Advanced Wheel Bearing Diagnostics

Advanced Wheel Bearing Diagnostics

Can a bump set a wheel speed sensor code?

A fellow employee contacted me with a diagnostic conundrum recently. When he was driving to work, the ABS and ESC light would come on just a quarter-mile from his house, and a message in the driver information center told him that these critical safety systems were not available. It did not matter the time of day or the weather. 

Was it a self-check gone wrong? No. The ABS system did a self-check during start-up and at 7 mph. The ABS system passed those checks when I drove. 

When scanned, the ABS had a C1011 code for a left front wheel speed sensor for erratic performance. On my test drive, I could not replicate the condition.

The day after my test drive, he called me from our office parking lot saying the lights were on again and that I should grab my scan tool. Sure enough, code C1011 was set. I cleared the code and went for a test drive. I could not get the code to come back on and the data from the left front was not erratic and matched the right front in a straight line. Was this guy crazy? 

As it turns out, no, he’s fine. He just manages to hit the same bump every day.

I asked more about when the light comes on. He said that it came on right after he drove through an intersection with a big bump caused by uneven concrete blocks before the freeway. 

On the lift, the wheel bearing hub unit and ABS wheel speed sensor was secure with no damage. However, when the wheel was turned, we heard a grinding noise coming from the wheel bearing.

The hub unit had the reluctor ring embedded in the inboard seal. The wheel speed sensor was mounted in the knuckle. There was enough play and/or wear in the bearing that the air gap would change when he hit the bump. This change in air gap caused enough of a change in the signal to set the code. 

Mechanical Diagnosis

On a typical passenger vehicle weighing around 3,400 pounds, each pair of front-wheel bearings, as well as the rear-wheel or axle bearings, support around 850 pounds, depending on the weight balance and driveline configuration. If it’s a 6,000-pound SUV, each bearing might carry about 1,500 pounds. This load is concentrated on the relatively small bearing surfaces. To understand why active wheel speed sensors fail to report the correct speed of a wheel, it is necessary to understand what happens inside the bearing and the rolling elements, and how they change the relationship of the encoder ring and the sensor.

Wheel bearing failures are event-related. Curb strikes, potholes and other incidents that may cause damage to the bearing surfaces and seals are what kill bearings, rather than an abundance of miles.

Back in the day, experiencing some play when tugging at 12 o’clock and 6 o’clock was normal. On most modern vehicles, it would take the world’s strongest man to feel any play.

Nowadays, bearing play is best measured by placing a dial indicator against the hub and turning the wheel. Refer to the vehicle manufacturer’s specifications, but as a rule, no more than 0.005˝ of play is allowed for most sealed wheel bearing and hub assemblies. This is due to the air gap tolerances between the encoder ring and sensor.

Most bearing components are heat-treated to harden the metal. But this heat-treating can penetrate only so far into the metal. Once the bearing has worn through the treated layer, rapid and catastrophic wear occurs to the softer metal below. This type of fatigue failure is called “spalling,” and the damage causes the metal to come off in flakes. The material loss of the races and rolling elements creates play and noise. When the bearing play is great enough, the seal attached to the inner race becomes dislodged from the groove in the outer race. The movement damages the seal and the encoder ring that is embedded in the seal. 

After making our inspection and diagnosis, we turned to service on my co-worker’s car. The hub unit was replaced. The next day, he was in my office, happily announcing that this was the first time in weeks he had ABS and ESC the entire drive to work.

You May Also Like

Transmissions – CVT

Knowing what the common issues are & understanding the options available to isolate & fix problems are the keys to success.

All transmission models have unique problems that are common to that model; JATCO CVT units are no exception. Technicians are facing an uphill battle when it comes to diagnosing and repairing CVTs if they don’t have the right information on hand. Knowing what the common issues are and understanding the options available to isolate and fix problems are the keys to a successful repair.

Simplifying AWD Systems

No matter the manufacturer, there is almost always an all-wheel drive (AWD) option.

Manual Transmission Service

Parts that wear out must be replaced.

Transmission Fluid Hydraulics

You need to know how transmission fluid flows inside an automatic transmission.

How Well Do You Know Your Driveshaft?

There are three types of modern driveshafts with multiple configurations.

Other Posts

Live Axle Wheel Bearing Service

Replacing rear wheel bearings on a live axle rear suspension requires a few extra steps when compared to a unitized bearing.

Threadlocking Compounds For Vehicle Corners

To get the most out of these “liquid” tools, you first need to know how they work.

Tesla Wheel Bearing Replacement

The process of replacing the hub unit on a Tesla is the same as many cars and light trucks.

Steering Angle Sensor Service

Ninety percent of the time when a steering angle sensor code is active, it means the sensor needs to be calibrated.