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© 2011 Pearson Education, Inc.All Rights Reserved
Automotive Technology, Fourth EditionJames Halderman
HYDRAULIC VALVES AND SWITCHES
95
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
ObjectivesObjectives• The student should 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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
ObjectivesObjectives• The student should be able to:
– List testing procedures used to test hydraulic valves.
– Describe how the brake fluid level and brake light switches work.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
INTRODUCTIONINTRODUCTION
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
IntroductionIntroduction• The hydraulic system is the heart of the
braking system• 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
• Valves and switches need to be added to the system to meet all possible braking conditions
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
RESIDUAL CHECK RESIDUAL CHECK VALVEVALVE
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Residual Check ValveResidual Check Valve• Purpose and Function
– A residual check valve is used on some drum brake systems to keep a slight amount of pressure on the entire hydraulic system
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Residual Check ValveResidual Check Valve• Purpose and Function
– This valve is located in the master cylinder at the outlet for the drum brakes
– A check ball and spring permit all the brake fluid to return to the master cylinder until the designated pressure is reached
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Residual Check ValveResidual Check Valve• Purpose and Function
– 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
– Slight pressure also keeps the wheel cylinder sealing cups tight against the inside wall of the wheel cylinder
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Residual Check ValveResidual Check Valve• Purpose and Function
– Residual check valves are not used often 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-1 Most residual check valves are located under the tubing seats in the master cylinder outlet ports.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-2 The momentary drop in pressure created when the brakes are released can draw air into the hydraulic system.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-3 The use of cup expanders is the main reason why residual check valves are not used in most braking systems today.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
PRESSURE-PRESSURE-DIFFERENTIALDIFFERENTIAL
SWITCH (BRAKE SWITCH (BRAKE WARNING SWITCH)WARNING SWITCH)
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Pressure-Differential Switch (Brake Pressure-Differential Switch (Brake Warning Switch)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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Pressure-Differential Switch (Brake Pressure-Differential Switch (Brake Warning Switch)Warning Switch)
• Purpose and Function– The brake lines from both sections of the
master cylinder are sent to the switch, which lights the brake warning indicator lamp
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Pressure-Differential Switch (Brake Pressure-Differential Switch (Brake Warning Switch)Warning Switch)
• Purpose and Function– After the hydraulic system has been
repaired, if the lamp remains on, it may be necessary to do the following:1. Apply light pressure to the
brake pedal.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Pressure-Differential Switch (Brake Pressure-Differential Switch (Brake Warning Switch)Warning Switch)
• Purpose and Function– After the hydraulic system has been
repaired, if the lamp remains on, it may be necessary to do the following:2. Momentarily open the bleeder
valve on the side that did not have the failure until the warning light goes out.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-4 A red brake warning lamp.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-6 The pressure-differential switch piston is used to provide the electrical ground for the red brake warning light circuit.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
BRAKE FLUID LEVELBRAKE FLUID LEVELSENSOR SWITCHSENSOR SWITCH
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Fluid Level Sensor SwitchBrake Fluid Level Sensor Switch• Purpose and Function
– 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
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Fluid Level Sensor SwitchBrake Fluid Level Sensor Switch• Purpose and Function
– After refilling the master cylinder reservoir to the correct level, the red “brake” warning lamp should go out
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-8 A magnetic brake fluid level switch.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Fluid Level Sensor SwitchBrake Fluid Level Sensor Switch• Diagnosing a Red “Brake" Dash
Warning Lamp– Activation of the red brake dash warning
lamp can be for any one of several reasons:• Parking brake "On"• Low brake fluid
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Fluid Level Sensor SwitchBrake Fluid Level Sensor Switch• Diagnosing a Red “Brake" Dash
Warning Lamp– Activation of the red brake dash warning
lamp can be for any one of several reasons:• Unequal brake pressure
– The most likely cause of the warning lamp being on is low brake fluid caused by a leaking brake line, wheel cylinder, or caliper
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Fluid Level Sensor SwitchBrake Fluid Level Sensor Switch• Diagnosing a Red “Brake" Dash
Warning Lamp– Activation of the red brake dash warning
lamp can be for any one of several reasons– The first step in diagnosis is to determine
the cause of the lamp being on, then to repair the problem
• Check the level on the brake fluid
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Fluid Level Sensor SwitchBrake Fluid Level Sensor Switch• Diagnosing a Red “Brake" Dash
Warning Lamp– The first step in diagnosis is to determine
the cause of the lamp being on, then to repair the problem
• Disconnect the wire from the pressure-differential switch
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Fluid Level Sensor SwitchBrake Fluid Level Sensor Switch• Diagnosing a Red “Brake" Dash
Warning Lamp– 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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Fluid Level Sensor SwitchBrake Fluid Level Sensor Switch• Diagnosing a Red “Brake" Dash
Warning Lamp– 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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
PROPORTIONING PROPORTIONING VALVEVALVE
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Purpose and Function
– A proportioning valve improves brake balance during hard stops by limiting hydraulic pressure to the rear brakes
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Purpose and Function
– Inertia creates a weight shift toward the front of the vehicle during braking
– The weight shift unloads the rear axle, which reduces traction between the tires and the road
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Purpose and Function
– Unless application pressure to the rear wheels is limited, the rear brakes will lock
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Purpose 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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Purpose and Function
– Vehicles with front disc and rear drum brakes require a proportioning valve for two reasons:2. 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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Purpose and Function
– Before proportioning action will begin, brake system hydraulic pressure must reach a minimum level called the split point
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Purpose and Function
– Below the split point full system pressure is supplied to the rear brakes
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Purpose and Function
– Once system hydraulic pressure exceeds the split point, the rear brakes receive a fixed percentage of any further increase in pressure
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Purpose and Function
– Brake engineers refer to the ratio of front to rear brake pressure proportioning as the slope
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Purpose and Function
– The proportioning valves on most vehicles have a slope between 0.25 and 0.50
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-9 Many proportioning valves are mounted directly to the master cylinder in the outlet to the rear brakes.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-11 A Chrysler proportioning valve. Note that slope and split point are stamped on the housing.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-12 These two proportioning valves are found under the vehicle on this Dodge minivan.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Operation
– A simple proportioning valve consists of a spring-loaded piston that slides in a stepped bore and is exposed to pressure on both sides
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Operation
– 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
– This forces the piston back to the right and opens the proportioning valve
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-13 The proportioning valve piston can travel within the range shown without reducing pressure to the rear brakes.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-14 At the split point, the proportioning valve piston closes the fluid passage through the valve.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Height-Sensing Proportioning Valves
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Height-Sensing Proportioning Valves
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Height-Sensing Proportioning Valves
– A stepped cam is used to alter the split point of this height-sensing proportioning valve
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Height-Sensing Proportioning Valves
– 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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-16 A stepped cam is used to alter the split point of this height-sensing proportioning valve.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Height-Sensing Proportioning Valves
– Proportioning valve adjustment• 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Height-Sensing Proportioning Valves
– Proportioning valve adjustment• Always consult the factory service
information for the exact procedure
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Height-Sensing Proportioning Valves
– Proportioning valve adjustment• Some trucks require the use of special
plastic gauges available from the dealer
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Diagnosis and
Testing– If 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Diagnosis and
Testing– A pressure gauge can also be used to
check for proper operation– Install one gauge into the brake line from
the master cylinder and a second gauge to the rear brake outlet of the proportioning valve
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Diagnosis and
Testing– With the brake pedal depressed, observe
the two gauges– Both gauges should register an increasing
pressure as the brake pedal is depressed until the split point
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Diagnosis and
Testing– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Proportioning ValveProportioning Valve• Proportioning Valve Diagnosis and
Testing– If the pressures do not react as described,
the proportioning valve should be replaced– The same procedure can be used on a
diagonal split-type system
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
ELECTRONIC BRAKEELECTRONIC BRAKEPROPORTIONINGPROPORTIONING
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Electronic Brake ProportioningElectronic Brake Proportioning• Purpose and Function
– Many newer antilock braking systems eliminate the need for a conventional brake proportioning valve
– Pressure to the rear brakes is reduced as needed
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Electronic Brake ProportioningElectronic Brake Proportioning• Parts and Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Electronic Brake ProportioningElectronic Brake Proportioning• Parts and Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
METERING VALVEMETERING VALVE(HOLD-OFF) (HOLD-OFF) OPERATIONOPERATION
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Purpose and Function
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Purpose and Function
– This allows the front and rear brakes to apply at the same time for even stopping
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Purpose and Function
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Parts and Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Parts and Operation
– Brake fluid is free to flow through this passage to compensate for changes in system fluid volume
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Parts and Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Parts and Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Parts and Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Parts and Operation
– This opens a passage around the outside of the piston and allows fluid to flow through the valve to the front brake calipers
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Parts and Operation
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Parts and Operation
– Excess fluid returns to the master cylinder through this passage and the valve is ready for another brake application
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-19 A metering valve under light brake pedal application.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-20 A metering valve during a normal brake application.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Systems Without Metering Valves
– There are three reasons why braking systems that are diagonal split (found on most front wheel drive vehicles) do not use a metering valve:
• A diagonally split dual braking system would require a separate metering valve for each hydraulic circuit
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Systems Without Metering Valves
– There are three reasons why braking systems that are diagonal split (found on most front wheel drive vehicles) do not use a metering valve:
• It is desirable to apply the front brakes as soon as possible when the brake pedal is depressed
• A metering valve would create a slight delay
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Systems Without Metering Valves
– There are three reasons why braking systems that are diagonal split (found on most front wheel drive vehicles) do not use a metering valve:
• 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Systems Without Metering Valves
– Most rear-wheel-drive vehicles without metering valves are equipped with four-wheel disc brakes
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Systems Without Metering Valves
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Systems Without Metering Valves
– Some vehicles also have antilock brake systems that prevent the wheels from locking at any time
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Metering Valve Diagnosis and Testing
– A defective valve can leak brake fluid and/or cause the front brakes to apply before the rear brakes
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Metering Valve Diagnosis and Testing
– Inspect the metering valve for these two conditions:
• Look around the bottom of valve for brake fluid leakage (ignore slight dampness)
• Replace the metering valve assembly if it is leaking
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Metering Valve Diagnosis and Testing
– Inspect the metering valve for these two conditions:
• As the pressure builds to the front brakes, the metering valve stem should move
• If it does not, replace the valve
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Metering Valve Diagnosis and Testing
– Pressure gauges can be used for more accurate testing
– Install one gauge in the pressure line coming from the master cylinder and another in the outlet line leading to the front brakes
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Metering Valve Diagnosis and Testing
– 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Metering Valve Diagnosis and Testing
– The master cylinder outlet gauge should show an increase in pressure as the brake pedal is depressed further
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Metering Valve Diagnosis and Testing
– Once 75 to 300 PSI is reached, the gauge showing pressure to the front brakes should match the pressure from the master cylinder
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Metering Valve (Hold-Off) OperationMetering Valve (Hold-Off) Operation• Metering Valve Diagnosis and Testing
– If the pressures do not match these ranges, the metering valve assembly should be replaced
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
COMBINATION VALVECOMBINATION VALVE
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Combination ValveCombination Valve• The function of a proportioning valve is
combined with one or more other valves into one unit called a combination valve
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Combination ValveCombination Valve• A typical combination valve consists of
the following components all in one replaceable unit:– Metering valve– Proportioning valve
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Combination ValveCombination Valve• A typical combination valve consists of
the following components all in one replaceable unit:– Pressure-differential switch
• Some combination valves have only two functions
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-21 Typical two-function combination valves.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
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.
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
BRAKE LIGHT SWITCHBRAKE LIGHT SWITCH
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Light SwitchBrake Light Switch• The brake light switch is used to turn on
the brake lights at the back of the vehicle
• 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
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Light SwitchBrake Light Switch• A brake light switch can be called:
– Stop light switch– Brake pedal position (BPP) switch
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Light SwitchBrake Light Switch• A brake light switch can be called:
– Brake on/off (BOO) switch (Fords)• Brake light switches are normally open
and close when the brakes are applied, completing the brake light circuit
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Light SwitchBrake Light Switch• Most newer vehicles use the brake
switch as an input to the body computer for many functions including:– Brake lights– Antilock brake system (ABS) input signal
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Brake Light SwitchBrake Light Switch• Most newer vehicles use the brake
switch as an input to the body computer for many functions including:– Traction control is disabled when the brake
pedal is depressed– Electronic stability control (ESC) system
input signal
95 HYDRAULIC VALVES AND SWITCHES
Automotive Technology, Fourth EditionJames Halderman
© 2011 Pearson Education, Inc.All Rights Reserved
Figure 95-23 Typical brake light switches.