Halderman ch095 lecture

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  • Figure 95-1 Most residual check valves are located under the tubing seats in the master cylinder outlet ports.
  • Figure 95-2 The momentary drop in pressure created when the brakes are released can draw air into the hydraulic system.
  • Figure 95-3 The use of cup expanders is the main reason why residual check valves are not used in most braking systems today.
  • Figure 95-4 A red brake warning lamp.
  • Figure 95-5 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 ground circuit for the red brake warning lamp.
  • Figure 95-6 The pressure-differential switch piston is used to provide the electrical ground for the red brake warning light circuit.
  • Figure 95-7 A movable contact brake fluid level switch. When the brake fluid level and float drop, the rod-mounted contact completes the electrical circuit which turns on the red brake warning lamp.
  • Figure 95-8 A magnetic brake fluid level switch.
  • Figure 95-9 Many proportioning valves are mounted directly to the master cylinder in the outlet to the rear brakes.
  • Figure 95-10 Typical proportioning valve pressure relationship. Note that, at low pressures, the pressure is the same to the rear brakes as is applied to the front brakes. After the split point, only a percentage (called the slope) of the master cylinder pressure is applied to the rear brakes.
  • Figure 95-11 A Chrysler proportioning valve. Note that slope and split point are stamped on the housing.
  • Figure 95-12 These two proportioning valves are found under the vehicle on this Dodge minivan.
  • Figure 95-13 The proportioning valve piston can travel within the range shown without reducing pressure to the rear brakes.
  • Figure 95-14 At the split point, the proportioning valve piston closes the fluid passage through the valve.
  • Figure 95-15 A height-sensing proportioning valve provides the vehicle with variable brake balance. The valve allows higher pressure to be applied to the rear brakes when the vehicle is heavily loaded and less pressure when the vehicle is lightly loaded.
  • Figure 95-16 A stepped cam is used to alter the split point of this height-sensing proportioning valve.
  • Figure 95-17 A proportioning valve pressure test can be performed using two pressure gauges—one to register the pressure from the master cylinder and the other gauge to read the pressure being applied to the rear brakes. This test has to be repeated in order to read the pressure to each rear wheel.
  • Figure 95-18 A metering valve when the brakes are not applied. Notice the brake fluid can flow through the metering valve to compensate for brake fluid expansion and contraction that occurs with changes in temperature.
  • Figure 95-19 A metering valve under light brake pedal application.
  • Figure 95-20 A metering valve during a normal brake application.
  • Figure 95-21 Typical two-function combination valves.
  • Figure 95-22 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 proportioning valve section resembles the grip section of a handgun.
  • Figure 95-23 Typical brake light switches.
  • Halderman ch095 lecture

    1. 1. HYDRAULIC VALVES AND SWITCHES 95
    2. 2. Objectives <ul><li>The student should be able to: </li></ul><ul><ul><li>Prepare for the Brakes (A5) ASE certification test content area “A” (Hydraulic System Diagnosis and Repair). </li></ul></ul><ul><ul><li>Describe the operation of a residual check valve. </li></ul></ul><ul><ul><li>Explain how a proportioning valve works. </li></ul></ul><ul><ul><li>Discuss the need and use of a metering valve. </li></ul></ul>
    3. 3. Objectives <ul><li>The student should be able to: </li></ul><ul><ul><li>List testing procedures used to test hydraulic valves. </li></ul></ul><ul><ul><li>Describe how the brake fluid level and brake light switches work. </li></ul></ul>
    4. 4. INTRODUCTION
    5. 5. Introduction <ul><li>The hydraulic system is the heart of the braking system </li></ul><ul><li>Using hydraulics is a very efficient way to transfer the force that the driver exerts on the brake pedal to apply the friction materials at the individual wheel brakes </li></ul><ul><li>Valves and switches need to be added to the system to meet all possible braking conditions </li></ul>
    6. 6. RESIDUAL CHECK VALVE
    7. 7. Residual Check Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>A residual check valve is used on some drum brake systems to keep a slight amount of pressure on the entire hydraulic system </li></ul></ul>
    8. 8. Residual Check Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>This valve is located in the master cylinder at the outlet for the drum brakes </li></ul></ul><ul><ul><li>A check ball and spring permit all the brake fluid to return to the master cylinder until the designated pressure is reached </li></ul></ul>
    9. 9. Residual Check Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>Slight pressure also keeps the wheel cylinder sealing cups tight against the inside wall of the wheel cylinder </li></ul></ul>
    10. 10. Residual Check Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>Residual check valves are not used often on late-model vehicles equipped with front disc/rear drum brakes </li></ul></ul><ul><ul><li>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 </li></ul></ul>
    11. 11. Figure 95-1 Most residual check valves are located under the tubing seats in the master cylinder outlet ports.
    12. 12. Figure 95-2 The momentary drop in pressure created when the brakes are released can draw air into the hydraulic system.
    13. 13. Figure 95-3 The use of cup expanders is the main reason why residual check valves are not used in most braking systems today.
    14. 14. PRESSURE-DIFFERENTIAL SWITCH (BRAKE WARNING SWITCH)
    15. 15. Pressure-Differential Switch (Brake Warning Switch) <ul><li>Purpose and Function </li></ul><ul><ul><li>A pressure-differential 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 </li></ul></ul>
    16. 16. Pressure-Differential Switch (Brake Warning Switch) <ul><li>Purpose and Function </li></ul><ul><ul><li>The brake lines from both sections of the master cylinder are sent to the switch, which lights the brake warning indicator lamp </li></ul></ul>
    17. 17. Pressure-Differential Switch (Brake Warning Switch) <ul><li>Purpose and Function </li></ul><ul><ul><li>After the hydraulic system has been repaired, if the lamp remains on, it may be necessary to do the following: </li></ul></ul><ul><ul><ul><li>Apply light pressure to the brake pedal. </li></ul></ul></ul>
    18. 18. Pressure-Differential Switch (Brake Warning Switch) <ul><li>Purpose and Function </li></ul><ul><ul><li>After the hydraulic system has been repaired, if the lamp remains on, it may be necessary to do the following: </li></ul></ul><ul><ul><ul><li>Momentarily open the bleeder valve on the side that did not have the failure until the warning light goes out. </li></ul></ul></ul>
    19. 19. Figure 95-4 A red brake warning lamp.
    20. 20. Figure 95-5 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 ground circuit for the red brake warning lamp.
    21. 21. Figure 95-6 The pressure-differential switch piston is used to provide the electrical ground for the red brake warning light circuit.
    22. 22. BRAKE FLUID LEVEL SENSOR SWITCH
    23. 23. Brake Fluid Level Sensor Switch <ul><li>Purpose and Function </li></ul><ul><ul><li>A brake fluid level sensor or switch in the master cylinder reservoir, will light the red brake warning lamp on the dash if low brake fluid level is detected </li></ul></ul><ul><ul><li>A float-type sensor or a magnetic reed switch are commonly used and provide a complete electrical circuit when the brake fluid level is low </li></ul></ul>
    24. 24. Brake Fluid Level Sensor Switch <ul><li>Purpose and Function </li></ul><ul><ul><li>After refilling the master cylinder reservoir to the correct level, the red “brake” warning lamp should go out </li></ul></ul>
    25. 25. Figure 95-7 A movable contact brake fluid level switch. When the brake fluid level and float drop, the rod-mounted contact completes the electrical circuit which turns on the red brake warning lamp.
    26. 26. Figure 95-8 A magnetic brake fluid level switch.
    27. 27. Brake Fluid Level Sensor Switch <ul><li>Diagnosing a Red “Brake&quot; Dash Warning Lamp </li></ul><ul><ul><li>Activation of the red brake dash warning lamp can be for any one of several reasons: </li></ul></ul><ul><ul><ul><li>Parking brake &quot;On&quot; </li></ul></ul></ul><ul><ul><ul><li>Low brake fluid </li></ul></ul></ul>
    28. 28. Brake Fluid Level Sensor Switch <ul><li>Diagnosing a Red “Brake&quot; Dash Warning Lamp </li></ul><ul><ul><li>Activation of the red brake dash warning lamp can be for any one of several reasons: </li></ul></ul><ul><ul><ul><li>Unequal brake pressure </li></ul></ul></ul><ul><ul><li>The most likely cause of the warning lamp being on is low brake fluid caused by a leaking brake line, wheel cylinder, or caliper </li></ul></ul>
    29. 29. Brake Fluid Level Sensor Switch <ul><li>Diagnosing a Red “Brake&quot; Dash Warning Lamp </li></ul><ul><ul><li>Activation of the red brake dash warning lamp can be for any one of several reasons </li></ul></ul><ul><ul><li>The first step in diagnosis is to determine the cause of the lamp being on, then to repair the problem </li></ul></ul><ul><ul><ul><li>Check the level on the brake fluid </li></ul></ul></ul>
    30. 30. Brake Fluid Level Sensor Switch <ul><li>Diagnosing a Red “Brake&quot; Dash Warning Lamp </li></ul><ul><ul><li>The first step in diagnosis is to determine the cause of the lamp being on, then to repair the problem </li></ul></ul><ul><ul><ul><li>Disconnect the wire from the pressure-differential switch </li></ul></ul></ul>
    31. 31. Brake Fluid Level Sensor Switch <ul><li>Diagnosing a Red “Brake&quot; Dash Warning Lamp </li></ul><ul><ul><li>NOTE: Brake systems use either a pressure-differential switch or a low brake fluid switch to light the dash red “brake” lamp, but usually not both. </li></ul></ul>
    32. 32. Brake Fluid Level Sensor Switch <ul><li>Diagnosing a Red “Brake&quot; Dash Warning Lamp </li></ul><ul><ul><li>NOTE: Some older 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 brake system inspection. </li></ul></ul>
    33. 33. PROPORTIONING VALVE
    34. 34. Proportioning Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>A proportioning valve improves brake balance during hard stops by limiting hydraulic pressure to the rear brakes </li></ul></ul>
    35. 35. Proportioning Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>Inertia creates a weight shift toward the front of the vehicle during braking </li></ul></ul><ul><ul><li>The weight shift unloads the rear axle, which reduces traction between the tires and the road </li></ul></ul>
    36. 36. Proportioning Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>Unless application pressure to the rear wheels is limited, the rear brakes will lock </li></ul></ul>
    37. 37. Proportioning Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>Vehicles with front disc and rear drum brakes require a proportioning valve for two reasons: </li></ul></ul><ul><ul><ul><li>Disc brakes require higher hydraulic pressure for a given stop than do drum brakes. In a disc/drum system, the front brakes always need more pressure than the rear brakes. </li></ul></ul></ul>
    38. 38. Proportioning Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>Vehicles with front disc and rear drum brakes require a proportioning valve for two reasons: </li></ul></ul><ul><ul><ul><li>Drum brakes require less pressure to maintain a fixed level of stopping power than they did to establish that level. In a disc/drum system, the rear brakes will always need less pressure than the front brakes. </li></ul></ul></ul>
    39. 39. Proportioning Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>Before proportioning action will begin, brake system hydraulic pressure must reach a minimum level called the split point </li></ul></ul>
    40. 40. Proportioning Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>Below the split point full system pressure is supplied to the rear brakes </li></ul></ul>
    41. 41. Proportioning Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>Once system hydraulic pressure exceeds the split point, the rear brakes receive a fixed percentage of any further increase in pressure </li></ul></ul>
    42. 42. Proportioning Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>Brake engineers refer to the ratio of front to rear brake pressure proportioning as the slope </li></ul></ul>
    43. 43. Proportioning Valve <ul><li>Purpose and Function </li></ul><ul><ul><li>The proportioning valves on most vehicles have a slope between 0.25 and 0.50 </li></ul></ul>
    44. 44. Figure 95-9 Many proportioning valves are mounted directly to the master cylinder in the outlet to the rear brakes.
    45. 45. Figure 95-10 Typical proportioning valve pressure relationship. Note that, at low pressures, the pressure is the same to the rear brakes as is applied to the front brakes. After the split point, only a percentage (called the slope) of the master cylinder pressure is applied to the rear brakes.
    46. 46. Figure 95-11 A Chrysler proportioning valve. Note that slope and split point are stamped on the housing.
    47. 47. Figure 95-12 These two proportioning valves are found under the vehicle on this Dodge minivan.
    48. 48. Proportioning Valve <ul><li>Proportioning Valve Operation </li></ul><ul><ul><li>A simple proportioning valve consists of a spring-loaded piston that slides in a stepped bore and is exposed to pressure on both sides </li></ul></ul>
    49. 49. Proportioning Valve <ul><li>Proportioning Valve Operation </li></ul><ul><ul><li>The valve is located in the center of the piston and is opened or closed depending on the position of the piston in the stepped bore </li></ul></ul>
    50. 50. Proportioning Valve <ul><li>Proportioning Valve Operation </li></ul><ul><ul><li>When the brakes are first applied, hydraulic pressure passes through the proportioning valve to the rear brakes and the piston moves to the left against the spring tension </li></ul></ul>
    51. 51. Proportioning Valve <ul><li>Proportioning Valve Operation </li></ul><ul><ul><li>As pressure to the front brakes increases, enough force is developed on the master cylinder side of the piston to overcome pressure trapped in the rear brake circuit </li></ul></ul><ul><ul><li>This forces the piston back to the right and opens the proportioning valve </li></ul></ul>
    52. 52. Proportioning Valve <ul><li>Proportioning Valve Operation </li></ul><ul><ul><li>Before pressure in the two circuits can equalize, the force developed on the larger piston area in the rear circuit moves the piston back to the left and closes the valve </li></ul></ul>
    53. 53. Proportioning Valve <ul><li>Proportioning Valve Operation </li></ul><ul><ul><li>As long as system pressure continues to increase, the piston will repeatedly cycle back and forth, opening and closing the proportioning valve, and maintaining a fixed proportion of full system pressure to the rear brakes </li></ul></ul>
    54. 54. Proportioning Valve <ul><li>Proportioning Valve Operation </li></ul><ul><ul><li>When the brakes are released, the spring returns the piston all the way to the right, which opens the valve and allows fluid to pass in both directions </li></ul></ul>
    55. 55. Figure 95-13 The proportioning valve piston can travel within the range shown without reducing pressure to the rear brakes.
    56. 56. Figure 95-14 At the split point, the proportioning valve piston closes the fluid passage through the valve.
    57. 57. Proportioning Valve <ul><li>Height-Sensing Proportioning Valves </li></ul><ul><ul><li>A height-sensing proportioning valve varies the amount of pressure that can be sent to the rear brakes depending on the height of the rear suspension </li></ul></ul>
    58. 58. Proportioning Valve <ul><li>Height-Sensing Proportioning Valves </li></ul><ul><ul><li>The valve provides the vehicle with variable brake balance, with higher pressure applied to the rear brakes when the vehicle is heavily loaded and less pressure when the vehicle is lightly loaded </li></ul></ul>
    59. 59. Proportioning Valve <ul><li>Height-Sensing Proportioning Valves </li></ul><ul><ul><li>A stepped cam is used to alter the split point of this height-sensing proportioning valve </li></ul></ul>
    60. 60. Proportioning Valve <ul><li>Height-Sensing Proportioning Valves </li></ul><ul><ul><li>CAUTION: Some vehicle manufacturers warn that service technicians should never install replacement air lift shock absorbers or springs that may result in a vehicle height different than specified by the vehicle manufacturer. If the ride height is increased, the front to rear brake proportional relationship will be changed and could reduce stopping distances. </li></ul></ul>
    61. 61. Figure 95-15 A height-sensing proportioning valve provides the vehicle with variable brake balance. The valve allows higher pressure to be applied to the rear brakes when the vehicle is heavily loaded and less pressure when the vehicle is lightly loaded.
    62. 62. Figure 95-16 A stepped cam is used to alter the split point of this height-sensing proportioning valve.
    63. 63. Proportioning Valve <ul><li>Height-Sensing Proportioning Valves </li></ul><ul><ul><li>Proportioning valve adjustment </li></ul></ul><ul><ul><ul><li>Height-sensing proportioning valves should be adjusted when replaced to ensure that the proper pressure is applied to the rear brakes in relation to the loading of the vehicle </li></ul></ul></ul>
    64. 64. Proportioning Valve <ul><li>Height-Sensing Proportioning Valves </li></ul><ul><ul><li>Proportioning valve adjustment </li></ul></ul><ul><ul><ul><li>Always consult the factory service information for the exact procedure </li></ul></ul></ul>
    65. 65. Proportioning Valve <ul><li>Height-Sensing Proportioning Valves </li></ul><ul><ul><li>Proportioning valve adjustment </li></ul></ul><ul><ul><ul><li>Some trucks require the use of special plastic gauges available from the dealer </li></ul></ul></ul>
    66. 66. Proportioning Valve <ul><li>Proportioning Valve Diagnosis and Testing </li></ul><ul><ul><li>If rear brakes tend to lock during braking, the proportioning valve should be checked for proper operation </li></ul></ul><ul><ul><li>If the proportioning valve is height sensing, verify the proper vehicle ride (trim) height and adjustment of the operating lever </li></ul></ul>
    67. 67. Proportioning Valve <ul><li>Proportioning Valve Diagnosis and Testing </li></ul><ul><ul><li>A pressure gauge can also be used to check for proper operation </li></ul></ul><ul><ul><li>Install one gauge into the brake line from the master cylinder and a second gauge to the rear brake outlet of the proportioning valve </li></ul></ul>
    68. 68. Proportioning Valve <ul><li>Proportioning Valve Diagnosis and Testing </li></ul><ul><ul><li>With the brake pedal depressed, observe the two gauges </li></ul></ul><ul><ul><li>Both gauges should register an increasing pressure as the brake pedal is depressed until the split point </li></ul></ul>
    69. 69. Proportioning Valve <ul><li>Proportioning Valve Diagnosis and Testing </li></ul><ul><ul><li>After the split point, the gauge connected to the proportioning valve (rear brakes) should increase at a slower rate than the reading on the gauge connected to the master cylinder </li></ul></ul>
    70. 70. Proportioning Valve <ul><li>Proportioning Valve Diagnosis and Testing </li></ul><ul><ul><li>If the pressures do not react as described, the proportioning valve should be replaced </li></ul></ul><ul><ul><li>The same procedure can be used on a diagonal split-type system </li></ul></ul>
    71. 71. Figure 95-17 A proportioning valve pressure test can be performed using two pressure gauges—one to register the pressure from the master cylinder and the other gauge to read the pressure being applied to the rear brakes. This test has to be repeated in order to read the pressure to each rear wheel.
    72. 72. ELECTRONIC BRAKE PROPORTIONING
    73. 73. Electronic Brake Proportioning <ul><li>Purpose and Function </li></ul><ul><ul><li>Many newer antilock braking systems eliminate the need for a conventional brake proportioning valve </li></ul></ul><ul><ul><li>Pressure to the rear brakes is reduced as needed </li></ul></ul>
    74. 74. Electronic Brake Proportioning <ul><li>Parts and Operation </li></ul><ul><ul><li>Electronic brake proportioning (EBP) in the antilock braking system is accomplished by monitoring front- and rear-wheel speeds, and reducing pressure to the rear brakes as needed using the ABS solenoids when there is a difference in wheel deceleration rates </li></ul></ul>
    75. 75. Electronic Brake Proportioning <ul><li>Parts and Operation </li></ul><ul><ul><li>The dynamic rear proportioning function is enabled at all times unless there is a failure of the antilock brake controller or two wheel speed sensors on the same axle both fail at the same time </li></ul></ul>
    76. 76. METERING VALVE (HOLD-OFF) OPERATION
    77. 77. Metering Valve (Hold-Off) Operation <ul><li>Purpose and Function </li></ul><ul><ul><li>A metering valve is used on all front-disc, rear-drum-brake-equipped vehicles to prevent full operation of (holds off) the disc brakes until between 75 to 125 PSI is sent to the rear drum brakes to overcome rear-brake return spring pressure </li></ul></ul>
    78. 78. Metering Valve (Hold-Off) Operation <ul><li>Purpose and Function </li></ul><ul><ul><li>This allows the front and rear brakes to apply at the same time for even stopping </li></ul></ul>
    79. 79. Metering Valve (Hold-Off) Operation <ul><li>Purpose and Function </li></ul><ul><ul><li>Most metering valves also allow for the pressure to the front brakes to be gradually blended up to the metering valve pressure to prevent front brake locking under light pedal pressures on icy surfaces </li></ul></ul>
    80. 80. Metering Valve (Hold-Off) Operation <ul><li>Parts and Operation </li></ul><ul><ul><li>A metering valve consists of a piston controlled by a strong spring and a valve stem controlled by a weak spring </li></ul></ul>
    81. 81. Metering Valve (Hold-Off) Operation <ul><li>Parts and Operation </li></ul><ul><ul><li>When the brakes are not applied, the strong spring seats the piston and prevents fluid flow around it, while the weak spring holds the valve stem to the right and opens a passage through the center of the piston </li></ul></ul>
    82. 82. Metering Valve (Hold-Off) Operation <ul><li>Parts and Operation </li></ul><ul><ul><li>Brake fluid is free to flow through this passage to compensate for changes in system fluid volume </li></ul></ul>
    83. 83. Metering Valve (Hold-Off) Operation <ul><li>Parts and Operation </li></ul><ul><ul><li>When the brakes are applied and pressure in the front brake line reaches 3 to 30 PSI (20 to 200 kPa), metering valve stem moves to the left, which closes the passage through the piston and prevents fluid flow to the front brakes </li></ul></ul>
    84. 84. Metering Valve (Hold-Off) Operation <ul><li>Parts and Operation </li></ul><ul><ul><li>While the fluid flow to the front calipers is shut off, the rear brake shoes move into contact with the drums, braking begins, and hydraulic pressure throughout the brake system increases </li></ul></ul>
    85. 85. Metering Valve (Hold-Off) Operation <ul><li>Parts and Operation </li></ul><ul><ul><li>When the pressure at the metering valve reaches 75 to 300 PSI, the tension of the strong spring is overcome and the valve stem and piston move farther to the left </li></ul></ul>
    86. 86. Metering Valve (Hold-Off) Operation <ul><li>Parts and Operation </li></ul><ul><ul><li>This opens a passage around the outside of the piston and allows fluid to flow through the valve to the front brake calipers </li></ul></ul>
    87. 87. Metering Valve (Hold-Off) Operation <ul><li>Parts and Operation </li></ul><ul><ul><li>When the brakes are released, the strong spring seats the piston and prevents fluid flow around it, while the weak spring opens the fluid passage through the center of the piston </li></ul></ul>
    88. 88. Metering Valve (Hold-Off) Operation <ul><li>Parts and Operation </li></ul><ul><ul><li>Excess fluid returns to the master cylinder through this passage and the valve is ready for another brake application </li></ul></ul>
    89. 89. Figure 95-18 A metering valve when the brakes are not applied. Notice the brake fluid can flow through the metering valve to compensate for brake fluid expansion and contraction that occurs with changes in temperature.
    90. 90. Figure 95-19 A metering valve under light brake pedal application.
    91. 91. Figure 95-20 A metering valve during a normal brake application.
    92. 92. Metering Valve (Hold-Off) Operation <ul><li>Systems Without Metering Valves </li></ul><ul><ul><li>There are three reasons why braking systems that are diagonal split (found on most front wheel drive vehicles) do not use a metering valve: </li></ul></ul><ul><ul><ul><li>A diagonally split dual braking system would require a separate metering valve for each hydraulic circuit </li></ul></ul></ul>
    93. 93. Metering Valve (Hold-Off) Operation <ul><li>Systems Without Metering Valves </li></ul><ul><ul><li>There are three reasons why braking systems that are diagonal split (found on most front wheel drive vehicles) do not use a metering valve: </li></ul></ul><ul><ul><ul><li>It is desirable to apply the front brakes as soon as possible when the brake pedal is depressed </li></ul></ul></ul><ul><ul><ul><li>A metering valve would create a slight delay </li></ul></ul></ul>
    94. 94. Metering Valve (Hold-Off) Operation <ul><li>Systems Without Metering Valves </li></ul><ul><ul><li>There are three reasons why braking systems that are diagonal split (found on most front wheel drive vehicles) do not use a metering valve: </li></ul></ul><ul><ul><ul><li>Until all the clearance in the brake system is taken up, there is not enough pressure in the brake hydraulic system for the front disc brakes to overcome the engine torque applied to the driven front wheels </li></ul></ul></ul>
    95. 95. Metering Valve (Hold-Off) Operation <ul><li>Systems Without Metering Valves </li></ul><ul><ul><li>Most rear-wheel-drive vehicles without metering valves are equipped with four-wheel disc brakes </li></ul></ul>
    96. 96. Metering Valve (Hold-Off) Operation <ul><li>Systems Without Metering Valves </li></ul><ul><ul><li>The clearance between the pads and rotors is approximately the same at all four wheels, so it is not necessary to delay front brake actuation </li></ul></ul>
    97. 97. Metering Valve (Hold-Off) Operation <ul><li>Systems Without Metering Valves </li></ul><ul><ul><li>Some vehicles also have antilock brake systems that prevent the wheels from locking at any time </li></ul></ul>
    98. 98. Metering Valve (Hold-Off) Operation <ul><li>Metering Valve Diagnosis and Testing </li></ul><ul><ul><li>A defective valve can leak brake fluid and/or cause the front brakes to apply before the rear brakes </li></ul></ul>
    99. 99. Metering Valve (Hold-Off) Operation <ul><li>Metering Valve Diagnosis and Testing </li></ul><ul><ul><li>Inspect the metering valve for these two conditions: </li></ul></ul><ul><ul><ul><li>Look around the bottom of valve for brake fluid leakage (ignore slight dampness) </li></ul></ul></ul><ul><ul><ul><li>Replace the metering valve assembly if it is leaking </li></ul></ul></ul>
    100. 100. Metering Valve (Hold-Off) Operation <ul><li>Metering Valve Diagnosis and Testing </li></ul><ul><ul><li>Inspect the metering valve for these two conditions: </li></ul></ul><ul><ul><ul><li>As the pressure builds to the front brakes, the metering valve stem should move </li></ul></ul></ul><ul><ul><ul><li>If it does not, replace the valve </li></ul></ul></ul>
    101. 101. Metering Valve (Hold-Off) Operation <ul><li>Metering Valve Diagnosis and Testing </li></ul><ul><ul><li>Pressure gauges can be used for more accurate testing </li></ul></ul><ul><ul><li>Install one gauge in the pressure line coming from the master cylinder and another in the outlet line leading to the front brakes </li></ul></ul>
    102. 102. Metering Valve (Hold-Off) Operation <ul><li>Metering Valve Diagnosis and Testing </li></ul><ul><ul><li>When depressing the brake pedal, both gauges should read the same until about 3 to 30 PSI (20 to 200 kPa) when the metering valve shuts, delaying the operation of the front brakes </li></ul></ul>
    103. 103. Metering Valve (Hold-Off) Operation <ul><li>Metering Valve Diagnosis and Testing </li></ul><ul><ul><li>The master cylinder outlet gauge should show an increase in pressure as the brake pedal is depressed further </li></ul></ul>
    104. 104. Metering Valve (Hold-Off) Operation <ul><li>Metering Valve Diagnosis and Testing </li></ul><ul><ul><li>Once 75 to 300 PSI is reached, the gauge showing pressure to the front brakes should match the pressure from the master cylinder </li></ul></ul>
    105. 105. Metering Valve (Hold-Off) Operation <ul><li>Metering Valve Diagnosis and Testing </li></ul><ul><ul><li>If the pressures do not match these ranges, the metering valve assembly should be replaced </li></ul></ul>
    106. 106. COMBINATION VALVE
    107. 107. Combination Valve <ul><li>The function of a proportioning valve is combined with one or more other valves into one unit called a combination valve </li></ul>
    108. 108. Combination Valve <ul><li>A typical combination valve consists of the following components all in one replaceable unit: </li></ul><ul><ul><li>Metering valve </li></ul></ul><ul><ul><li>Proportioning valve </li></ul></ul>
    109. 109. Combination Valve <ul><li>A typical combination valve consists of the following components all in one replaceable unit: </li></ul><ul><ul><li>Pressure-differential switch </li></ul></ul><ul><li>Some combination valves have only two functions </li></ul>
    110. 110. Figure 95-21 Typical two-function combination valves.
    111. 111. Figure 95-22 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 proportioning valve section resembles the grip section of a handgun.
    112. 112. BRAKE LIGHT SWITCH
    113. 113. Brake Light Switch <ul><li>The brake light switch is used to turn on the brake lights at the back of the vehicle </li></ul><ul><li>A properly adjusted light switch will activate the brake lights as soon as the brake pedal is applied and before braking action begins at the wheels </li></ul>
    114. 114. Brake Light Switch <ul><li>A brake light switch can be called: </li></ul><ul><ul><li>Stop light switch </li></ul></ul><ul><ul><li>Brake pedal position (BPP) switch </li></ul></ul>
    115. 115. Brake Light Switch <ul><li>A brake light switch can be called: </li></ul><ul><ul><li>Brake on/off (BOO) switch (Fords) </li></ul></ul><ul><li>Brake light switches are normally open and close when the brakes are applied, completing the brake light circuit </li></ul>
    116. 116. Brake Light Switch <ul><li>Most newer vehicles use the brake switch as an input to the body computer for many functions including: </li></ul><ul><ul><li>Brake lights </li></ul></ul><ul><ul><li>Antilock brake system (ABS) input signal </li></ul></ul>
    117. 117. Brake Light Switch <ul><li>Most newer vehicles use the brake switch as an input to the body computer for many functions including: </li></ul><ul><ul><li>Traction control is disabled when the brake pedal is depressed </li></ul></ul><ul><ul><li>Electronic stability control (ESC) system input signal </li></ul></ul>
    118. 118. Figure 95-23 Typical brake light switches.
    119. 119. TECH TIP <ul><li>Always Inspect Both Front and Rear Brakes </li></ul><ul><ul><li>If a vehicle tends to lock up the rear brakes during a stop, many technicians may try to repair the problem by replacing the proportioning valve or servicing the rear brakes. Proportioning valves are simple spring-loaded devices that are usually trouble free. </li></ul></ul>BACK TO PRESENTATION <ul><li>If the rear brakes lock up during braking, carefully inspect the rear brakes looking for contaminated linings or other problems that can cause the rear brakes to grab. Do not stop there— always inspect the front brakes, too. If the front brakes are rusted or corroded, they cannot operate efficiently and greater force must be exerted by the driver to stop the vehicle. </li></ul><ul><li>Even if the proportioning valve is functioning correctly, the higher brake pedal pressure by the driver could easily cause the rear brakes to lock up. </li></ul><ul><li>A locked wheel has less traction with the road than a rotating wheel. As a result, if the rear wheels become locked, the rear of the vehicle often “comes around” or “fishtails,” causing the vehicle to skid. Careful inspection of the entire braking system is required to be assured of a safe vehicle. </li></ul>
    120. 120. TECH TIP <ul><li>Push-In or Pull-Out Metering Valve? </li></ul><ul><ul><li>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). </li></ul></ul>BACK TO PRESENTATION <ul><li>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. </li></ul><ul><li>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. </li></ul>
    121. 121. TECH TIP <ul><li>No Valves Can Cause a Pull </li></ul><ul><ul><li>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. </li></ul></ul><ul><li>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. </li></ul>BACK TO PRESENTATION

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