Modern Tire Dealer

APR 2019

Magazine for the professional tire industry

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Page 37 of 85

B r a k e s M T D A p r i l 2 0 1 9 36 is could cause the driver to feel ABS false activation when coming to a slow stop on dry pavement. False activation is usually described as a pulse in the brake pedal when not expected. e pulsation comes from the ABS valves cycling the supposedly locked-up wheel. is is due to the difference in signal strength from the wheel speed sensors (WSS) side to side. e problem is usually associated with air gap difference or wiring and/or connec- tor integrity. In many cases, removing the WSS from the other side, if possible, and cleaning the mounting surface may repair the problem. e rust buildup actually lis the WSS from the bearing, increasing the air gap and weakening the signal. Another possible issue is play in the bearing causing sine wave frequency change and or AC voltage variation. A cracked ABS tone ring (located on the outer CV joint, on the hub or inside the rotor, depending on design) can also cause the ABS warning light to illuminate due to small variations in the signal caused by the crack(s). PAD BEDDING/BREAK-IN As the pads mate with the rotor disc surface, and depending on the type of pad formula, a small amount of friction material is trans- ferred to the disc surface, which increases braking efficiency. If the replacement pads differ from the original pads (in terms of friction material formulation), and are to be used with the original rotors (when changing pads but not the rotors), an initial break-in is recommended. Drive the vehicle through a few moder- ate braking exercises in order for the new pads to "wipe" the disc surfaces of the previous embedded friction material. en continue to drive with moderate braking (at speeds of around 35 mph to 40 mph) in order to "bed" the new pads to the disc surfaces. However, some brake pad manufacturers note that no bedding- in procedure is necessary with certain pads, so don't make assumptions. Read the instructions (if provided) that came with the new pads. Regardless of the type of pad formula, it's still not a bad idea to perform a few moderate braking procedures before the need to perform more extreme braking, if for no other reason than to "mate" the pads to the disc surfaces. A note regarding rotor disc surface preparation is applicable when discussing the installation of any new set of brake pads. e rotor disc surfaces should be clean. at does not mean simply wiping the surfaces down with a rag. In order to remove residue (dirt and potential oils/grease from used rotors, and packing/shipping rust inhibitors on new rotors), the disc surfaces should be washed and scrubbed thoroughly with a fast-drying solvent, and if time permits, this should be followed up with a hot scrubbing bath with a detergent (Dawn dish washing detergent is a favorite). A hot soapy bath followed by a thorough hot rinse will aid in removing any contami- nants that the solvent didn't remove. Also, if you do treat the rotors to a hot bath and hot rinse, it's a good idea to immediately follow this with a cold water rinse (this will reduce the chance of light surface rusting on the bare metal), followed by blow-drying with compressed air. EPB SERVICE Servicing a vehicle equipped with EPB (electronic parking brake) presents its own challenges. In order to retract caliper pistons prior to caliper removal, specific steps are required to avoid problems during both caliper removal and installation. Using a scan tool is typically going to be the manufacturer's recommended method to retract the calipers to their service position to allow rear brake service or pad replace- ment. e scan tool is then used again to exit the service mode and automatically adjust the pads to the correct clearance aer the rear brake service is performed. Refer to the February 2018 issue of MTD's sister publication Auto Service Professional (find it on www.autoservi- for a technical article by Jeff Taylor that provides detailed look at EPB systems. BRAKE LIGHT SWITCH While it won't affect braking performance, a faulty brake light switch will result in the loss of brake light activation. If a no-brake-light concern is found, naturally you'll inspect bulbs and fuses. However, if the brake light switch requires replacement, be aware that in some models (Chryslers, for example) feature a one-time-use switch. is type of switch is designed to set its adjustment during initial installation. If installation steps are not followed properly, or if a previously functioning switch is removed, it must be discarded and replaced. When dealing with this type of switch, don't be in a rush. Take your time to perform the proper installation steps. Otherwise you'll waste time and money by buying and installing additional switches. ese switches are generally labeled with a warn- ing ("Do not re-use", etc.). ■ Whenever dealing with brake system connections, never use an open-ended wrench. Always use a dedicated line wrench to avoid damaging/deforming connection fitting hexes. Check rotor disc thickness with a micrometer. This allows you to deter- mine of the rotor has worn beyond the recommended minimum thickness limit. If brake vibration (pedal pulsa- tion) is a concern, in addition to check- ing rotor lateral runout, a check of rotor thickness for variations can help to determine rotor condition. Measure for thickness at a minimum of four evenly spaced clock locations. Especially where alloy wheels are used, apply a thin film of a high-heat anti- seize paste to the hub surface to reduce the chance of dissimilar metals mated together sticking together (electrolysis), making future wheel removal difficult.

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