Auto Service Professional

FEB 2016

Magazine for the auto service professional

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Page 36 of 67

39 | February 2016 sensors (WSS) side to side. The problem is usually associated with air gap difference or wiring and/or connector integrity. In many cases, removing the WSS from the other side, if possible, and cleaning the mounting surface may repair the problem. The rust build-up actually lifts the WSS from the bearing, increasing the air gap and weakening the signal. Another pos- sible issue is play in the bearing causing sine wave frequency change and or AC voltage variation. The new bearing will have little to no play, while the remain- ing hub has acceptable play but can still affect signal strength. Again, the differ- ence in signal from side to side may be enough to trigger false activation. If WSS is integral and not serviceable, replacing the hub bearings in pairs may be the only answer. The ABS system is activating as designed, so no warning light will be illu- minated in most cases. At least discuss this with the customer to prevent unnecessary surprise repairs in the future and prevent the dreaded "It never did this before you worked on it" conversation. In some cases, the issue may not be caused by an air gap concern. Citing cer- tain 1996-2005 Audi models as an example, the ABS light may be on under light brak- ing when the wheels are not locking up. Check for damaged wheel speed sensors or mismatched tires. Out of specifcation wheel speed sensor air gaps are not the likely issue. The most likely problem will be tiny cracks in the ABS tone rings found at the outer CV joint, or inside the rotor or wheel hub. The best way to identify a crack or dam- aged tone ring is to use a lab scope. With the scope attached to the sensor, turn the ABS ring. A normal wheel speed sensor will show a smooth, round, uninterrupted sine wave that increases in amplitude and frequency as the wheel speed increases. If a crack in the tone ring is present, a notch or a fat spot on the wave form will be present. If a scope isn't available, clean the tone ring and inspect carefully with a bright light. Cracks normally form at the base of the teeth. ‚óŹ The Chassis Uneven pad wear, where the pads on the same axle show different rates of wear, indicate a sticking caliper, where the caliper is not able to respond and return properly, causing one pad to wear more quickly. Pad wear should be viewed as a diagnostic ele- ment. Instead of blaming the pads, look for the cause of the uneven wear. Tapered pad wear indicates caliper misalignment and is not to be blamed on the pads. Closely inspect the caliper mounting to check for a bent bracket, debris or faulty hardware that causes the pad to be cocked and not parallel to the disc surface. Photo courtesy of NAPA

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