chap70 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 70, the reader should be able to: • • • Prepare for ASE Brakes (A5) certification test content area “A” (Hydraulic System Diagnosis and Repair) Describe the operation of a residual check valve Explain how a proportioning valve works 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 70, the reader should be able to: • • • Discuss the need and use of a metering valve List testing procedures used to test hydraulic valves Describe how the brake fluid level and brake light switches work 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: brake fluid level sensor • brake light switch combination valve electronic brake proportioning • expander height-sensing proportioning valve metering valve pressure-differential switch • proportioning valve residual check valve • slope • split point 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 RESIDUAL CHECK VALVE A residual check valve has been used on some drum brake  systems to keep a slight amount of pressure on the entire  hydraulic system for drum brakes (5 to 12 psi).  The check valve is located in the master cylinder at the outlet for  the drum brakes.  The ball and spring in the  check valve permit all the  brake fluid to return to the  master cylinder until the  designated pressure is  reached Figure 70–1 Most residual check valves are located under the tubing seals in the master cylinder outlet ports 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 This slight pressure prevents air leaks from entering into the  hydraulic system in the event of a small hole or leak.  With a low pressure kept on the hydraulic system, any small hole  will cause fluid to leak out rather than permit air to enter the  system This slight pressure also keeps the wheel cylinder sealing cups  tight against the inside wall of the wheel cylinder See Figure 70–2 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 70–2 The momentary drop in pressure created when the brakes are released can draw air into the hydraulic 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 Residual check valves are often not used on late model vehicles  equipped with front disc/rear drum brakes The residual check valve has been eliminated by equipping the  wheel cylinder internal spring with a sealing cup expander to  prevent sealing cup lip collapse.  Figure 70–3 The use of cup expanders is the main reason why residual check valves are not used in most braking systems today 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-DIFFERENTIAL SWITCH (BRAKE WARNING SWITCH) A pressure­differential switch is used on all vehicles built after  1967 with dual master cylinders to warn the driver of a loss of  pressure in one of the two separate systems by lighting the  dashboard red brake warning indicator lamp.  The brake lines from both front  and rear sections of the master  cylinder are sent to this switch Figure 70–4 A red brake warning lamp It lights the brake warning  indicator lamp in the event of a “difference in pressure.” See Figure 70–5 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 70–5 A leak in the hydraulic system causes unequal pressures between the two different brake circuits This difference in pressures causes the plunger inside the pressure-differential switch to move, which completes the electrical circuit for the red brake warning lamp (Courtesy of Ford Motor Company) If the lamp remains on: Apply light pressure to the brake pedal Momentarily open the bleeder valve on the side that did not fail 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 70–19 A metering valve under light brake pedal application 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 small amount of pressure applied to the calipers before the  metering valve closes is enough to take up any clearance, but not enough to generate braking force While the fluid flow to the front calipers is shut off, rear brake  shoes move into contact with the drums, braking begins, and  hydraulic pressure throughout the brake system increases When the pressure at the metering valve reaches 75 to 300 psi,  tension of the strong spring is overcome and the valve stem and  piston move farther to the left This opens a passage around the outside of the piston and allows  fluid to flow through the valve to the front brake calipers See Figure 70–20 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 Figure 70–20 A metering valve during a normal brake application 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 No Valves Can Cause a Pull When diagnosing a pull to one side during braking, some technicians tend to blame the metering valve, proportional valve, the pressure differential switch, or the master cylinder itself Just remember that if a vehicle pulls during braking that the problem has to be due to an individual wheel brake or brake line The master cylinder and all the valves control front or rear brakes together or diagonal brakes and cannot cause a pull if not functioning correctly NOTE: Neither the metering valve nor the proportioning valve can cause  a pull to one side if defective. The metering valve controls both front  brakes, and the proportioning valve controls both rear brakes. A defective  master cylinder cannot cause a pull either. Therefore, if a vehicle pulls to  one side during a stop, look for problems in the individual wheel brakes,  hoses, or suspension 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 SYSTEMS WITHOUT METERING VALVES There are three reasons front­wheel­drive vehicles do not use  metering valves:  Front­wheel­drive vehicles usually have a more costly and  complicated diagonally split dual braking system that requires a separate metering valve for each hydraulic circuit.  Front­wheel­drive vehicles have a forward weight bias that  requires front brakes to supply up to 80% of the total braking  power. A metering valve would create a delay Until all the clearance in the brake system is taken up, there  will not be enough pressure in the brake hydraulic system for  the front disc brakes to overcome the engine torque applied to  the driven front 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 Engine torque and a heavy front weight bias help prevent front  wheel lockup from being a problem during light braking or when  the brakes are first applied Most rear­wheel­drive vehicles without metering valves are  equipped with four­wheel disc brakes. Because the clearance  between the pads and rotors is approximately the same at all four  wheels, there is no need to delay front brake actuation Some of these vehicles also have antilock brake systems that  prevent the wheels from locking at any time Other rear­wheel­drive vehicles without metering valves have a  predominantly forward weight bias, like front­wheel­drive  vehicles, and therefore benefit from having the front brakes applied  sooner 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 METERING VALVE DIAGNOSIS AND TESTING A defective metering valve can leak brake fluid and/or cause the  front brakes to apply before the rear brakes. Most noticed on  slippery surfaces such as on snow or ice or on rain­slick roads.  If the front brakes lock up during these conditions, the front  wheels cannot be steered. Inspect the metering valve for these   conditions: Look around the bottom of the metering valve for brake fluid  leakage. (Ignore slight dampness.) Replace the metering valve  assembly if it is leaking As the pressure builds to the front brakes, the metering valve  stem should move. If it does not, replace the valve More accurate testing of the metering valve can be accomplished  using pressure gauges.  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 Install two gauges, one in the pressure line from the master cylinder,  the other in the outlet line leading to the front brakes When pressing the brake pedal, both gauges should read the same  until 3 to 30 psi (20 to 200 kPa) when the metering valve shuts,  delaying operation of the front brakes. The master cylinder gauge  should show an increase in as the brake pedal is pressed further Once 75 to 300 psi is reached, the gauge showing pressure to the  front brakes should match the pressure from the master cylinder If the pressures do not match these ranges, the metering valve  assembly should be replaced NOTE: Braking systems that are diagonal split, such as those found on most front­wheel­drive vehicles, do not use a metering valve. A metering valve is only used on front/rear split braking systems such as those found on most rear­wheel­drive vehicles 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 Push-In or Pull-Out Metering Valve? Whenever bleeding the air out of the hydraulic brake system, the metering valve should be bypassed The metering valve stops the passage of brake fluid to the front wheels until pressure exceeds about 125 PSI (860 kPa) It is important not to push the brake pedal down with a great force so as to keep from dispersing any trapped air into small and hard-to-bleed bubbles To bypass the metering valve, the service technician has to push or pull a small button located on the metering valve An easy way to remember whether to push in or to pull out is to inspect the button itself If the button is rubber coated, then you push in If the button is steel, then pull out Special tools allow the metering valve to be held in the bypass position Failure to remove the tool after bleeding the brakes can result in premature application of the front brakes before the rear drum brakes have enough pressure to operate 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 COMBINATION VALVE Most manufacturers combine the  function of a proportioning valve  with one or more other valves  into one unit called a  combination valve.  On a rear­wheel­drive vehicle,  a typical combination valve  consists of these components,  all in one replaceable unit: Metering valve Proportioning valve Pressure­differential switch Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Figure 70–2 Typical two-function combination valves ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Some valves have two functions and contain pressure­differential and  metering valve; others combine the pressure­differential with the  proportioning valve Figure 70–22 Combination valve containing metering, pressure-differential (warning switch), and proportioning valves all in one unit This style is often called a “pistol grip” design because the valve section resembles the grip section of a hand gun 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 LIGHT SWITCH The brake light switch turns on brake lights at the back of the  vehicle when brakes are applied. A properly adjusted light switch  will activate brake lights as soon as the pedal is applied and  before braking action actually begins at the wheels Mechanical switches operating  directly off the brake pedal arm are most often used Brake light switches are normally  open; when brakes apply, the switch  closes, completing the light circuit Figure 70–23 Typical brake light switches CAUTION:  Check service information for procedures to follow when  replacing and/or adjusting a brake switch to ensure proper operation 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 Residual check valves are used in older vehicles to keep a slight  amount of pressure on the system to help prevent air from  entering the system when the brake pedal is released A pressure­differential switch is used to turn on the red brake  warning lamp in the event of a hydraulic pressure failure Brake fluid level sensors are used in many vehicles to warn the  driver that the brake fluid level is low Proportioning valves are used to limit the maximum fluid  pressure sent to the rear wheel brakes during heavy braking to  help prevent rear wheel lockup 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.) Metering valves are used on some vehicles to keep the front disc  brakes from locking up on slippery surfaces Combination valves include two or more hydraulic valves in  one assembly 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 ... Many Japanese vehicles energize the relay that turns off the red “brake” warning lamp from the output terminal? ?of? ?the alternator. If a quick inspection? ?of? ?the brake system seems to indicate that everything is okay, check for correct charging voltage before continuing a more detailed... creating a pressure difference? ?of? ?at? ?least 150 psi) Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc... 1967 with dual master cylinders to warn the driver? ?of? ?a loss? ?of? ? pressure in one? ?of? ?the two separate systems by lighting the  dashboard red brake warning indicator lamp.  The brake lines from both front  and rear sections? ?of? ?the master