the lip, it needs to be replaced. There’s no way you can service just the seal.
Just keep that in mind when you’re dealing with one of these vehicles and a customer complaint that their ABS light is on and an erratic or intermittent wheel speed sensor code. I’m Andrew Markel. Thank you very much.
This video is sponsored by BCA Bearings by NTN