Antilock Brakes, Vehicle Stability Control, and Power Assist

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Antilock Brakes, Vehicle Stability Control, and Power Assist

Chapter ObjectivesAt the conclusion of this chapter you should be able to:

Explain the purpose and operation of antilock brake and traction control systems.

Identify components of the antilock brake and traction control systems.

Depressurize the antilock brake system.

Identify the types of power brake assist.

Check the power brake assist operation.

KEY TERMSaccumulator

EBCM

electrohydraulic unit

hold mode

hydraulic assist

pressure differential

pressure increase mode

release mode

tire slip

vacuum assist

vacuum check valve

wheel speed sensors

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Copyright 201 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.

386 Chapter 16 • Antilock Brakes, Vehicle Stability Control, and Power Assist

Antilock brakes have been widely available as an option on many vehicles since the 1980s and have

become standard equipment on passenger cars and light trucks. Antilock brake systems operate in conjunction with the regular service brakes. They use a computer and sensors to monitor wheel speeds, and if necessary, to take control of brake the application. This prevents the tires from locking up and causing a loss of vehicle control. Vehicle stability control systems are designed to work with the antilock brake system to reduce oversteer and understeer, and to reduce engine torque to assist in main-taining vehicle control.

ABS/VSC Principles and OperationAntilock brake systems (ABS) and vehicle stability control (VSC) systems have become standard equip-ment on modern cars and light trucks sold in the United States. Although ABS has been in use for many years, the requirement to include ABS and VSC as standard equipment is mandated for all passenger cars and light trucks with the 2012 model year. Because of the regu-lation requiring ABS and VSC, you will need to under-stand how these systems operate.

ABS PrinCiPleSContrary to popular belief, the purpose of ABS is not to make the car or truck stop in the shortest possible distance. The purpose of ABS is to enable the driver to maintain steering control during braking in the event of wheel lockup. This is done by the ABS pumping the brakes for the driver. An average driver may be able to pump the brakes two to three times per second, while the ABS system can pump the brakes over a dozen

times per second. This allows the ABS to pump and release the brakes to prevent the tires from staying locked up.

The ability of the ABS is subject to the limits of physics. If you are driving on a wet road in a non-ABS-equipped car and an obstacle, such as a fallen tree, blocks the road ahead, overbraking that locks the front wheels will cause the car to continue in a straight line regard-less of the position of the steering wheel. Because the ABS allows the wheels to continue turning, the driver can steer around the obstacle. However, if the same sce-nario is repeated on an ice-covered road, the ABS cannot provide sufficient braking if there is no traction between the tire and the road. In this case the car may continue in a straight line, and into the tree, regardless of whether the ABS controls wheel speed.

The amount of traction between the tire and the ground is called tire slip. A freely rolling wheel has zero tire slip, while a locked wheel moving over the pavement has 100 percent slip. This is illustrated in Figure 16-1 and Figure 16-2. Maximum braking occurs with the wheel rotating. The antilock system, by moni-toring wheel speed and controlling hydraulic pressure to the wheel brakes, can maintain a low tire slip rate, typi-cally below 20 percent, which keeps the wheel rotating and not skidding over the pavement.

Antilock brake systems use sensors, called wheel speed sensors, to monitor the rotational speed of each wheel. These sensors provide this information to an electronic brake control module, often called an EBCM. The EBCM controls the operation of an electrohydrau-lic unit, which contains electric motors, solenoids, and valves that are used to control the flow of brake fluid to each wheel brake.

A is the distance covered without slipB is the distance slippedC is the total stopping distance

C

A B

Wheel

Brakingstart point

Brakingstop point

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Figure 16-1 An illustration of wheel slip. A tire with zero slip is rolling freely, a tire with 100 percent slip is locked. A wheel slowing faster than the other wheels has a slip rate between zero and 100 percent.

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Chapter 16 • Antilock Brakes, Vehicle Stability Control, and Power Assist 387

ABS Components. Although there are many types of ABS systems found on modern cars and trucks, most share many of the same types of components and oper-ate in similar ways. The typical ABS system contains the following components:

• EBCM—This is the control unit for the ABS system. An example is shown in Figure 16-3. Data from the wheel speed sensors is used by the EBCM to determine if one or more wheels are rotating at a lower speed than other wheels.

Figure 16-2 The ABS monitors wheel slip rate through the wheel speed sensors.

cL

25 percent slip rateat 60 mph equals 540revolutions permintue.

Zero percent slip rateat 60 mph equals 720revolutions perminute.



A 28-inch-diameterwheel/tire willspin about 720times per mile. Ifthe wheel/tire islocking up, it willspin fewer timesper mile. In thisexample, 25% or180 times less.(720 * 25% = 180)

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Figure 16-3 An example of an ABS unit.

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LF wheelspeed sensor

LR wheelspeed sensor

RF wheelspeed sensor

RR wheelspeed sensor

Stopswitch

Stop/hazard

fuse

EBCMIGN

19

LR w

heel

spee

d se

nsor

RF

whe

elsp

eed

sens

or

RR

whe

elsp

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sens

or

LF w

heel

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Bra

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appl

ied

18 16 17 24 22 21 20 25

B

A

S

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B

A

S

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B

A

S

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B

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Figure 16-4 An illustration of the wheel speed sensors connected to the ABS control unit.

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Figure 16-5 This ABS electrohydraulic unit is attached directly to the master cylinder.

An illustration of the EBCM and speed sensors is shown in Figure 16-4. Based on wheel speed input and its software, the EBCM commands the electrohydraulic unit to hold, open, or close the wheel brake hydraulic circuits. The EBCM typ-ically has self-diagnostic capability, meaning it can determine faults in system operation, provide diagnostic data, and store and provide diagnostic trouble codes (DTCs).

• Electrohydraulic unit—This usually contains electric motors, solenoids, and valves that con-trol the flow of brake fluid to the wheel brakes based on input from the EBCM. The electrohy-draulic unit can be mounted to the master cylin-der, as in Figure 16-5, or mounted remotely, as in Figure 16-6.

• Wheel speed sensors (WSS)—Two types of wheel speed sensors are currently in use, analog and digi-tal sensors. Analog sensors consist of a permanent magnet and a winding of wire. This type of sensor produces an AC voltage signal as the teeth on a tone ring pass by the sensor. An example of this type of

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Chapter 16 Antilock Brakes, Vehicle Stability Control, and Power Assist 389

sensor is shown in Figure 16-7. As the tone ring passes the sensor, it produces positive and negative voltage within the sensor, as shown in Figure 16-8.These pulses are AC voltage. The faster the wheel rotational speed, the more pulses are generated each second. The EBCM interprets these pulses as wheel speed. Some vehicles use magnetoresistive (MR) sensors. MR sensors produce a digital DC voltage signal. These sensors can also produce signals at very low speeds and can differentiate which way the wheel is rotating. Regardless of which sensor is used, both provide a signal to the EBCM each time a segment of the tone ring moves past the sensor. An example of a tone ring is shown in Figure 16-9.The EBCM measures the frequency of each sensor’s output and determines the wheel speed.

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FIGURE 16-6 Many ABS units are mounted remotely.

Reluctor

Inductioncoil

ALNICOpermanent

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FIGURE 16-7 An inside view of a wheel speed sensor. The tone ring induces a magnetic field into the coil as it passes the sensor.

Output

Permanentmagnet

Gear pulser

At high speed

At low speedZerovoltage

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FIGURE 16-8 The magnetic field in the sensor creates an AC voltage, which produces positive and negative pulses. These pulses are read by the computer and converted into wheel speed data.

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FIGURE 16-9 An example of a wheel speed sensor and reluc-tor or tone ring.




Operation of the ABS. There are many different antilock brake systems in use on modern cars and trucks from several different manufacturers, such as Bosch, Teves, Bendix, Kelsey-Hayes, and Delphi. Each system has specific operating, diagnosing, and servicing pro-cedures. The following is a general explanation of ABS operation. For specific systems, refer to the service and repair information from the vehicle manufacturer.

ABS without VSC. On vehicles with ABS but without VSC, the ABS remains in a passive or rest state until the brake pedal is depressed. When the EBCM detects input from the brake pedal switch, it begins to actively monitor wheel speed data from the wheel speed sensors (WSS). If the wheel speed data is consistent with normal decel-eration, then no action is taken. If the WSS data shows one or more wheels are decelerating much faster or have stopped rotating, as indicated in Figure 16-10, then the ABS will act to correct the condition. How the ABS con-trols each wheel brake depends on the type of system.

In general, there are three modes of operation: hold, release, and pressure increase.

• Hold mode, also called isolate mode, limits any further pressure increase in a wheel brake circuit. Figure 16-11 shows how pressure is held.

• Release mode, also called pressure decay or dump mode, is used when the EBCM senses that even after holding pressure to a wheel brake, the wheel is still slowing too rapidly. The EBCM commands the pres-sure in the circuit be released so that the wheel can begin to rotate again. Figure 16-12 shows release mode. Pressure increase mode is used to reapply pressure to the brake circuit to slow the wheel again. Figure 16-13 shows how pressure is reapplied.

This cycle of hold, release, and apply allows a wheel to unlock and resume its rotation and allows the system to reapply brake pressure to slow the wheel again. This cycle takes place very rapidly, a dozen or more times per second during an ABS event.

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Figure 16-10 This illustrates the signals from the four wheel speed sensors. One wheel is spinning much slower than the other three.

Towheel

cylinder

Frommastercylinder

ISO/HoldSolenoid activated

DumpSolenoid activated

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Figure 16-11 In hold mode, pressure from the master cylinder is blocked to prevent the wheel from locking.

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ABS with VSC. Vehicles with vehicle stability control, (VSC), use the ABS to monitor wheel speeds to deter-mine if a wheel or wheels are rotating faster than others. A detailed explanation of the VSC is covered later in this chapter.

Safety Precautions. Since the ABS system uses fluid under high pressure to rapidly pump the brakes, caution must be used when servicing the brake system. Never open a hydraulic line or bleeder screw with the ignition on, as this can cause injury from the release of high-pressure fluid. Always follow the manufacturer’s service precautions and procedures when you are servic-ing the brake system and ABS system.

Integral ABS systems require that the high-pressure accumulators be discharged before any service is per-formed. This is usually accomplished with the iginition off and by pumping the brake pedal at least 40 times. Failure to fully discharge the pressure can result in

damage to the ABS system and injury if the system is opened.

ABS SerViCeThe antilock brake system itself does not require normal service. When problems occur, it is often the result of a component that is either not functioning correctly or is not working at all. When this happens, diagnose the fault and replace the component. Problems can also be caused by damaged wiring and connections. To ensure the actual cause of a concern is located, an inspection of the system and careful diagnosis is needed.

When you are performing service on the service brakes, an inspection of ABS components should be made. In addition, though not technically an ABS component, brake fluid condition can affect the ABS components and operation. Because of this, some vehicle manufacturers recommend periodic brake fluid replace-ment as part of an inspection and maintenance program.

To wheel cylinder

From mastercylinder

Accumulator and pump

Inlet Outlet

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Figure 16-12 In dump mode, pressure from the wheel brake is released by the ABS to allow the wheel to spin.

To wheel cylinder

From mastercylinder

Accumulator and pump

Inlet Outlet

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Figure 16-13 As the wheel starts to spin again, the brakes need to reapply. This is called pressure increase mode.

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Flushing and Bleeding the ABS System. On many cars and trucks, the brake system can be flushed and bled using the same methods as used on non-ABS-equipped vehicles.

Because the brake fluid absorbs moisture and becomes contaminated with copper, it needs to be periodically flushed and replaced. This is especially important on ABS-equipped vehicles because the solenoids, motors, and check valves operate within very small tolerances. Moisture in the fluid leads to rust and corrosion for-mation, which can cause the close-fitting parts of the ABS electrohydraulic unit to stick or seize. Many vehi-cle manufacturers now specify brake fluid replacement intervals to prevent damage to the hydraulic system and ABS components. In most cases, flushing the hydraulic system on an ABS-equipped vehicle can be done using a power bleeder, vacuum bleeder, or manual bleeding methods, just as on non-ABS vehicles. It is important to refer to the manufacturer’s service information for spe-cific procedures before you work on the ABS system.

On ABS systems that use high-pressure accumulators, the pressure must be discharged before any service or bleeding is performed. Follow the manufacturer’s ser-vice guidelines to relieve the system pressure, and follow the flushing and bleeding procedures exactly to prevent damage to the ABS system.

Some vehicles will require the use of a scan tool to bleed the ABS system properly. Figure 16-14 shows some of the steps in bleeding a General Motors vehicle while using a scan tool. Flushing and bleeding the brake system is discussed in detail in Chapter 11. Once the sys-tem is flushed and bled, ensure that the fluid is full and test-drive the vehicle. The brake pedal should be firm and have adequate reserve. If the pedal is spongy, air may be trapped in the system, requiring you to rebleed the brake system.

Traction Control and Vehicle Stability Control SystemsTraction control systems, in various forms, began to appear on vehicles in the 1990s. The range of functions of the types of traction control (TC) systems varied widely based on the type of vehicle. Traction control has since evolved into vehicle stability control (VSC) systems. VSC provides increased monitoring abilities and control over the vehicle to allow the on-board computer system to actively manage certain functions to help the driver maintain control.

TrACTiOn COnTrOl SySTemSEarly traction controls systems were often very limited. Some General Motors products utilized a Low Traction warning light that would illuminate if the ABS system detected that the drive wheels were rotating faster than the nondrive wheels. While not really a traction control system, it did alert the driver so that he or she could adjust for a low-traction condition. Other systems inte-grated the ABS and powertrain control systems to use the brakes, engine, and transmission to reduce wheel spin.

Many traction control systems were essentially soft-ware add-ons to the ABS system. Since the ABS was already used to monitor wheel speeds when braking, by revising the software, the ABS began to monitor wheel speeds during regular driving. By doing this, the ABS could detect if a drive wheel had a faster rotation speed than other wheels. This is illustrated in Figure 16-15. If this occurred, the ABS could then apply the brake on the wheel with reduced traction.

Other TC systems use a more active approach to reduc-ing unwanted wheel spin. On some vehicles, if excessive wheel spin is detected, the powertrain control module reduces engine power by cutting back fuel delivery, retarding ignition timing, and upshifting the transmis-sion to reduce torque to the drive wheels. When a vehicle with this ability is on ice or a similar low-traction surface, even with the throttle wide open, the engine speed drops to idle and power to the drive wheels drops to near zero, this stops the wheel spin and allows the tires to regain traction. Another benefit of TC systems is that, by reducing wheel spin, performance cars can accelerate even faster if the drive wheels are not subject to excessive spinning at takeoff.

VehiCle STABiliTy COnTrOl (VSC)An outgrowth of TC is vehicle stability control or VSC. VSC systems utilize traction control functions but add monitoring of steering input and yaw rate. In 2006 the National Highway Traffic Safety Administration (NHTSA) issued a proposal to phase in VSC on all new vehicles under 10,000 lb gross vehicle weight (GVW). Starting in 2008, with full compliance by 2012, all new

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Figure 16-14 Bleeding some antilock systems requires using a scan tool.

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vehicles sold in the United States weighing less than 10,000 lb GVW will have had VSC as standard equip-ment. The rationale is that VSC can save approximately 10,000 lives per year. This number represents fatal inju-ries from single-vehicle accidents and rollover accidents from SUVs where a VSC system can help the driver to maintain vehicle control and prevent the accident from occurring.

VSC Components. To comply with the VSC stan-dard, vehicles covered under the rule must meet the following requirements:

brake torque. This necessitates the use of an ABS system to monitor wheel speed and to control wheel brake operation.

by monitoring steering input and wheel speeds. For example, when oversteer occurs, the rear of the vehicle starts to break loose, which can cause a spin-out. By monitoring steering angle, the VSC system can determine where the driver is attempting to point the vehicle. Inside of the vehicle, yaw sensor data shows where the vehicle is actually headed. As the vehicle turns, the inside and outside wheels rotate at different speeds. If the speed difference is excessive, the computer can apply the brakes and reduce engine torque as necessary to reduce the oversteer, allowing the driver to regain control.

change in rotation from around the vehicle’s center of gravity, shown in Figure 16-16

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FIGURE 16-15 Traction and stability control systems use the wheel speed sensors to monitor wheel speed at all times. This illustration shows one wheel spinning faster than the others, indicating a low-traction condition.

X axis islateralacceleration

Z axis is along the thrust angle

Y axis is yaw

The Z axis is along the thrust angle and represents linear acceleration anddecleration.The X axis is the sideways lateral acceleration when cornering.The Y axis is represented by yaw, and is the rotational force around theX axis.

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FIGURE 16-16 Yaw is motion around the vehicle’s center.




Angle at which the tiresare pointed

Slip rate is theangle betweenthe tire’s actualdirection andthe direction itis pointed. Asshown here,understeer isthe effect.

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Figure 16-17 Slip angle is the difference between where the tires are pointed and the actual direction of travel.

installed at the center of gravity, often in the cen-ter console of the vehicle. Slip rate is similar to yaw but is measured as the difference between the lateral (sideways) speed and the longitudinal (along the cen-terline of the vehicle) speed. Slip rate is shown in Figure 16-17. Vehicles use a lateral acceleration sen-sor, as shown in Figure 16-18, to measure slip rate.

• Monitorsdriver steering input.Asteeringanglesensor and, on some models, a steering torque sensor are used to determine where the driver is trying to point the vehicle. Steering angle sensors are mounted on the steering column, as shown in Figure 16-19. Can modify engine torque. This allows the powertrain control module (PCM) to modify fuel delivery, spark timing, and transmission

gearing to reduce or increase torque to the drive wheels as necessary.

What Does that Light on My Dash Mean? Since both traction control (TC) and vehicle stabil-ity control (VSC) systems rely on the antilock brake system for wheel speed input, a failure of a wheel speed sensor will disable the ABS and TC/VSC sys-tems. This means that many times the ABS and TRAC warning lights on the instrument panel will illuminate when a fault is present, as shown in Figure 16-20. The service brakes still remains working but without ABS function, and the TC/VSC system will be unable to provide assistance to the driver if the situation should arise.

XXXXXXX

Switchpart numberArrow indicates front of

switch for proper mounting

Mercuryswitches

G1

G2G3

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Figure 16-18 An illustration of a lateral acceleration sensor, used to determine vehicle motion.

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remain illuminated on the instrument panel with the engine running. If the ABS and TC/VSC lights are illu-minated, begin by connecting a scan tool to the data link connector (DLC) to check for diagnostic trouble codes (DTCs). Not all vehicles provide ABS/VSC data through the standard OBDII computer connector located under the driver’s side of the dash; you may need special adap-tors to connect to the ABS/VSC data link connector.

If there are stored codes, you will need to diagnose what caused the code to set. A very common problem with ABS/VSC systems is faulty wheel speed sensors (WSS). If a code or codes are set for WSS circuits, inspect the wiring and connections for the sensors. With a scan tool connected, you will probably be able to examine ABS/VSC data, including individual wheel speeds. Raise the vehicle so the tires are off the ground, and spin each tire while you watch the WSS data. If no wheel speed is indicated, the sensor and/or its wiring are likely the cause. Detailed testing requires locating the appropriate service information for the vehicle. Follow the vehicle manufac-turer’s diagnosis and testing procedures to determine the cause of the fault. Vehicles with unwanted ABS activa-tion may not have any stored trouble codes. A common cause of this complaint is rust buildup around the WSS in the wheel hubs. The rust buildup creates false speed data, which is interpreted by the EBCM as a difference in wheel speed. This causes the EBCM to activate and attempt to control a slipping wheel. The sensor and tone ring may be able to be cleaned and reused. Severe rusting requires replacement of the sensor or hub assembly.

Power Assist Types and ComponentsThere are two types of external power brake assist systems, vacuum assist and hydraulic assist. Vacuum assist uses vacuum supplied by the engine or vacuum pump. Hydraulic assist is supplied by the power steering system to a hydraulic power brake booster. Some vehicles use an integral brake master cylinder, ABS, and power assist unit. The most common of these is the vacuum assist since vacuum is readily available from gasoline-powered engines. Hydraulic assist is often used on diesel-powered vehicles. However, there are exceptions to this as some gasoline-powered vehicles use hydraulic-assisted brakes, and some diesel-powered vehicles have a vacuum pump to operate a vacuum booster.

VACuum ASSiSTAs discussed in Chapter 4, vacuum is air pressure that is less than atmospheric pressure. A pressure/vacuum chart is shown in Figure 16-21. Because pressure moves from high to low, this movement can be used to generate force, which is applied to the brake pushrod to reduce the amount of force supplied by the driver.

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Figure 16-19 An example of a steering angle sensor. These sensors provide information about driver input to the computer. This information, combined with that from other sensors, is used to determine how to correct for unwanted vehicle motion.

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Figure 16-20 An example of warning lights for the ABS and traction control.

Unwanted ABS activation is a common customer com-plaint, especially during low speed stops. The ABS fault light may flash when the problem occurs. To confirm the complaint, drive the vehicle on dry pavement and perform several slow speed stops, from about 15 mph, and note if the ABS activates. When the ABS is active, the brake pedal will pulsate quickly and a rapid tap-ping or knocking sound occurs that coincides with the pulsating pedal. If there is unwanted ABS activation, you will need to inspect the brake system.

Inspection of the TC/VSC systems generally begins with observing if any malfunction indicator lamps

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Components. Vacuum for the assist unit is supplied by the engine or by a vacuum pump. Gasoline-powered engines produce vacuum in the intake manifold, as shown in Figure 16-22. Typically, a large-diameter vacuum hose connects the assist unit to the intake manifold, shown in Figure 16-23 and Figure 16-24. A vacuum check valve is located either in the vacuum hose or at the connection to the booster assembly. Figure 16-25 shows a check valve installed at the booster. The check valve allows vacuum to the booster but stops the release of vacuum from the booster back to the engine. This allows the booster to store a vacuum reserve to provide assist for a couple of pedal applications in the event that the engine stalls. An illustration of the check valve is shown in Figure 16-26.

Engines without throttle plates, such as diesels and some BMW engines, use a vacuum pump to supply vacuum to the assist unit. Vacuum pumps may be belt driven, gear driven, cam driven, or electric. An example of a belt-driven vacuum pump is shown in Figure 16-27.

The vacuum power booster is bolted to the firewall in the engine compartment. The brake master cylinder is attached to the front of the booster. The brake pushrod

29.9"

20"

10" Vacuum

0"0 psi

5 psi19.7 psi

14.7 psi

10 psi

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auge

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Figure 16-21 Vacuum is pressure that is less than atmospheric pressure.

Spark plug

Intakevalve

Exhaustvalve

Combustionchamber

PistonCrankshaft

Fuelinjector

Air-fuelmixture

INTAKE STROKE

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Figure 16-22 Vacuum is formed as the piston moves down in the cylinder. This reduces the pressure in the intake manifold.

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Intake manifold

Vacuum hose

Master cylinder

Vacuumbooster

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FIGURE 16-23 An illustration of a vacuum booster connected to an engine.

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FIGURE 16-24 An example of a vacuum booster and mastercylinder.

FIGURE 16-25 The vacuum check valve traps vacuum in the booster. This allows for a couple of assisted stops in the event the engine stalls.

Seat Disc valve

Spring

Todiaphragm

housing

Tointake

manifold

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FIGURE 16-26 An inside view of a vacuum check valve.

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FIGURE 16-27 An example of a vacuum pump.©

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Master cylinder

Brake pedal

mountingbracket

Brakepedal

assembly

Brakepedal

Bulkhead

Frontbulkhead

Power brakebooster unit

Clevis

Pushrod

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Figure 16-28 This illustrates the connection of the brake pedal to the vacuum booster.

Floatingcontrolvalve

Air valve

Airpassage

Filter

Valve seat

Powerpistonreturnspring

Reactionretainer

Reactionplate

Reactionlever

Air valvespring ©

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Figure 16-29 An illustration of the inside of a vacuum booster.

connects to the booster under the dash. An illustration is shown in Figure 16-28.

Inside the vacuum booster is a large rubber diaphragm, power piston, return spring, and control valve to control booster operation. The pushrod at the front of the power piston applies the brake force to the primary master cyl-inder piston. Figure 16-29 illustrates the construction and components of a vacuum booster. Figure 16-30 shows the components of a typical vacuum booster assembly that has been disassembled. In vehicles where space limitations pre-vent the vehicle manufacturer from using a large- diameter vacuum booster, a tandem- or dual- diaphragm booster may be used. An illustration of a dual-diaphragm booster is shown in Figure 16-31. A dual-diaphragm booster uses two small-diameter diaphragms to achieve a larger surface area and provide power assist just as a single large- diameter diaphragm booster does. However, even though the dual-diaphragm booster is smaller in diameter, it is also longer since it is using two chambers.

Operation. A vacuum brake booster operates using the principle of pressure differential. Since pressure,

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Front housing

Front diaphragm

plate

Front diaphragm

Rear housing

Reaction disc C o n t r o 1

valve

Filter

Rear diaphragm

Rear plate diaphragm

FIGURE 16-31 A tandem booster uses two diaphragms of smaller diameter instead of one large diaphragm.

like temperature, moves from high to low, the booster can provide a mechanical advantage by using atmo¬spheric pressure and vacuum.

Figure 16-32 shows an illustration of a typical vac¬uum booster with the brakes released. During normal operation with the engine running, vacuum is present

on both sides of the booster diaphragm. Since the pres¬sure is equal on both sides, the diaphragm remains in place. When the brake pedal is pressed, the pushrod moves a valve off its seat, allowing atmospheric pres¬sure to enter the rear of the diaphragm chamber. This is shown in Figure 16-33. Since the pressure at the rear of

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Reaction Reaction P ' a t e

FIGURE 16-32 With the brakes unapplied, vacuum is present on both sides of the diaphragm. No movement or assist is provided because of the equal pressure.

FIGURE 16-33 When the brakes are applied, atmospheric pressure pushes on the back of the diaphragm, which applies force to the pushrod and master cylinder.



the diaphragm is higher than the vacuum at the front of the diaphragm, the diaphragm flexes forward. The move-ment of the diaphragm forces the pushrod into the master cylinder with greater force than that which was applied by the driver only.

The force applied to the pushrod is determined by the surface area of the diaphragm and the pressure applied to each square inch of the diaphragm’s surface. For exam-ple, the surface area of the booster in the diaphragm in Figure 16-34 is 144 square inches. The pressure on the diaphragm can be calculated by subtracting the pressure on the vacuum side from atmospheric pressure. If 10 inches of manifold vacuum is supplied to the booster, and from

Figure 16-21 we know that 10 inches of vacuum is equal to 10 psi of absolute pressure, then the pressure on the dia-phragm is equal to 4.7 psi (14.7 - 10 = 4.7). The result is 4.7 psi difference times the surface area of 144 square inches equals slightly less than 677 pounds of force.

As you can see, the vacuum brake booster is capable of providing a significant increase in brake application force over what the driver alone can produce. Because of this increased force, brake pedals on vehicles with power assist will be shorter than those on vehicles with-out power assist. Nonpower assist vehicles have longer brake pedals and a greater brake pedal ratio, as discussed in Chapter 10.

Powerpiston

Filter

Returnspring

Housingdivider

Reactionretainer

Pistonrod Front housing

Front housingseal

Vacuum checkvalve

Grommet

6.77 * 6.77 * 3.14 = 143.9square inches of surfacearea

Radius6.77 inchesPower piston

bearing

Diaphragmretainer

Primarydiaphragm

Primarysupport plate

Secondarypiston

bearing

Silencer

Silencer

Boot

Rearhousing

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Figure 16-34 The amount of assist provided by the booster depends on the size of the diaphragm and the difference in pressure.

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hydrAuliC ASSiSTThere are two types of hydraulic assist systems, which are generally referred to by their product names, the Powermaster and Hydro-boost power brake systems. These are used because some vehicles, due to space limitations or to their diesel-powered engines, cannot use a vacuum power assist unit. Instead, these vehicles generate power brake assist by using hydraulic fluid pressure.

Powermaster Assist. The Powermaster system is a self-contained master cylinder and power brake booster assembly that uses the brake fluid for power assist as well as standard hydraulic brake function. The Power-master unit, as shown in Figure 16-35, has an electrically operated pump, an accumulator, pressure switch, and master cylinder.

The accumulator stores brake fluid under pressure, which is used to provide the power assist. The pump is activated when the key is turned on, and it maintains the pressure in the accumulator, which is approximately 500 psi. The pressure switch is used to turn the pump on when accumulator pressure drops below a speci-fied amount. When the brakes are applied, pressurized fluid from the accumulator acts upon the power piston

at the rear of the master cylinder, providing power assist. When the brakes are released, fluid returns to the reservoir.

One advantage of this system is that if the engine stalls, as long as the key is on, the pump can generate pressure and the system can maintain power assist.

Hydro-Boost. Hydro-boost is the name of the hydraulic brake assist used on General Motors prod-ucts, although similar systems have been used by Ford and other manufacturers. Unlike the Powermaster sys-tem, Hydro-boost operates by using the power steering system to provide assist for the brakes. Because of this, Hydro-boost-equipped vehicles cannot maintain brake assist if the engine stops running. A Hydro-boost system is shown in Figure 16-36.

Components. The power steering pump supplies the hydraulic pressure for the Hydro-boost system. The Hydro-boost unit is mounted between the firewall and the master cylinder, as shown in Figure 16-37, and is connected to the power steering pump and power steer-ing gearbox via the power steering hoses.

Operation. When the brakes are not applied, fluid enters and exits the Hydro-boost unit, as shown in

Accumulatorball

Reservoir Pressureswitch

Mastercylinder

Electricmotor

Pump

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Figure 16-35 This type of power assist uses an electric pump and accumulator. The pump runs to generate pressure and the accumulator stores brake fluid under high pressure.

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FIGURE 16-36 This system uses the power steering pump to supply high-pressure fluid for power brake assist.

Brake pedal

FIGURE 16-37 An inside view of a Hydro-boost assembly.

Figure 16-38. When the brakes are applied, the spool valve closes the fluid return port, building pressure in the power chamber. This exerts force on the power piston, providing brake assist as shown in Figure 16-39.

• Vacuum Booster Inspection. A visual inspection of the vacuum brake booster should include checking the

vacuum supply hose, the vacuum check valve, and for leaks around the master cylinder.

The booster's vacuum supply hose is a large-diameter hose connected to the engine. Inspect the hose and valve for vacuum leaks with the engine running at idle. A vacuum leak, especially a large leak, sounds like a hiss or whistle. A hissing sound inside of the passenger compartment when



Pumppressure

Spool valve

Non-pressurized fluid

Spool valve

Power piston

Fluid

Body

Seals

(Lowpressure)Return toreservoir

(Pumppressure)Steering

gear

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Figure 16-38 When the brakes are not applied, fluid to the booster simply returns to the power steering system.

the brakes are not applied is often due to a leaking power assist unit. Listen for a vacuum leak around where the brake pushrod goes into the booster, as shown in Figure 16-40.

The vacuum valve keeps vacuum trapped in the booster. This is so that if the engine stalls, there will be sufficient vacuum stored in the booster to allow power assist to stop the vehicle. To check the vacuum valve, run the engine for a minute, shut the engine off, and carefully remove the valve from the booster. If there is a rush of air when the valve is removed, then it is working properly. Another way to test the valve is to run and then shut off the engine. Pump the brake pedal a couple of time. If the

effort to press the pedal is normal for two or three appli-cations, the check valve is working.

Hard Brake Pedal and No Power Assist. When a vacuum booster develops an internal leak, or if the vac-uum supply to the booster is insufficient, the customer complaint is typically of a very hard brake pedal requir-ing significantly increased effort to apply the brakes and increased stopping distances. These symptoms are some-times accompanied by a hissing noise from under the dash, or in the event of a vacuum leak to the booster, a hiss from the engine compartment. Sometimes, a leaking

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(Restricted (No flow) flow) Return to Pump Steering reservoir pressure gear

Spool valve

Power cavity

Pressurized fluid

Output pushrod

Pedal rod

Power piston

| | Spool valve

[ / J Power piston

I I Fluid

Reaction rod

I I Body

Seals

FIGURE 16-39 With the brakes applied, fluid is routed to the rear of the power piston to provide power assist.

booster creates enough of a vacuum leak to cause the engine to run rough, stumble, or hesitate. This is because the vacuum leak affects the amount of air entering the engine. Too much air for the fuel supplied by the fuel system causes a rough idle or running condition.

When the diaphragm inside the booster ruptures, vac¬uum will occur on both sides of the diaphragm. When the brakes are applied, atmospheric pressure cannot build in the rear of the chamber due to the ruptured dia¬phragm. This means the pressure on both sides of the diaphragm is equal, and consequently, no power assist is provided.

• Check Vacuum Booster Operation. With the engine off, pump the brake pedal several times until the pedal is very hard to press. Keep your foot on the brake pedal and start the engine. The pedal should drop a couple of inches once the engine starts. This indicates vacuum is supplied to the booster, and the booster is working normally. If the pedal remains very hard, check the vacuum to the booster. If the vacuum is sufficient, then the booster is defective.

• Hybrid Vehicles and Integral ABS Units. Some vehicles use a combination master cylinder, brake booster, and ABS unit. An example is shown in Figure 16-41.

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Master cylinder

A ruptured diaphragm usuallyresults in a vacuum leak into thepassenger compartment whenthe brakes are applied. Thiscauses a hiss when the brakepedal is pressed.

Frontbulkhead

Power brakebooster unit

Pushrod

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Figure 16-40 A leaking vacuum booster will have a hard brake pedal. Sometimes, the leak-ing vacuum can be heard inside the passenger compartment as a hiss with the brakes released.

Pump andmotor assembly

Returnhose

High-pressureline

Pressureswitch

Master cylinderand booster

Main valvesolenoid

connector

Valve block

Accumulator

Fluid levelswitch

Reservoir

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Accumulato

Figure 16-41 Some vehicles use an integral master cylinder, ABS, and power assist unit. This combines all three functions into one unit.

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These units, like the Powermaster system, use electric pumps to supply pressure for the system. One significant difference is that integral systems store brake fluid under very high pressure in accumulators.

Because integral units store fluid under high pressure, the system pressure must be discharged before work is performed. With the ignition off, pump the brake pedal at least 40 times to discharge the accumulators. The pedal should become very firm once the system is depressur-ized. Do not turn the ignition on while you are working on the hydraulic system as this can cause the pump to operate and build pressure in the system.

On Toyota Prius models, any time the battery is con-nected, even if the power switch is off, the brake control system activates when the brake pedal is depressed. The system also activates when any door switch turns on.

Do not operate the brake pedal or the doors while the battery is connected and you are servicing brake system components.

Some vehicles use a backup power source for the brake system. This often consists of capacitors that store a 12-volt charge in the event the regenerative braking becomes inoperative. This allows the brake system to function long enough to slow and stop the vehicle if a malfunction occurs with the 12-volt battery supply. If the backup power source voltage is depleted, the brake sys-tem will continue to function as a conventional hydrau-lic brake system. Because some models use capacitors as power backup, there may be an electrical charge left in the brake control system. Do not try to service the brake control system or the backup power supply until the capacitors have discharged.

SUMMARY

The ABS system prevents the wheels from staying locked by rapidly pumping the brakes for the driver.

The amount of traction between the tire and the ground is called tire slip.

Wheel speed sensors monitor the rotational speed of each wheel.

The EBCM commands the electrohydraulic unit to hold, open, or close wheel brake hydraulic circuits depending on wheel slip.

The electrohydraulic unit contains electric motors, solenoids, and valves that control the flow of brake fluid to the wheel brakes based on input from the EBCM.

Hold mode, also called isolate mode, is used to limit any further pressure increase in a wheel brake circuit.

Release mode, also called pressure decay or dump mode, is used when the wheel is still slowing too rapidly.

Pressure increase mode is used to reapply pressure to the brake circuit to slow the wheel.

Never open a hydraulic line or bleeder screw with the ignition on as this can cause injury from the release of high-pressure fluid.

Integral ABS systems require that the high-pressure accu-mulators be discharged before any service is performed.

Some vehicles will require the use of a scan tool to properly bleed the ABS system.

REVIEW QUESTIONS

1. A vacuum is used to keep a reservoir of vacuum in the power brake booster in the event the engine stops running.

2. If a wheel locks during braking, wheel speed sensor data will show a in the number of rotations for the wheel.

3. ABS units that combine functions of the master cylinder, ABS control, and power assist are called

systems.

4. Traction control systems are designed to reduce to slipping drive wheels.

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408 Chapter 16 • Antilock Brakes, Vehicle Stability Control, and Power Assist408 Chapter 16 • Antilock Brakes, Vehicle Stability Control, and Power Assist

8. Which of the following is not a common ABS configuration?

a. Rear wheel antilock c. Front wheel antilock

b. Four wheel antilock d. Three wheel antilock

9. A vehicle has illuminated red BRAKE and yellow ABS lights. Technician A says a thorough inspection of the brake system should be performed. Technician B says to check for stored DTCs. Who is correct?

a. Technician A c. Both A and B

b. Technician B d. Neither A nor B

10. Which of the following tools can be used to check AC voltage-generating wheel speed sensors?

a. Oscilloscope c. Digital multimeter

b. Scan tool d. All of the above

5. Vehicles with stability control use multiple to determine vehicle movement.

6. Two technicians are discussing bleeding the hydraulic brake system on an ABS-equipped vehicle. Technician A says special bleeding procedures may be required to purge all of the air from the system. Technician B says some antilock systems require using a scan tool to bleed the system. Who is correct?

a. Technician A c. Both A and B

b. Technician B d. Neither A nor B

7. All of the following are modes of ABS operation except:

a. Pressure increase c. Hold mode

b. Release mode d. Calibration mode


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Antilock Brakes, Vehicle Stability Control, and Power Assist

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