Halderman ch095 lecture

Automotive Technology, Fourth EditionJames Halderman

HYDRAULIC VALVES AND SWITCHES

95 HYDRAULIC VALVES AND SWITCHES

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.

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.

INTRODUCTIONINTRODUCTION

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

RESIDUAL CHECK RESIDUAL CHECK VALVEVALVE

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

– 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

– 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

– 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

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.

PRESSURE-PRESSURE-DIFFERENTIALDIFFERENTIAL

SWITCH (BRAKE SWITCH (BRAKE WARNING SWITCH)WARNING SWITCH)

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

• 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

• 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.

• 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.

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.

BRAKE FLUID LEVELBRAKE FLUID LEVELSENSOR SWITCHSENSOR SWITCH

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

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

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.

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

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

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

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

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.

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.

PROPORTIONING PROPORTIONING VALVEVALVE

Proportioning ValveProportioning Valve• Purpose and Function

– A proportioning valve improves brake balance during hard stops by limiting hydraulic pressure to the rear brakes

– 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

– Unless application pressure to the rear wheels is limited, the rear brakes will lock

– 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.

– 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.

– Before proportioning action will begin, brake system hydraulic pressure must reach a minimum level called the split point

– Below the split point full system pressure is supplied to the rear brakes

– Once system hydraulic pressure exceeds the split point, the rear brakes receive a fixed percentage of any further increase in pressure

– Brake engineers refer to the ratio of front to rear brake pressure proportioning as the slope

– The proportioning valves on most vehicles have a slope between 0.25 and 0.50

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.

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

– 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

– 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

– 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

– 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

– 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

– 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

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.

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

– 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

– A stepped cam is used to alter the split point of this height-sensing proportioning valve

– 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.

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.

– 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

– Proportioning valve adjustment• Always consult the factory service

information for the exact procedure

– Proportioning valve adjustment• Some trucks require the use of special

plastic gauges available from the dealer

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

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

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

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

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

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.

ELECTRONIC BRAKEELECTRONIC BRAKEPROPORTIONINGPROPORTIONING

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

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

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

METERING VALVEMETERING VALVE(HOLD-OFF) (HOLD-OFF) OPERATIONOPERATION

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

– 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

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

– 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

– Brake fluid is free to flow through this passage to compensate for changes in system fluid volume

– 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

– 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

– 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

– This opens a passage around the outside of the piston and allows fluid to flow through the valve to the front brake calipers

– 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

– Excess fluid returns to the master cylinder through this passage and the valve is ready for another brake application

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.

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

• 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

• 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

– Most rear-wheel-drive vehicles without metering valves are equipped with four-wheel disc brakes

– 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

– Some vehicles also have antilock brake systems that prevent the wheels from locking at any time

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

– 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

– 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

– 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

– 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

– The master cylinder outlet gauge should show an increase in pressure as the brake pedal is depressed further

– Once 75 to 300 PSI is reached, the gauge showing pressure to the front brakes should match the pressure from the master cylinder

– If the pressures do not match these ranges, the metering valve assembly should be replaced

COMBINATION VALVECOMBINATION VALVE

Combination ValveCombination Valve• The function of a proportioning valve is

combined with one or more other valves into one unit called a combination valve

Combination ValveCombination Valve• A typical combination valve consists of

the following components all in one replaceable unit:– Metering valve– Proportioning valve

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

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.

BRAKE LIGHT SWITCHBRAKE LIGHT SWITCH

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

Brake Light SwitchBrake Light Switch• A brake light switch can be called:

– Stop light switch– Brake pedal position (BPP) switch

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

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

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

Figure 95-23 Typical brake light switches.

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