Auto Service Professional

APR 2018

Magazine for the auto service professional

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Page 47 of 61

46 | ASP April 2018 Technical Measure rotor disc thickness at eight equidistant points along the rotor (12 o'clock, at 45 degrees, 90 degrees, 135 degrees, 180 degrees, 225 degrees, 270 degrees and 315 degrees). Record all measurements. e total thickness variation equals the maximum thickness minus the minimum thickness. If total variation is beyond the specification for that vehicle, the rotor may be resurfaced (only using an on-the-car caliper mounted lathe), or replaced. We've addressed this issue many times before, but it always bears repeating. A pulsating/bouncing brake pedal is likely due to a warped brake rotor or excessive lateral runout. is causes the brake pads to move in and out of their bores as they contact the uneven rotor surface. Checking for runout is simple. First make sure that the brake rotor is fully secured to the hub using all of that wheel position's fasteners, and torque all fasteners to the specification listed for the wheel mounting. You must simulate the installed wheel condi- tion in order to achieve an accurate runout reading. Mount a dial indicator (clamp-on or magnetic base style) and place the indicator plunger 90 degrees to the rotor surface, about 1/2-inch from the rotor outer edge. Adjust the indicator with about 0.050-inch preload, and then zero the gauge. Slowly rotate the rotor/hub a full 360 degrees and note runout. If more than about 0.0025-inch runout is found, correction is needed. Before assuming that you need to replace a warped rotor, place matchmarks at one of the wheel fastener locations (mark the stud and that fastener location on the rotor hat). Remove the rotor and re-install at about a 45-degree clock posi- tion and re-check runout. You may be able to minimize/correct the runout by changing the clock position of the rotor relative to the hub. Wheel fastener torque can affect rotor runout. Excessive and/or uneven applied fastener torque, especially with regard to a thin-hat rotor design, can easily distort the rotor. While shop time is valuable, the use of an impact wrench is not recommended for wheel installation. Especially in the case of alloy wheels and thin-hat brake rotors, use only a calibrated torque wrench, and always follow the proper tightening sequence in order to evenly spread the clamping load across the hub face. Another possible cause of a brake "pulse" may be the result of front wheel hub service on a vehicle equipped with ABS, when only If the vehicle is equipped with ABS, the OE system bleeding steps must be followed to ensure that the ABS module functions properly. If in doubt, always refer to the vehicle's service manual. ABS wheel speed sensors pick up wheel speed signals from a tone ring that is built into the hub or CV outer joint. A specific air gap exists between the tone ring and sensor. If the tone ring is part of the hub and only one hub is replaced during service, the new hub will feature a tight per-spec air gap, while the opposite hub may be worn resulting in excessive air gap. If one front hub needs to be replaced, it's best to replace both. A differential in the signals can cause a false ABS light activation. Rust/debris buildup at the air gap location also can cause issues.

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