When engineers came up with the idea of putting the wheel speed sensor (WSS) and tone ring inside a sealed wheel bearing hub assembly, it sounded like a great idea. The sealed environment would help protect the sensor and make it less vulnerable to damage or contamination from the outside world.
The first applications date back to 1999 on Chevy/GMC and Ford trucks. Since then, the sealed wheel bearing hub assemblies with the integral ABS wheel-speed sensors have been used on a growing number of other cars and trucks.
But, there have been some problems. In late 2004 and early 2005, GM in Canada had to recall 1999-2002 model year Chevrolet Silverado, Tahoe, Suburban and Avalanche trucks, GMC Sierra and Yukon, and Cadillac Escalade models because of problems with the WSS in the sealed hubs.
Unlike other WSS setups where the tone ring is on the outside of the outer CV joint housing, axle or hub, the WSS tone ring is built into the wheel bearing assembly. On the later applications, the WSS tone ring is exposed and very vulnerable to corrosion from road salt and road splash.
The tone ring also can be damaged if somebody uses a pry bar to separate a halfshaft from the steering knuckle, or drops the halfshaft on the floor. A difference of only a few thousandths of an inch in the height of the teeth on the tone ring can affect the WSS signal.
The signal pulse is generated when a tooth passes under the magnetic tip of the sensor. The tooth passing through the sensor’s magnetic field causes the sensor to act like a little generator and produce an alternating current signal that increases in frequency and amplitude in direct proportion to wheel speed. If you look at the signal on an oscilloscope, it should look like a nice, even sine wave with all the up and down humps in the waveform evenly spaced and at the same height.
Magnetic sensors also can pick up metallic wear debris from the rotors and pads, causing the sensors to read erratically and confuse the anti-lock brake system. Cleaning the sensors and resetting the air gap often can restore normal operation. But, if the tone ring is corroded or damaged, it must be replaced.
In the case of the sealed hub assemblies with the WSS and tone ring inside, the hub provides protection against external corrosion – or at least it is supposed to. But nothing is perfect, and when moisture seeps into the hub, it corrodes the tone ring. The result is an uneven WSS signal that confuses the ABS system.
The fix was to pull out the sensor, clean the sensor and tone ring, and dump some zinc anti-corrosion treatment into the hub to prevent further corrosion.
The first signs of trouble may be the ABS system kicking in when braking at low speed, and/or the ABS warning light coming on. Until you hook up a scan tool and pull the codes, though, there’s no telling why the light is on. The fault may be a bad WSS or it might be something else, so don’t jump to conclusions – especially if there are no codes to guide you.
Reading ABS codes requires an ABS code reader, scan tool or scanner software that can access the ABS system. An inexpensive OBD II code reader or an entry-level scan tool designed for a do-it-yourselfer won’t work here. You need a professional tool designed for ABS diagnostics, or a digital storage oscilloscope to look at the WSS waveforms.
If the ABS light is on and you find a code for a WSS, check the sensor wires for breaks or a loose/corroded connector. Broken wires probably are the leading cause of WSS-related failures.
On some vehicles, the wires tend to be brittle and break as a result of fatigue from road vibration and/or steering maneuvers. Replacing the WSS wiring harness usually is the recommended fix for these situations. You could try to patch the broken wire, but crimp connectors are vulnerable to road splash and corrosion, and solder is usually too rigid and will crack again. Better to replace the wiring harness than to risk a comeback.
If there are no codes, but the vehicle owner complains about the ABS system engaging when braking (noise, vibrations and pedal pulsations), the problem is likely a bad WSS. But which one? That’s where a scope can really help you identify which sensor is acting up. Connect the scope to the sensor leads and spin the tire by hand. If you get a good, clean signal, move on to the next WSS and so on, until you find the one that is generating a bad signal.
You also can measure bearing play by placing a dial indicator against the hub and turning the wheel. Refer to the vehicle manufacturers’ specifications but, as a rule, no more than 0.005 inches of play is allowed for most sealed wheel bearing and hub assemblies.
If one wheel bearing is loose or noisy, pay close attention to all of the other hubs on the vehicle, especially if the vehicle has a lot of miles on it, has been driven through axle-deep water, or has been flooded. Chances are some of the other wheel bearings also may be going bad.
You need to make sure you have diagnosed the fault correctly on vehicles with the sealed hub and ABS WSS because the replacement parts are not cheap.
Hub replacement is fairly simple. Remove the wheel, caliper and rotor, then remove the wheel bearing and hub assembly from the spindle or axle. Do not use an impact wrench for removal or installation. Use a torque wrench and tighten all bolts and nuts to specification.
Many hub units for FWD applications come with a new hub nut. Use it, and be sure to torque it to specification with a torque wrench – never an impact wrench.
After repairs have been made, confirm the problem has been fixed by clearing any ABS codes and doing a short test drive to make sure the ABS light does not come on and there is no bearing noise.
This article appeared in the December 2012 edition of Tire Review. You can read the entire issue on your phone or tablet by downloading the Tire Review app.
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