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CHAPTER 6 Hydraulic Valves and Switches

CHAPTER 6 Hydraulic Valves and Switches. OBJECTIVES. After studying Chapter 6, the reader will be able to: Prepare for the Brakes (A5) ASE certification test content area “A” (Hydraulic System Diagnosis and Repair). Describe the operation of a residual check valve.

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CHAPTER 6 Hydraulic Valves and Switches

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  1. CHAPTER 6 Hydraulic Valves and Switches

  2. OBJECTIVES After studying Chapter 6, the reader will be able to: • Prepare for the Brakes (A5) ASE certification test content area “A” (Hydraulic System Diagnosis and Repair). • Describe the operation of a residual check valve. • Explain how a proportioning valve works. • 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.

  3. 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 KEY TERMS

  4. FIGURE 6–1 Most residual check valves are located under the tubing seals in the master cylinder outlet ports. 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).

  5. RESIDUAL CHECK VALVE • This residual check valve is located in the master cylinder at the outlet for the drum brakes. • The check ball and spring in the residual check valve permit all the brake fluid to return to the master cylinder until the designated pressure is reached. • 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.

  6. FIGURE 6–2 The momentary drop in pressure created when the brakes are released can draw air into the hydraulic system. RESIDUAL CHECK VALVE

  7. FIGURE 6–3 The use of cup expanders is the main reason why residual check valves are not used in most braking systems today. RESIDUAL CHECK VALVE • 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.

  8. PRESSURE-DIFFERENTIAL SWITCH (BRAKE WARNING SWITCH) • PURPOSE AND FUNCTION • 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.

  9. FIGURE 6–4 A red brake warning lamp. PRESSURE-DIFFERENTIAL SWITCH (BRAKE WARNING SWITCH)

  10. FIGURE 6–5 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. PRESSURE-DIFFERENTIAL SWITCH (BRAKE WARNING SWITCH)

  11. FIGURE 6–6 The pressure-differential switch piston is used to provide the electrical ground for the red brake warning light circuit. PRESSURE-DIFFERENTIAL SWITCH (BRAKE WARNING SWITCH) • The brake lines from both the front and the rear sections of the master cylinder are sent to this switch, which lights the brake warning indicator lamp in the event of a “difference in pressure” between the two sections.

  12. BRAKE FLUID LEVEL SENSOR SWITCH • Many master cylinders, especially systems that are a diagonal split, usually use a brake fluid level sensor or switch in the master cylinder reservoir. • This sensor will light the red brake warning lamp on the dash if low brake fluid level is detected. • 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. • After refilling the master cylinder reservoir to the correct level, the red “brake” warning lamp should go out.

  13. FIGURE 6–7 A movable contact brake fluid level switch. BRAKE FLUID LEVEL SENSOR SWITCH

  14. FIGURE 6–8 A magnetic brake fluid level switch. BRAKE FLUID LEVEL SENSOR SWITCH

  15. BRAKE FLUID LEVEL SENSOR SWITCHDIAGNOSING A RED “BRAKE” DASH WARNING LAMP • Activation of the red brake dash warning lamp can be for any one of several reasons: • 1. Parking Brake “On.” The same dash warning lamp is used to warn the driver that the parking brake is on. • 2. Low Brake Fluid. This lights the red dash warning lamp on vehicles equipped with a master cylinder reservoir brake fluid level switch. • 3. Unequal Brake Pressure. The pressure-differential switch is used on most vehicles with a front/rear brake split system to warn the driver whenever there is low brake pressure to either the front or rear brakes.

  16. BRAKE FLUID LEVEL SENSOR SWITCHDIAGNOSING A RED “BRAKE” DASH WARNING LAMP • The most likely cause of the red “brake” warning lamp being on is low brake fluid caused by a leaking brake line, wheel cylinder, or caliper. • Therefore, the first step in diagnosis is to determine the cause of the lamp being on, then to repair the problem. • STEP 1 Check the Level of the Brake Fluid. • STEP 2 Disconnect the Wire from the Pressure-Differential Switch.

  17. http://www.youtube.com/watch?v=XLf7FZSPgUg&feature=relmfu

  18. FIGURE 6–9 Many proportioning valves are mounted directly to the master cylinder in the outlet to the rear brakes. PROPORTIONING VALVEPURPOSE AND FUNCTION • A proportioning valve improves brake balance during hard stops by limiting hydraulic pressure to the rear brakes.

  19. PROPORTIONING VALVEPURPOSE AND FUNCTION • Vehicles with front disc and rear drum brakes require a proportioning valve for two reasons. • 1. 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. • 2. Once braking has begun, 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.

  20. FIGURE 6–10 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. PROPORTIONING VALVEPURPOSE AND FUNCTION

  21. FIGURE 6–11 A Chrysler proportioning valve. Note that slope and split point are stamped on the housing. PROPORTIONING VALVEPURPOSE AND FUNCTION

  22. FIGURE 6–12 These two proportioning valves are found under the vehicle on this Dodge minivan. PROPORTIONING VALVEPURPOSE AND FUNCTION

  23. FIGURE 6–13 The proportioning valve piston can travel within the range shown without reducing pressure to the rear brakes. PROPORTIONING VALVEPROPORTIONING VALVE OPERATION • A simple proportioning valve consists of a spring-loaded piston that slides in a stepped bore.

  24. FIGURE 6–14 At the split point, the proportioning valve piston closes the fluid passage through the valve. PROPORTIONING VALVEPROPORTIONING VALVE OPERATION

  25. Always Inspect Both Front and Rear Brakes • 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 springloaded devices that are usually trouble free. 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. 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.

  26. Always Inspect Both Front and Rear Brakes • 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.

  27. http://www.youtube.com/watch?v=E9rmwTcE-XE

  28. PROPORTIONING VALVEHEIGHT-SENSING PROPORTIONING VALVES • Many vehicles use a proportioning valve that varies the amount of pressure that can be sent to the rear brakes depending on the height of the rear suspension. • This type of valve is called a height-sensing proportioning valve. • If the vehicle is lightly loaded, the rear suspension is high, especially during braking. • In this case, the amount of pressure allowed to the rear brakes is reduced. • This helps prevent rear-wheel lockup and possible skidding. • Besides, a lightly loaded vehicle requires less braking force to stop than a heavily loaded vehicle.

  29. FIGURE 6–15 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. PROPORTIONING VALVEHEIGHT-SENSING PROPORTIONING VALVES

  30. FIGURE 6–16 A stepped cam is used to alter the split point of this height-sensing proportioning valve. PROPORTIONING VALVEHEIGHT-SENSING PROPORTIONING VALVES

  31. PROPORTIONING VALVEHEIGHT-SENSING PROPORTIONING VALVES • PROPORTIONING VALVE ADJUSTMENT • Height-sensing proportioning valves should be adjusted when replaced. • The proper adjustment ensures that the proper pressure is applied to the rear brakes in relation to the loading of the vehicle.

  32. PROPORTIONING VALVEPROPORTIONING VALVE DIAGNOSIS AND TESTING • A defective proportioning valve usually allows rear brake pressure to increase too rapidly, causing the rear wheels to lock up during hard braking. • When the rear brakes become locked, the traction with the road surface decreases and the vehicle often skids. • Whenever rear brakes tend to lock during braking, the proportioning valve should be checked for proper operation. • If the proportioning valve is height sensing, verify the proper vehicle ride (trim) height and adjustment of the operating lever.

  33. FIGURE 6–17 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. PROPORTIONING VALVEPROPORTIONING VALVE DIAGNOSIS AND TESTING

  34. ELECTRONIC BRAKE PROPORTIONING • 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. • The controller energizes the inlet valve solenoids for both rear brakes to hold pressure in the lines, and then energizes both rear outlet valve solenoids to release pressure as needed.

  35. METERING VALVE (HOLD-OFF) OPERATION • PURPOSE AND FUNCTION • A metering valve is used on all front-disc, rear-drum-brake-equipped vehicles. • The metering valve prevents the 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. • This allows the front and rear brakes to apply at the same time for even stopping. • 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.

  36. FIGURE 6–18 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. METERING VALVE (HOLD-OFF) OPERATION • PARTS AND OPERATION • A metering valve consists of a piston controlled by a strong spring and a valve stem controlled by a weak spring.

  37. FIGURE 6–19 A metering valve under light brake pedal application. METERING VALVE (HOLD-OFF) OPERATION

  38. METERING VALVE (HOLD-OFF) OPERATION • SYSTEMS WITHOUT METERING VALVES • Braking systems that are diagonal split, such as those found on most front-wheel-drive 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 rear-wheel-drive vehicles.

  39. FIGURE 6–20 A metering valve during a normal brake application. METERING VALVE (HOLD-OFF) OPERATION

  40. METERING VALVE (HOLD-OFF) OPERATION • 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. • This is most commonly noticed on slippery surfaces such as on snow or ice or on rain-slick roads. • If the front brakes lock up during these conditions, the front wheels cannot be steered. Inspect the metering valve for these two conditions: • 1. Look around the bottom of the metering valve for brake fluid leakage. (Ignore slight dampness.) Replace the metering valve assembly if it is leaking. • 2. As the pressure builds to the front brakes, the metering valve stem should move. If it does not, replace the valve.

  41. 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 hardto- 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.

  42. Push-In or Pull-Out Metering Valve? • 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.

  43. COMBINATION VALVE • Most vehicle manufacturers combine the function of a proportioning valve with one or more other valves into one unit called a combination valve. • On a typical rear-wheel-drive vehicle, a typical combination valve consists of the following components all in one replaceable unit: • Metering valve • Proportioning valve • Pressure-differential switch

  44. FIGURE 6–21 Typical two-function combination valves. COMBINATION VALVE

  45. FIGURE 6–22 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. COMBINATION VALVE

  46. 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.

  47. BRAKE LIGHT SWITCH • The job of the brake light switch is to turn on the brake lights at the back of the vehicle when the brakes are applied. • A properly adjusted light switch will activate the brake lights as soon as the brake pedal is applied and before braking action actually begins at the wheels. • Mechanical switches that operate directly off the brake pedal arm are most often used.

  48. FIGURE 6–23 Typical brake light switches. BRAKE LIGHT SWITCH

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