chap78 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 78, the reader should be able to: • Prepare for ASE Brakes (A5) certification test content area “E” (Miscellaneous Systems Diagnosis and Repair) • • Describe what is required of a parking brake Describe the parts and operation of the parking brake as used on a rear drum brake system 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 OBJECTIVES: After studying Chapter 78, the reader should be able to: • Describe how a parking brake functions when the vehicle is equipped with rear disc brakes • Explain how to adjust a parking brake properly 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: application cables control cables electric parking brake (EPB) • equalizer intermediate lever red brake warning lamp vacuum servo 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 PARKING BRAKE Before 1967, most vehicles had only a single master cylinder operating all four brakes If fluid leaked at just one wheel, the operation of all brakes was lost This required a separate method to stop the vehicle in case of an emergency After 1967, federal regulations required use of dual or tandem master cylinders where half of the braking system has its own separate hydraulic system In case one-half of the system fails, a dash brake warning lamp lets the driver know that a failure has occurred The term parking brake has replaced the term emergency brake since the change to dual master cylinder design 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 According to Federal Motor Vehicle Safety Standard (FMVSS) 135, the parking brake must hold a fully loaded (laden) vehicle stationary on a slope of 20% up or down grade The hand force required cannot exceed 80 lb (18 N) or a foot force greater than 100 lb (22 N) See Figure 78–1 for a typical parking brake system 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 Figure 78–1 Typical parking brake cable system showing the foot-operated parking brake lever and cable routing 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 PEDALS, LEVERS AND HANDLES Parking brakes are applied by a pedal, a lever, or a handle from inside the vehicle Foot pedals and floor-mounted levers are the common means of applying parking brakes See Figure 78–3 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 Figure 78–3 Typical hand-operated parking brake Note that the adjustment for the cable is underneath the vehicle at the equalizer All parking brakes are applied manually and the release procedure varies with the design of the parking brake control 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 All parking brake controls incorporate a ratchet mechanism to lock the brake in the applied position Figure 78–4 A ratchet mechanism is used to lock parking brakes in the applied position Some vehicles were equipped with a system that required the driver to depress the parking brake pedal to release the parking brake once it was set The rubber pad on the parking brake pedal usually states “push to release.” 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 Figure 78–26 A piston installation tool is required to fully install the piston into a General Motors rear disc brake caliper 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 Figure 78–27 A spanner wrench (or needle-nose pliers) can be used to rotate the caliper piston prior to installing the disc brake pads A notch on the piston must line up with a tab on the back of the brake pad to keep the piston from rotating when the parking brake is applied Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Figure 78–28 After removing parking brake lever and thrust bearing, remove the antirotation pin ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 78–29 Unscrew the thrust screw from the piston with an Allen (hex) wrench After removing the thrust screw, push the piston out of the caliper bore Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Figure 78–30 To test the piston adjuster, thread the thrust screw into the piston Hold the piston and pull the thrust screw outward 1/4 in (6 mm) The adjuster nut should not turn when the thrust screw retracts Replace the piston assembly if not functioning correctly ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 78–31 To adjust the parking brake cable on a Ford vehicle equipped with rear disc brakes, start by loosening the cable adjustment until the cables to the calipers are slack Tighten until the caliper lever moves Position a 1/4-in drill bit or dowel into the caliper alignment hole Adjustment is correct if the parking brake lever does not hit the 1/4-in dowel 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 Parking Brake Click Test When diagnosing any brake problem, apply the parking brake and count the “clicks.” This method works for both hand- and foot-operated parking brakes Most vehicle manufacturers specify a maximum of 10 clicks If the parking brake travel exceeds this amount, the rear brakes may be worn or out of adjustment If the rear brake lining is usable, check for the proper operation of the selfadjustment mechanism If the rear brakes are out of adjustment, the service brake pedal will also be low This 10-click test is a fast and easy way to determine if the problem is due to rear brakes CAUTION: Do not adjust the parking brake cable until the rear brakes have been thoroughly inspected and adjusted 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 PARKING BRAKE CABLE ADJUSTMENT Most manufacturers specify a minimum of or 4, and a maximum of to 10, clicks when applying the parking brake Most vehicle manufacturers specify that the rear brakes be inspected and adjusted correctly before attempting to adjust the parking brake cable Always follow the manufacturer’s procedure exactly General procedure for parking brake adjustment: Make certain that the rear service brakes are adjusted correctly and the lining is serviceable With the drums installed, apply the parking brake or clicks There should be a slight drag on both rear wheels 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 Adjust the cable at the equalizer (equalizes one cable’s force to both rear brakes) if necessary until there is a slight drag on both rear brakes Figure 78–32 After checking that rear brakes are OK and properly adjusted, parking brake cable can be adjusted Always follow manufacturer’s recommended procedure See Figure 78–33 Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Release the parking brake Both rear brakes should be free and not dragging Repair or replace rusted cables or readjust as necessary to ensure that the brakes are not dragging Figure 78–33 Many hand-operated parking brakes are adjusted inside the vehicle NOTE: Some vehicles are equipped with an automatic adjusting parking brake lever/cable Simply cycling the parking brake on/off/on three times is often all that is required to adjust the parking brake cable 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: The rear parking brake adjustment should always be checked whenever replacing the rear brake linings It may be necessary to loosen the parking brake cable adjustment to allow clearance to get the drum over the new linings This could happen because someone may have adjusted the parking brake cable during the life of the rear linings Figure 78–34 Always check that the brake shoes contact the anchor pin 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 To prevent possible parking brake cable adjustment problems when installing new rear brakes, always observe the following: Both brake shoes should make contact with the anchor pin at the top If not, check the parking brake cable for improper adjustment or improper installation of the brake shoes Feel the tension of the parking brake cable under the vehicle It should be slightly loose (with the parking brake “off”) Lubricate the parking brake cable to ensure that water or ice will not cause rust or freezing of the cable This is necessary because even though the parking brake lever is released inside the vehicle, a stuck parking brake cable could cause the linings to remain out against the drums 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 If the parking brake needs to be adjusted (will not hold on a hill or requires excessive lever movement), always check and adjust the rear brake adjustment before adjusting the parking brake cable Figure 78–35 A 1/8-in (3-mm) drill bit is placed through an access hole in the backing plate to adjust this General Motors leadingtrailing rear parking brake Adjust the parking brake cable until the drill can just fit between the shoe web and the parking brake lever Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 The Hose Clamp or Wrench Trick It is often difficult to remove a parking brake cable from the backing plate due to the design of the retainer The many fingers used to hold the cable to the backing plate can be squeezed all at once if a hose clamp is used to compress the fingers A wrench as shown here can also be used Figure 78–36 Many parking brake cables can be removed easily from the backing plate using a 1/2-in (13-mm) box-end wrench The wrench fits over the retainer finger on the end of the parking brake cable 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 How Does An Electric Parking Brake Work? Electric parking brake (EPB) systems are available using two different designs including: A cable-pulling type that uses an electric motor to pull the parking brake cable rather than a mechanical handle or foot pedal A more advanced unit uses a computer-controlled motor attached to the brake caliper to activate it Some vehicles use an electric parking brake (EPB) that can be activated when the vehicles stops and then goes off as soon as the gas pedal is pressed, preventing the car from moving while stopped Figure 78–37 An electric parking brake button on the center console of a Jaguar 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 Government regulation requires that the parking brake be able to hold a fully loaded vehicle on a 20% grade The typical parking brake uses either a hand-operated lever or a foot operated pedal to activate the parking brake On a typical drum brake system, the parking brake cable moves a parking brake lever attached to the secondary brake shoe The primary shoe is applied through force being transferred through the strut All parking brake cables should move freely The rear brakes should be adjusted properly before the parking brake is adjusted 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