chap77 pps Automotive technology at University of Cambridge

start Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 77, the reader should be able to: • • • • • Prepare for ASE Brakes (A5) certification test content area “C” (Disc Brake Diagnosis and Repair) List the items that should be checked during a visual inspection Describe the caliper disassembly procedure Describe the caliper assembly procedure List the steps necessary to reduce brake noise Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 KEY TERMS: abutments • antiseize compound bedded-in • brake assembly fluid • burnished constrained layer shims (CLS) lithium-based brake grease • loaded calipers minimum thickness • MOS2 (molybdenum disulfide) grease Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 KEY TERMS: polyalphaolefin (PAO) • phenolic caliper pistons reaction pads silicone grease • synthetic grease ways Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 DISC BRAKE DIAGNOSTIC PROCEDURE When diagnosing disc brake concerns, the first step is to verify  the customer complaint. This usually includes test driving the  vehicle to see if the complaint can be duplicated. If the problem  cannot be duplicated, then the repair or service cannot be verified CAUTION: Do not test­drive the vehicle on public roads if the red  brake warning light is on After verifying the customer concern, the brake system should be  carefully inspected and all parts and systems checked for proper  operation.  Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Step #1  Check the brake pedal height and verify proper operation.  If the brake pedal is low, check the parking brake and count the  number of “clicks.” There should be 3 to 7 clicks. If there are over  10 clicks, check the rear brakes Step #2  Safely hoist the vehicle and remove the wheels. Visually  check the following: • Flexible brake lines for wear or damage • Disc brake rotors for excessive rust or scoring • Disc brake calipers for leakage or damage Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Step #3  Remove disc brake calipers and check the disc brakepads  for proper lining thickness and check for cracks or other damage Step #4  Replace all components that do not meet factory specs Step #5  Test­drive the vehicle to verify that the repairs did correct  the customer concern Let The Owner Drive When verifying the customer complaint, ask the owner or driver of the vehicle to drive Often, the problem is best discovered if the vehicle is being driven exactly the same way as when the complaint first occurred For example, the tech may brake harder or softer than the driver so the problem may not be detected Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 VISUAL INSPECTION Even with operating wear­indicating sensors, a visual inspection  is very important.  Figure 77–1 Minimum thickness for various types of disc brake pads Disc brake pads can, of course, be replaced before they wear down to the factory-recommended minimum thickness Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 A lining thickness check alone should not be the only inspection  performed on a disc brake. A thorough visual inspection can only  be accomplished by removing the friction pads Figure 77–2 This cracked disc brake pad must be replaced even though it is thicker than the minimum allowed by the vehicle manufacturer Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 NOTE: Some disc brake pads use a heat barrier (thermo) layer between  the steel backing plate and the friction material. The purpose of the heat  barrier is to prevent heat from transferring into the caliper piston where it may cause the brake fluid to boil. Do not confuse the thickness of the  barrier as part of the thickness of the friction lining material. The barrier material is usually a different color and usually can be distinguished from the lining material Some disc brake pads may show more wear on the end of the pad  that first contacts the rotor as compared to the pad trailing end This wear is caused by force between the pad and the abutment  (slide area). In designs that place the caliper piston exactly in the  center of the leading edge of the pad that first contacts the rotor as  it is revolving through the caliper, pressures are often one­third  higher than the average pressure exerted on the entire pad. The  result of this is greater wear.  Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 DISC BRAKE SYMPTOM GUIDE Pulls to One Side during Braking  Incorrect or unequal tire pressures Front end out of alignment  Unmatched tires on the same axle  Restricted brake lines or hoses  Stuck or seized caliper or caliper piston Defective or damaged shoe and lining (grease or brake fluid on the lining, or a bent shoe) Malfunctioning rear brakes Loose suspension parts Loose calipers Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Brake Roughness or Chatter (Pedal Pulsates)  Excessive lateral runout of rotor Parallelism of the rotor not within specifications Wheel bearings not adjusted correctly Rear drums out­of­round Brake pads worn to metal backing plate Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Excessive Pedal Effort  Binding or seized caliper suspension Binding brake pedal mechanism Improper rotor surface finish Malfunctioning power brake Partial system failure Excessively worn shoe and lining Piston in the caliper stuck or sluggish Fading brakes due to incorrect lining Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Excessive Pedal Travel  Partial brake system failure Insufficient fluid in the master cylinder Air trapped in the system Bent shoe and lining Excessive pedal effort Excessive parking brake travel (four­wheel disc brakes, except Corvette) Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Dragging Brakes  Pressure trapped in the brake lines (to diagnose,  momentarily open the caliper bleeder valve to relieve  the pressure) Restricted brake tubes or hoses Improperly lubricated caliper suspension system Improper clearance between the caliper and torque  abutment surfaces Check valve installed in the outlet of the master  cylinder to the disc brakes Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Front Disc Brakes Very Sensitive to Light Brake Applications  Metering valve not holding off the front brake  application Incorrect lining material Improper rotor surface finish Check other causes listed under “PULLS” Rear Drum Brakes Skidding under Hard Brake Applications  Proportioning valve Contaminated rear brake lining Caliper or caliper piston stuck or corroded  Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 The Screwdriver Trick - Part A low brake pedal on GM vehicles equipped with rear disc brakes is a common customer complaint Often the reason is a lack of self-adjustment that should occur whenever the brake pedal (or parking brake) is released During brake release, the pressure is removed from the caliper piston and the spring inside the caliper piston is free to adjust Often this selfadjustment does not occur and a low brake pedal results A common trick used on the vehicle assembly line is to use a screwdriver to hold the piston against the rotor while an assistant releases the brake pedal As the brake pedal is released, the adjusting screw inside the caliper piston is free to move Sometimes, it may be necessary to tap on the caliper itself with a deadblow hammer to free the adjusting screw Repeat as necessary until the proper brake pedal height returns If this method does not work, replace the caliper assembly Step #1 Have an assistant depress the brake pedal Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 … Trick - Part Step #2 With a screwdriver through the hole in the top of the caliper, hold the piston against the rotor Step #3 While still holding the piston against the rotor, have the assistant release the brake pedal The adjusting screw adjusts when the brake pedal is released and a slight vibration or sound will be noticed as the brake is released This vibration or sound is created by the self-adjusting mechanism inside the caliper piston taking up the excess clearance Step #4 Repeat as until normal brake pedal height is achieved Figure 77–33 The screwdriver blade is used to keep the piston applied to allow self adjustment to occur when the brake pedal is released Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Pressure Testing Can Help Find Problems A stuck caliper or caliper slide is often difficult to see or diagnose as a problem because the movement of the broken pads is so little Using a pressure gauge between the caliper piston and the rotor (inboard) or between the rotor and the caliper (outboard) can tell the service tech if there is a difference between the left and the right side brakes (a) (b) Figure 77–34 (a) A brake pressure tester (b) The small “pads” can be placed between the caliper piston and the rotor to check for applied pressure and inserted between the caliper and the rotor on the outside of the rotor to test the pressure—the pressure should be the same if the caliper is able to slide on its pins or slides Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 PHOTO SEQUENCE Disc Brake Service Step-by-Step Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 PHOTO SEQUENCE Disc Brake Service (cont.) Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 PHOTO SEQUENCE Disc Brake Service Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman (cont.) ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY Caliper pistons are either chrome­plated steel or plastic  (phenolic). Any damaged piston must be replaced. Both the  square­cut O­ring and the dust boot must be replaced when the  caliper is disassembled All metal­to­metal contact points of the disc brake assembly  should be coated with an approved brake lubricant such as  synthetic grease, “moly” (molybdenum disulfide) grease, or  antiseize compound After a brake overhaul, the brake pedal should be depressed  several times until a normal brake pedal is achieved before  performing a thorough test­drive Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY (cont.) Many rear disc brake systems use an integral parking brake.  Regular use of the parking brake helps maintain proper rear  brake clearance Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 end Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458