Automotive Braking Systems

MASTER AUTOMOTIVE TECHNICIAN SERIES

SAMPLE CHAPTERS 8 and 12

Automotive Braking Systems

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Nicholas Goodnight Kirk VanGelder

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CONTENTS Chapter 1

Strategy-Based Diagnostics

Chapter 2

Principles of Braking

Chapter 3

Hydraulic Brake Systems

Chapter 4

Disc Brakes Systems

Chapter 5

Drum Brakes Systems

Chapter 6

Refinishing Brake Rotors and Drums

Chapter 7

Parking Brakes

Chapter 8

Power Assist Systems

Chapter 9

Wheel Bearing Service

Chapter 10

Hybrid Vehicle Braking Systems

Chapter 11

Advanced Braking Systems: Electronic Brake Control

Chapter 12 Electronic Stability Control Systems

MASTER AUTOMOTIVE TECHNICIAN SERIES

SAMPLE CHAPTERS 8 and 12

We Support

Education Foundation

Automotive Braking Systems Nicholas Goodnight Kirk VanGelder Automotive Braking Systems, published as part of the CDX Master Automotive Technician Series, teaches students the knowledge and skills they need to effectively maintain, diagnose, and repair braking systems. Employing a “strategy-based diagnostic” approach, it guides students through the basic components of automotive brake design before discussing strategies for navigating challenges in the shop. n Outcome focused with clear objectives, assessments, and seamless coordination with tasksheets n Introduces foundational concepts of braking systems before advancing to more complex diagnostic topics n Covers all types of braking systems—from disc and drum brakes to hybrid vehicle brakes and electronic brake control—in logical, understandable modules n Includes the latest braking technology and innovations, from collision avoidance to regenerative braking and lane assist n Contains detailed illustrations of complex parts and systems to enhance student comprehension n Offers instructors an intuitive, methodical course structure and helpful support tools Pairing essential theory with practical advice from experienced technicians, Automotive Braking Systems prepares students for ASE certification and success in the shop.

About CDX Master Automotive Technician Series Organized around the principles of outcome-based education, CDX Learning Systems prepares students for workplace success by aligning training content and assessments to required professional competencies.

The CDX Master Automotive Technician Series will eventually cover all areas of ASE certification.

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CHAPTER 8

Power-Assist Systems Learning Objectives ■ ■

8-1 Explain vacuum booster operation.

8-2 Summarize the operation of hydroboost power-assist systems.

■

8-3 Diagnose a power-assist system failure.

You Are the Automotive Technician the customer comes in with a 2013 Chevy Silverado 2500hD with a Duramax engine in it.they report that the truck does not seem to have the power assist that it normally has, which is causing the driver to barely stop the vehicle. along with the braking issue, the driver reports needing to increase their steering effort at low speeds. What should the technician check first?

1. the service brakes.

2. the serpentine belt tension and condition.

3. the power steering pump operation.

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Chapter 8 Power-Assist Systems

▶▶ Introduction The need for increased braking pressure was apparent with the introduction of the disc brakes to the automotive world. Previously, the drum brakes on vehicles would ­naturally multiply the force that was applied to brakes since the servo action would help with the application of the brake shoe. The limitations of drum brakes resulted in the design of disc brakes. This progression in design is a direct result of the increasing ­requirements of safety and the National Highway Traffic Safety Administration (NHTSA). The NHTSA regulates the stopping distance of vehicles that operate on roads in the United States. As the ­distance the ­vehicle needed to stop within decreased, the technology to get the vehicle to stop needed to be f­urther developed. With these technologies, there are tradeoffs that must be dealt with as they become apparent, which gave rise to power assist.

▶▶ Vacuum

Booster Operation

As the need for power assist rose, the design for a simple, compact component that would meet the needs of the increased pressure to operate the disc brake system was developed. The vacuum brake booster uses the engine vacbrake booster chamber brake fluid uum created by the operation of the engine to help with in reservoir boosting the force applied to the master cylinder. The term poppet valve master cap spring “vacuum” here refers to any pressure that is lower than atmocylinder reservoir spheric pressure. A ­vacuum booster uses the difference in poppet valve seat atmospheric pressure and the vacuum created by the engine master (FIGURE 8-1). cylinder A vacuum booster uses a diaphragm to isolate ­atmospheric pressure from vacuum pressure. Vacuum or negative pressure is created by the operation of the engine. The negative pressure builds up on the engine side of the booster. The other side secondary primary filter piston piston of the diaphragm fills with atmospheric pressure when the hydraulic brake pedal is depressed. Once the brake pedal is depressed, pushrod primary secondary the atmospheric pressure contained inside the vehicle, which check brake brake valve is positive pressure, floods into the booster, creating a positive circuit circuit pressure on the opposite side of the ­diaphragm (FIGURE 8-2). to engine Once the pressure has built up to the tipping point, it starts to vacuum source push the ­diaphragm toward the negative pressure side, causdiaphragm ing the brake push rod located inside the booster to push the FIGURE 8-1 A vacuum booster at rest with no pressure on it from piston in the master cylinder with the force of the driver’s foot the driver.

8-1 Explain vacuum booster operation.

brake booster chamber contains constant vacuum

brake fluid in reservoir master cylinder reservoir

cap

piston return spring resists vacuum

poppet valve (shown open) allows air into pressure chamber

poppet valve seat follows poppet and closes automatically until brake pedal is moved farther

master cylinder

FIGURE 8-2 A vacuum

booster actuated with the driver pushing the brake pedal, which moves the valve within the booster, allowing atmospheric pressure to build on the opposite side of diaphragm.

secondary piston secondary brake circuit to engine vacuum source

primary piston hydraulic pushrod primary check brake valve circuit

filter

air enters through filter

brake pedal

piston is pushed brake booster pressure by atmospheric chamber contains atmospheric diaphragm ensures pressure, adding force pressure only when poppet valve is to pedal pressure an airtight seal opened by brake pedal movement

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Vacuum Booster Operation

on the pedal and the force of the diaphragm. This is how to gain the mechanical advantage to allow the driver to stop the ­vehicle with less effort.

To help with engine shut off and having power brakes for a few pumps after the engine stops running, a check valve is put on the brake booster so that the negative pressure/ vacuum will not be expended once it is no longer supplied by the engine ­(FIGURE 8-3).

This feature will allow the driver to operate the braking system with assist for a couple of applications.

Dual (Tandem) Diaphragm Booster The normal way to increase the force that a vacuum booster made was to make the booster larger so that the diaphragm had a larger surface area. As vehicles became more aerodynamic and more compact, this theory no longer worked, because of the l­imited space underneath the hood. To increase the force made by the vacuum booster, two smaller diaphragms were placed next to each other so that the total area of the diaphragms was bigger than a larger single-diaphragm booster (FIGURE 8-4).

When the tandem diaphragm booster is actuated, the air valve assembly allows the atmospheric pressure to flow into each diaphragm’s cavity to allow the movement of the master cylinder push rod toward the master cylinder (FIGURE 8-5).

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▶▶TECHNICIAN TIP The vacuum booster will not cause a low pedal on the braking system. Some people blame the power assist for their brake pedal not being where it is supposed to be, when in reality it is the hydraulic system that has an issue. When diagnosing a low brake pedal concern, make sure to verify the reported problem and inspect the foundation brakes before replacing any components.

Vacuum Booster Pushrod Adjustment The vacuum booster’s pushrod is sometimes an adjustable component that must be inspected and adjusted, if it is adjustable, before it is installed in the vehicle (FIGURE 8-6). When replacing the vacuum booster, the technician must make a measurement of how much the pushrod is protruding out of the booster so that they can adjust the new booster to be the same (FIGURE 8-7). Adjusting the push rod is usually as simple as holding the push rod with a pair of pliers and moving the self-locking nut into the required position. If this measurement is not checked and adjusted, the pushrod may push the piston too far into the master cylinder, causing it to bottom out and eventually break. If the push rod is too short after adjustment,

FIGURE 8-3 The check valve allows the booster to retain vacuum

pressure once the engine is off and will also allow the power assist to operate until the vacuum is exhausted.

FIGURE 8-4 The dual-diaphragm vacuum booster has two separate

FIGURE 8-5 The dual-diaphragm (tandem) vacuum booster consists

diaphragms that when combined, are larger than a single, larger vacuum booster. This increases the force that is made inside the booster to facilitate the driver in applying the brakes.

of two boosters that have been sized down to fit together in one housing. This design allows for a more forceful vacuum booster in a smaller space.

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Chapter 8 Power-Assist Systems

FIGURE 8-6 The pushrod on the vacuum booster must be inspected

FIGURE 8-7 Taking a measurement from the old booster will

before the component is replaced so that it is adjusted to the necessary position.

allow the technician to adjust the replacement booster to the same measurement so that no component breakage will happen.

it may not fully apply to the brakes when the driver presses firmly down on the brake pedal, which may cause an accident.

Vacuum Pump Some engines require the use of a vacuum pump, which creates a vacuum to operate the booster and other vacuum accessories (FIGURE 8-8). These types of engines make very little vacuum when they run, because either they have a special application ­camshaft or they are a diesel engine. A diesel engine usually does not use a venturi or butterflies to create the vacuum required to operate a vacuum-powered booster. The vacuum pump can be either engine driven or electric, depending on the application.

Engine-driven vacuum pumps have started to dwindle as electricand hydraulic-assisted power brake units have come into favor by most original equipment ­manufacturers (OEMs).

FIGURE 8-8 A vacuum pump is just a simple pump that creates

negative pressure (vacuum) by rotating the pump, creating suction inside the pump.

Checking a Vacuum-Type Power Booster Unit for Leaks and Inspecting the Check Valve

Vacuum leaks in the power booster require increased driver foot pressure to activate the brakes. Because the vacuum normally comes from the intake manifold, a leaky power booster can affect the operation of the engine by changing the air-fuel mixture. Power boosters can leak internally or externally. Perform the external leak test before the internal leak test to avoid confusion in identifying the cause of the leak. To perform an external leak test, follow these steps: 1. Start the engine, and allow it to run for at least 10 seconds.

2. With the foot off the brake pedal, turn the engine off.

3. Wait at least 10 minutes, and then apply the brake pedal with moderately firm pressure (20–30 lb [9.1–13.6 kg]). Note the feel and pedal reserve height.

4. Apply the brake pedal a couple more times with the same moderately firm pressure.

Each application should result in a higher and firmer brake pedal as the vacuum is released from the booster.

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Vacuum Booster Operation

211

If it does operate properly, then there are no substantial external leaks in the system. If it does not hold vacuum, inspect the booster for external vacuum leaks. The cause could be any of the following: ■■ ■■ ■■ ■■ ■■

the vacuum check valve and/or grommet the front or rear seals of the unit the atmospheric valve at the rear of the booster the case where the halves are crimped together a hole worn or rusted through the case.

Upon verifying that there are no external leaks, move on to the second step, which is to perform an internal leak test: 1. Begin by starting the engine and letting it idle.

2. Apply the brake pedal with firm pressure (30–50 lb [13.6–22.7 kg]).

3. Without moving the foot on the pedal, shut off the engine and observe the brake pedal for approximately one minute.

If the pedal stays steady, there are no internal leaks. If the brake pedal rises, there is an internal leak in the diaphragm, the control valve, or the check valve.

The third step is to perform a test for check valve operation: ■■ ■■

Start the engine, and allow it to run for 10 seconds to evacuate the booster.

Turn off the engine, wait at least 10 minutes, and then remove the check valve from the booster.

There should be a large rush of air into the booster. If there is, the check valve is holding a vacuum and is OK. If there is not, test the check valve by blowing air through it. Air should flow from the booster side of the check valve to the engine side only. If the check valve is OK and the tests indicate a leak, use a stethoscope and listen for leaks around the outside of the booster, including the control valve at the rear of the booster, which is under the dash.

To inspect the vacuum-type power booster unit for leaks and inspect the check valve for proper operation, follow the steps in SKILL DRILL 8-1.

SKILL DRILL 8-1 Checking Vacuum-Type Power Booster Unit for Leaks and Inspecting the Check Valve 1. The first step is to perform an external leak test. Start the engine, and allow it to run for at least 10 seconds. With the foot off the brake pedal, turn the engine off. Wait at least 10 minutes, and then apply the brake pedal with moderately firm pressure (20–30 lb [9.1–13.6 kg]). Note the feel and pedal reserve height. Apply the brake pedal a couple more times with the same moderately firm pressure. Each application should result in a higher brake pedal as the vacuum is released from the booster.

Continued

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Chapter 8 Power-Assist Systems

2. The second step is to perform an internal leak test. Begin by starting the engine and letting it idle. Apply the brake pedal with firm pressure (30–50 lb [13.6–22.7 kg]). Without moving the foot on the pedal, shut off the engine and observe the brake pedal for approximately one minute. If it stays steady, there are no internal leaks. If the brake pedal rises, there is an internal leak.

3. The third step is to perform a test for check valve operation.

Start the engine, and allow it to run for 10 seconds to evacuate the booster.

4. Turn off the engine, wait at least 10 minutes, and then remove the check valve from the booster. There should be a large rush of air into the booster if the check valve is holding a vacuum properly. If there is not, test the check valve by blowing air through it.

▶▶ Hydraulic 8-2 Summarize the operation of hydroboost power-assist systems.

Power-Assist Operation

Hydraulic power-assist brake boosters are usually sold under the name hydroboost. This type of power assist gets its pressure from the power steering pump. This hydraulic pressure is used to assist in the application of the brakes (FIGURE 8-9). This system is usually used on vehicles that create too little vacuum to operate a conventional vacuum booster style of power assist. These vehicles include those that are equipped with a diesel engine or a forced induction version of a gasoline engine. The design of diesel engines does not allow for a lot of vacuum, because of the absence of throttle plates, so if the manufacturer would like to use a vacuum booster in this application, they will have to install a vacuum

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Hydraulic Power-Assist Operation

213

Typical General Motors Hydroboost System Hydroboost to Pump

Master Cylinder

Pump

Hydroboost Unit Pump to Hydroboost

Hydroboost to Gear

Gear to pump

Gear

FIGURE 8-9 A typical

hydroboost system, which consists of a power steering pump, power steering gear, distribution block, hydraulic lines and hoses, and the hydroboost unit.

pump. In a forced induction engine application, the engine does not make enough vacuum as the vehicle accelerates, but it makes more than enough as the vehicle decelerates. This uneven application of ­vacuum could cause a power-assist-related issue, which is why some ­applications use hydroboost.

Hydroboost Operational Stages The hydroboost has three operational stages: unapplied, applied, and hold positions.

■■

Unapplied: The unapplied position is when the brakes are not being applied, the spool valve within the unit is in a neutral position, and the pressure is building up inside the accumulator (FIGURE 8-10). The lever attached to the input pushrod that is connected to the brake pedal keeps the spool valve in a position where the pressure from the power ­steering pump is being redirected to the power s­ teering gear. This is the bypass condition where there is no ­power-assist function happening.

ACCUMULATOR PISTON ACCUMULATOR CAP

RESERVE SYSTEM PRESSURE

2 FUNCTION VALVE BALL CHECK

NITROGEN GAS

PUMP PRESSURE ACCUMULATOR DUMP VALVE PUMP PRESSURE

TO STEERING GEAR SPOOL & SLEEVE ASSEMBLY BOOST PRESSURE CHAMBER

RETURN TO PUMP RESERVIOR SPOOL PLUG

LEVER PEDAL ROD

OUTPUT ROD

INPUT ROD END INPUT ROD PISTON

HOUSING

HOUSING COVER

FIGURE 8-10 When the power assist is not needed, the unit allows the power steering pressure to bypass

the unit and be applied exclusively to the steering gear box.

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Chapter 8 Power-Assist Systems ACCUMULATOR PISTON ACCUMULATOR CAP

RESERVE SYSTEM PRESSURE

BALL CHECK

2 FUNCTION VALVE

NITROGEN GAS

PUMP PRESSURE ACCUMULATOR DUMP VALVE PUMP PRESSURE

TO STEERING GEAR SPOOL & SLEEVE ASSEMBLY

RETURN TO PUMP RESERVIOR

BOOST PRESSURE CHAMBER

SPOOL PLUG

LEVER PEDAL ROD

OUTPUT ROD

INPUT ROD END INPUT ROD PISTON

HOUSING

HOUSING COVER

FIGURE 8-11 When power assist is required, the power steering pressure is applied to the power piston inside the

unit. The power piston moves the reaction rod toward the master cylinder, causing the brakes to be applied with increased force.

■■

■■

Applied: When the brake pedal is pressed and the input push rod moves the lever inside the hydroboost unit, pressurized power steering fluid is applied to the lever attached to the power piston (FIGURE 8-11). As the pressure pushes the power piston, it moves the reaction rod into the master cylinder, thus increasing the applied pressure to the master cylinder. This is how the brake system gets power assist.

Hold: In the hold position, the hydroboost unit directs the flow of the pressure from the power steering pump through the spool valve, bypassing the actuation lever ­(FIGURE 8-12). This function allows for pedal feedback to the driver and also does not allow for pressure increase unless the pedal moves.

Hydroboost Diagnosis When diagnosing a hydroboost system for poor brake assist, the first step is to determine whether the power steering pump is operating correctly. Since the hydroboost gets its power from the power steering pump, a missing belt, low fluid, or line restriction could cause the hydroboost to not operate correctly, which could cause a no-assist condition (FIGURE 8-13). Once it is determined that the power steering pump is operating, the technician must then determine whether the pump is putting out any pressure. This could be as simple as determining whether the steering assist is working correctly. If it is not, the steering wheel will be hard to turn from lock to lock.

With the engine off, the accumulator on the hydroboost unit should have enough reserve capacity to allow for power assist for two to three braking events. If the accumulator does not hold enough pressure to allow for power brake application, it has failed and the unit should be replaced (FIGURE 8-14).

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Hydraulic Power-Assist Operation

215

ACCUMULATOR PISTON ACCUMULATOR CAP

2 FUNCTION VALVE

RESERVE SYSTEM PRESSURE

BALL CHECK NITROGEN GAS

PUMP PRESSURE ACCUMULATOR DUMP VALVE PUMP PRESSURE

TO STEERING GEAR SPOOL & SLEEVE ASSEMBLY BOOST PRESSURE CHAMBER

RETURN TO PUMP RESERVIOR SPOOL PLUG

LEVER PEDAL ROD

OUTPUT ROD

INPUT ROD END INPUT ROD PISTON

HOUSING

HOUSING COVER

FIGURE 8-12 When the hydraulic booster is in the hold position, the spool valve directs the flow through

the unit without increasing the pressure on the master cylinder.

FIGURE 8-13 Sometimes the

obvious issues with the power steering system can cause the symptoms that the customer reported. A thorough visual inspection is the first step in diagnosis.

Inspecting and Testing a Hydraulically Assisted Power Brake System Operation problems can be caused by a number of issues, such as leaks inside the booster unit, a worn power steering pump, a slipping or broken pump drive belt, badly contaminated power steering fluid, or leaky hose connections. Most of these issues can be verified with a visual inspection.

To inspect and test a hydraulically assisted power brake system for leaks and proper operation, follow the steps in SKILL DRILL 8-2.

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Chapter 8 Power-Assist Systems

TO RESERVOIR

FROM PUMP

TO GEAR

ACCUMULATOR CHARGING CHECK VALVE

PRIMARY VALVE

ACCUMULATOR VALVE BOOST CAVITY SECONDARY VALVE

HOUSING

RATIO LEVER

OUTPUT ROD

INPUT BRACKET TRAVEL LIMITER

NITROGEN GAS

ACCUMULATOR BODY

POWER PISTON

INPUT ROD

INPUT ROD SPRING

HOUSING COVER

ACCUMULATOR PISTON

FIGURE 8-14 The accumulator feeds the built up pressure into the hydroboost unit to provide power assist for brake application.

SKILL DRILL 8-2 Inspecting a Hydraulically Assisted Power Brake System 1. First, do the performance test. Begin with the vehicle engine off.

Apply and release the brake pedal five or six times. Hold the brake pedal down with moderately firm pressure (20–30 lb [9.1–13.6 kg]).

Start the engine and observe the brake pedal. It should drop or rise an inch or so if the booster is providing boost.

2. Second, perform the accumulator leak test. Start the engine, apply the brake pedal, and note the pedal feel and applied height.

Release the brake pedal, turn off the engine, and wait at least 10 minutes.

Continued

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Diagnosis and Service

217

3. Apply the brake pedal with moderately firm pressure (20–30 lb [9.1–13.6 kg]). Note the feel and brake pedal height. Apply the brake pedal a couple more times with the same moderately firm pressure. Each application should result in a higher brake pedal as the accumulator pressure is released from the booster.

Power Booster Testing in the Electrohydraulic Braking System Testing the operation of this system can be performed just as the hydraulic booster ­system was tested. Carry out the same steps to performance test the electrohydraulic braking (EHB) system. Follow these tests up with the accumulator leak test, as described previously.

These tests will identify any operational issues with the system. If issues are found, follow the manufacturer’s recommended procedure to diagnose the fault.

▶▶ Diagnosis

and Service

When determining what needs to be done to the vehicle’s braking system, the technician must look at the whole system to determine how to fix it. Working in a systematic fashion will increase the possibility of correctly diagnosing the problem the first time and will decrease the time it takes to discover the underlying issue. Starting inside the vehicle cabin will lay the foundation for a proper repair.

8-3 Diagnose a power-assist system failure.

Inspecting the Brake Pedal Brake pedal height, free play, and travel are critical for proper brake operation. Having the proper brake pedal height helps to ensure that the brake pedal has enough starting height to fully apply force to the brakes, even if one-half of the hydraulic system is rendered useless by a leak. In other words, there has to be a specified distance in which the brake pedal can travel before it contacts the floor or anything else.

To measure brake pedal height, follow the steps in SKILL DRILL 8-3.

Free play is the amount of clearance between the brake pedal linkage and the master cylinder piston. To measure it, apply very light hand pressure to the brake pedal, measuring how far the pedal travels before resistance starts to be felt (FIGURE 8-15).

FIGURE 8-15 Measuring free play.

SKILL DRILL 8-3 Measuring Brake Pedal Height 1. Research the procedure and specifications for measuring brake pedal height, travel, and free play for the vehicle being worked on. Some manufacturers specify how much travel the brake pedal should have, whereas others specify how much reserve pedal should remain when the brake pedal is fully applied. Know which process applies to the vehicle being worked on.

2. Remove any removable floor mats or anything lying on the floor near the brake pedal.

3. With the engine off, measure the brake pedal height between the two specified points, using a measuring stick.

4. Compare this reading to the specifications, and determine any necessary actions.

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Chapter 8 Power-Assist Systems

SKILL DRILL 8-4 Measuring Brake Pedal Free Play 1. Research the specified procedure for measuring pedal free play.

2. Use light hand pressure to apply the brake pedal until all clearances are taken up, all the while measuring the distance the brake pedal moved.

3. Compare this reading to the specifications, and determine any necessary actions.

To measure brake pedal free play, follow the steps in SKILL DRILL 8-4.

Brake pedal travel is sometimes called reserve pedal. Travel is the distance the brake pedal travels from its rest position to its applied height. Travel is measured by reading brake pedal height and subtracting the height from the floor (FIGURE 8-16). For example, if the brake pedal height is 8" (203.2 mm) and the travel takes it down to 5" (127 mm) off the floor, then the travel is 3" (76.2 mm). Reserve pedal is the measurement from the floor to the height of the applied brake pedal and represents how much reserve is left for the brake pedal to travel if needed (FIGURE 8-17).

To measure brake pedal travel, follow the steps in SKILL DRILL 8-5.

Diagnosing Power Brake Systems All power brake systems should be inspected and tested whenever the customer reports that the brakes are dragging, the brake pedal is harder than normal to push, or the pedal height has changed or if the engine operation changes more than a minimal amount when the brake pedal is applied. Vacuum boosters should also be tested if it is determined that the vehicle has an unlocated vacuum leak. Single-diaphragm and dual-diaphragm vacuum brake boosters are diagnosed in the same manner. The following tests can be performed on single-diaphragm and dual-diaphragm boosters:

▶▶TECHNICIAN TIP Be sure to understand and observe all legislative and personal safety procedures when carrying out tasks. If a technician is unsure of what these procedures are, they should ask their supervisor.

■■

■■ ■■ ■■

Brake pedal free travel: Test to determine whether there is proper brake pedal linkage clearance.

Performance/operation test: Test to determine whether the booster is operational.

External leak test: Test for leaks to the atmosphere.

Internal leak test: Test for leaks between the booster chambers.

FIGURE 8-16 Travel is measured by reading brake petal height and

FIGURE 8-17 Reserve pedal is the measurement from the floor to the

subtracting the height from the floor.

applied brake pedal.

SKILL DRILL 8-5 Measuring Brake Pedal Travel 1. Research the specified procedure for measuring the brake pedal travel or reserve pedal height.

2. Start the engine. This allows the booster to operate normally.

3. Apply the brake pedal with the specified force.

4. Measure the brake pedal travel or reserve height.

5. Compare this reading to the specifications, and determine any necessary actions.

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Diagnosis and Service

Testing the Power Booster

219

▶▶TECHNICIAN TIP

Power booster testing starts with a brake pedal free travel test and then follows up with a performance test to determine whether the booster is operating properly. If not, then an external leak test and internal leak test can determine why the booster isn’t working ­properly. The brake pedal free travel is critical for proper brake operation. The proper amount of free travel ensures that the brake pedal linkage allows the master cylinder pistons to return to their proper rest position and uncover the compensating ports. Insufficient free travel can cause the brakes to drag due to trapped fluid pressure in front of each piston and not being able to return to the reservoir through the blocked compensating ports. Excessive free travel is not good either, because it reduces the amount of reserve pedal for braking in the event of a hydraulic brake system leak.

As important as brake pedal free travel is, it normally does not need to be adjusted, because the components are locked in place. The situations that would call for adjusting it include the following: Someone changed the adjustment setting; the brake pedal linkage has been repaired or adjusted; the linkage has worn over time, leading to increased free travel; or the power booster is being replaced. Just changing the master cylinder does not ­normally require brake pedal free travel adjustment; however, it is good practice to verify that it is within specifications. If adjustment is needed, the manufacturer usually incorporates a locking adjustment rod between the power booster and the brake pedal.

After verifying that the free travel is correct, the power booster needs to be performance tested. To do so, operate the booster to test its ability to provide boost to the master cylinder. To test pedal free travel and to performance test the vacuum booster, follow the steps in SKILL DRILL 8-6.

Holding the brake pedal in a steady manner should not affect the operation of the engine. If the engine runs rough when the brake pedal is held down or changes substantially when the brake pedal is released, it could indicate a vacuum leak in the booster. Perform the external and internal leak tests to identify any faults.

SKILL DRILL 8-6 Perform Brake Pedal Free Play and Performance Test on a Vacuum Power Booster 1. To test brake pedal free travel, start with the engine off and depress the brake pedal several times to remove any vacuum or hydraulic pressure from the power booster. Measure the distance of the brake pedal free travel by depressing the brake pedal by hand until all of the slack is taken up.

2. Performance test the booster by beginning with the vehicle engine off. Apply and release the brake pedal five or six times to bleed off any vacuum or hydraulic pressure in the power booster.

Hold the brake pedal down with moderately firm pressure (20–30 lb [9.1–13.6 kg]).

Continued

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Chapter 8 Power-Assist Systems

3. Start the engine and observe the brake pedal. On vacuumassisted vehicles, if the pedal drops about an inch, the booster is providing boost. If not, the booster is not providing boost, and thus the following tests must be performed. On hydraulically assisted vehicles, when starting the engine with a foot on the brake pedal, the pedal should either rise or fall about an inch (depending on the vehicle) if the booster is providing assist.

Checking Vacuum Supply to Vacuum-Type Power Booster The brake booster must have an adequate amount of vacuum to operate correctly. ­Insufficient vacuum requires the driver to increase foot pressure to activate the brakes.

Excessive vacuum is not usually a problem, because the booster is designed to work using maximum engine vacuum. Many manufacturers specify a minimum of 16" of mercury (16 inHg; 406 mm of Hg) of intake manifold vacuum. If the reading is insufficient, check for vacuum leaks or restrictions in the supply hose or for an improperly tuned engine. To check vacuum supply to a vacuum-type power booster, follow the steps in SKILL DRILL 8-7.

SKILL DRILL 8-7 Solve Vacuum Supply Issues on a ­Vacuum-Type Power Booster 1. With the engine off, remove the inlet hose from the vacuumtype booster.

2. Connect a vacuum gauge to the vacuum supply end of the hose.

Continued

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3. Start the engine, and read the vacuum supply available to the vacuum-type booster. Vacuum should be greater than 16 inHg (406 mm of Hg) on most vehicles.

Lack of Power Assist in the Braking System When diagnosing a power-assist failure on a braking system, the technician must first make some basic vehicle checks. Verifying that the engine is in operational condition is one of the main issues with power-assist failure (FIGURE 8-18). Inspecting the belts, hoses, ­fluids, and accessory components will help in diagnosing the root cause of the power-assist issue ­(FIGURE 8-19). Before elevating the diagnostic routine, the technician must verify the sources of the motivation for the various power-assist systems.

The vacuum booster power-assist system needs a strong vacuum source from the engine to make it operate correctly. If the vacuum feed line from the intake manifold is broken, collapsed, or disconnected, the assist function will not operate (FIGURE 8-20).

After verifying the vacuum source to the vacuum booster, the technician can move on to diagnosing the internal issue with the booster.

The hydroboost power-assist system is a fed with the power steering system pressure to supply the force needed to boost the brake pressure (FIGURE 8-21). Verify that the power steering system is producing enough pressure to operate the hydroboost system and the steering system. To verify the pressure, the technician can use a pressure flow analyzer to determine the output of the power steering pump (FIGURE 8-22). Once it is determined that the power steering pump is outputting enough pressure, the technician can move on to diagnosing the hydroboost unit.

▶▶TECHNICIAN TIP Engines with modified camshafts regularly have decreased amounts of manifold vacuum due to the high-duration camshaft. This lowered vacuum results in higher foot pressure required to stop the vehicle. An auxiliary vacuum pump may be needed to provide adequate braking.

Also, vehicles operated at high altitude always have less vacuum.The general rule of thumb is that 1 inHg (25 mm of Hg) is lost for every 1,000'' (305 m) of altitude gained.

FIGURE 8-18 When inspecting the brake system, the technician needs

FIGURE 8-19 A broken belt can cause a brake-assist issue inside

to look at the various pieces of the engine compartment to make sure that they are in operational condition.

a hydroboost system as the pressure is created within the power steering system.

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Chapter 8 Power-Assist Systems

High Side Port In

Low Side Port

Low side Power Steering Pump

High Side Port Out

T Fitting Driver Side

Passenger Side

High Pressure Low Pressure

FIGURE 8-20 A broken vacuum line to the power booster will cause

FIGURE 8-21 The hydroboost system is a form of brake-assist system

it to become inoperative.

that is run off of the power steering system.

FIGURE 8-22 The power

steering pressure analyzer is used to verify that the power steering pump produces output high enough to operate both the power steering system and the hydroboost system at the same time.

To test the pre the pressure inside a hydroboost power-assist system and a vehicle power steering system, follow the steps in SKILL DRILL 8-8.

SKILL DRILL 8-8 Testing the Pressure Inside a Hydroboost Power-Assist System and a Vehicle Power Steering System 1. Locate the power steering gear box or rack and pinion on the vehicle.

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2. Remove the pressure line from the steering gear box or rack and pinion.

3. Install the power steering pressure and flow analyzer in-line between the pressure line and the steering box.

4. Start the vehicle, and read the pressure output from the power steering pump on the gauge without moving the steering wheel.

5. Rotate steering wheel and watch the pressure build as the technician gets closer to the steering stop.

Continued

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Chapter 8 Power-Assist Systems

6. Once the pressure under the conditions that are explained in the service information has been read, compare the reading with the specification.

▶▶Wrap-Up Ready for Review ▶▶ ▶▶ ▶▶

▶▶ ▶▶

Review Questions

Verify the vacuum source to booster before diagnosing the fault.

Verify power steering pressure to the hydroboost before diagnosing it as faulty.

Inspect brake pedal free play so that if a mechanical problem is detected, it can be remedied before replacing non-faulty components.

Engine inspection is recommended before diagnosing a brake power-assist fault.

The power braking system is used to increase the potential pressure applied to the hydraulic brakes.

Key Terms Freeplay Is the amount of movement before the actuation of the master cylinder. hydroboost A power-assist system that uses the power steering pump to help with the power assist of the hydraulic braking system.

National Highway Traffic Safety Administration (NHTSA) A US federal agency that is in charge of making sure manufactures comply with safety protocols when developing their vehicles. tandem diaphragm booster A brake booster that uses two diaphragms inside of it so that the amount of pressure created will equal that of a very large single-diaphragm booster. vacuum pump An engine-driven or electric pump that generates vacuum so that vacuum-run components will be able to operate on a vehicle.

1. Why was the power-assist unit introduced to the modern automobile? a. The manufacture wanted to sell more parts. b. The use of disc brakes requires more pressure to stop the vehicle. c. The driver of the vehicle couldn’t operate the vehicle without it. d. So that 16-year-olds could drive the vehicle.

2. A vacuum pump is used when the engine does not _______________. a. create enough positive pressure to operate b. run correctly without suction within the engine c. create enough vacuum, because it does not have a butterfly or throttle body d. have a serpentine belt drive system 3. When the technician is replacing a vacuum booster, how far should they adjust the pushrod? a. Adjust it all the way out so that it will be sure to contact the master cylinder. b. Adjust it all the way in so that it will not cause component breakage. c. Leave it the way it came. d. Measure the old one and adjust the new one to match the old one.

4. A dual-diaphragm vacuum booster is used instead of a single-diaphragm vacuum booster because ______________________. a. there is limited room inside the engine compartment b. the vehicle designer thought it looked better

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Wrap-Up

c. it’s more futuristic d. a single-diaphragm vacuum booster is meant only for big vehicles 5. The customer reports that their power steering and power brakes do not work on their vehicle. What could potentially be the cause? a. The hydraulic brake system. b. The power steering belt. c. The alternator. d. The water pump.

6. What is the first step in checking the vacuum booster? a. Look at the color of the booster. b. Determine whether the booster is the correct one for the vehicle. c. Check the vacuum source to the booster. d. Put a pressure gauge on the master cylinder.

7. When replacing the vacuum booster on a vehicle, the technician must first remove __________________. a. the steering wheel b. the master cylinder c. the brake light d. the brake pads 8. When measuring the brake pedal height, the brake must be applied first in order to _________________. a. take up the slack in the brake pedal mechanism b. make sure the brakes operate correctly c. set the parking brake d. adjust the brake drums 9. The hydroboost system shares a component from what other system? a. The transmission. b. The engine. c. The power steering system. d. The suspension system.

10. When replacing the hydroboost unit, what should also be done? a. Flush the power steering system. b. Flush the cooling system. c. Replace the brake pads. d. Replace the brake lights.

ASE Technician A/Technician B Style Questions 1. Technician A says that a low pedal condition could be caused by the power booster. Technician B says that hydroboost is used on heavy-duty vehicles and vehicles with engines that do not make a lot of vacuum. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 2. Technician A says that older vehicles that had four-wheel drum brakes did not need a booster to help with brake application, because the design of drums increased the pressure with the rotation of the wheel. Technician B says

225

that only those people who are of small stature need a power brake booster. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 3. Technician A says that the vacuum booster uses brake fluid pressure to increase the pressure inside the system. Technician B says that a vacuum booster has a check valve built into it so that when the engine is shut off, the driver will still have some power brakes for a few actuations. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 4. Technician A says that low vacuum to the booster may cause the power assist to not work. Technician B says that the check valve inside the power booster is used to retain some vacuum inside the booster so that driver will have some power assist in the event that the engine dies. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 5. Technician A says that when diagnosing a hydroboost system, the technician should flush the power steering system.

Technician B says that a failure of a power steering pump can cause the power assist on a hydroboost system to fail as well. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 6. Technician A states that power-assist brakes are not needed on modern vehicles. Technician B says that power steering gearbox leaks can affect the hydroboost system. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 7. Technician A says that the vacuum line to a power booster is disconnected, and that could cause an engine performance issue. Technician B says that the vacuum line for the power booster is collapsed, which is the source of the ­power-assist condition. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 8. Technician A says that if the master cylinder is leaking into the power booster, this could cause power boost failure. Technician B says that using the wrong fluid in

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Chapter 8  Power-Assist Systems

the power steering system could cause hydroboost failure. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 9. Technician A says that to check the vacuum booster, the technician must depress the brake pedal until the assist is depleted. Technician B says that to check the booster, the technician needs to depress the brake pedal and start the vehicle and, the pedal should then drop if the booster is OK. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B

10. Technician A says that a failing water pump or belt tensioner may cause the hydroboost to operate sporadically.

Technician B says that using the wrong power steering pump can cause the vacuum booster to work poorly. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B

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CHAPTER 12

Electronic Stability Control Systems Learning Objectives ■ ■

12-1 Illustrate ESC operation.

12-2 Explain tCS operation.

■

12-3 Connect an ESC to other driver safety systems.

You Are the Automotive Technician a 2013 Ford Fusion was dropped off at your repair facility with a complaint of a electronic stability control light illuminated. they also have complained that at every stop sign the brake pedal pulsates and they roll through the stop. this have been happening more and more lately and they haven’t noticed anything else wrong with the vehicle. What should you do first as the automotive technician?

1. replace the electronic stability control module 2. replace the aBS sensors 3. Connect the scan tool to vehicle and retrieve codes from the ESC system

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Chapter 12 Electronic Stability Control Systems

▶▶ Introduction The electronic stability control (ESC) system is an integral part of maintaining control of a potentially out-of-control automobile. Using various sensors and electrohydraulic control modules helps the driver maintain control. This chapter will explore the different aspects of the ESC system.

▶▶ Electronic

Stability Control Operation

The antilock brake system (ABS) does a good job of preventing wheels from locking up under hard braking or poor traction conditions and allows the driver to maintain directional control of the vehicle. The traction control system (TCS) also does a good job of maintaining traction when the vehicle is driven in a relatively straight line. However, drivers can lose directional control of the vehicle while driving aggressively, while taking emergency steering actions, or when there are sudden changes in the traction of the road surface while in a turn. These situations can cause the vehicle to understeer (push) or oversteer (loose, or fishtail). It can also cause vehicles with a high center of gravity, such as an SUV (sport utility vehicle), to roll over (FIGURE 12-1). All of these situations can lead to serious collisions or accidents.

If any of these situations are imminent, the ESC system can independently activate individual wheel brake units as necessary to help keep the driver from losing control of the ­vehicle. ESC uses both ABS and TCS, but with a few enhancements to more actively interface with the vehicle’s operation in maintaining directional stability while the vehicle is being steered. A U.S. Insurance Institute for Highway Safety 2006 study estimated that if all vehicles were equipped with ESC, approximately 10,000 fatal accidents in the United States could be avoided each year. This finding led the U.S. Department of Transportation to require that all vehicles of less than 10,000 lb (4,536 kg) gross vehicle weight, and manufactured after September 1, 2011, be equipped with an ESC system that meets their minimum specifications.

The ESC system integrates a yaw sensor, a steering angle sensor, a ­lateral acceleration sensor, and sometimes a roll-rate sensor into the basic ABS and TCS (FIGURE 12-2). It also adds new programming parameters into the electronic brake control module (EBCM) to monitor the vehicle’s stability, as well as added output command capabilities to apply i­ndividual drive wheel and non–drive wheel brake units independent of the driver.

The yaw sensor measures the amount of directional rotation of the vehicle on its v­ ertical axis. In other words, it tells the EBCM the rate at which the vehicle is turning. The steering angle sensor tells the comFIGURE 12-1 ESC helps the driver maintain control of the vehicle while driving. puter what the driver’s directional intent is. If equipped, the roll-rate 12-1 Illustrate ESC operation.

INPUTS LHF Wheel Speed Sensor RHF Wheel Speed Sensor

OUTPUTS

LHR Wheel Speed Sensor

Hydraulic Unit Solenoids

RHR Wheel Speed Sensor

ABS Warning Lamp

Steering Angle Sensor Brake Switch

EBO Warning Lamp

ABS/ESC Control Unit

ESC Warning Lamp

ESP OFF Sensor Yaw Rate Sensor

ESC Function Lamp CAN

PCM

Roll Rate Sensor Master Cylinder Pressure Sensors FIGURE 12-2 A typical ESC schematic.

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Electronic Stability Control Operation

sensor tells the computer the rate of roll and the amount of roll that the vehicle is experiencing. The lateral acceleration sensor is used to measure the lateral acceleration acting on the car to help calculate its actual position. The lateral acceleration acts on a car sideways to the direction of travel, usually when the vehicle is approaching a spin. The EBCM continuously monitors these signals and compares them to preprogrammed scenarios, and it decides which, if any, brake units need to be applied and whether engine torque needs to be reduced to keep the vehicle stable. This process is a good example of the following computer feedback loop: Input → Control Logic Process → Output.

Electronic Stability Control Operation

297

SAFETY TIP Most standard passenger vehicles are designed with a bias toward understeer.

It is generally agreed that understeer is easier for the average driver to recover from. However, many performance vehicles are designed with a slight bias toward ­oversteer, which can be managed by an experienced driver while driving aggressively.

If the ESC system is activated, the EBCM (FIGURE 12-3) monitors the yaw sensor signal, the steering angle sensor signal, and the roll-rate sensor signal, as well as the wheel speed sensor signals. If the vehicle is beginning to understeer, oversteer, or roll, the EBCM detects it in ▶▶TECHNICIAN TIP the signal values. It then applies up to three wheel brake units to help bring the vehicle back The yaw sensor operates similarly to a within proper stability parameters. If that does not stop the stability issue, the EBCM will Wii or other video game controller. Its request a reduction in engine power through the powertrain control module to help slow internal circuitry senses movement and the vehicle further. On most vehicles, the EBCM illuminates a warning lamp on the dash sends a signal directly related to the or sounds a beeper that signifies when the ESC system has detected the start of a skid and movement it senses. reacted to it. This way the driver will be informed that he or she is on the verge of losing control of the vehicle.

On most vehicles, the ESC system defaults to “on” so that it is always active. Some vehicles have a switch on the dash or center console to temporarily deactivate the system.

This can be useful when driving in mud or sand when traction is nearly nonexistent and the ESC system cannot function effectively. Even though some ESC systems can be turned off, they may still monitor the operation of the vehicle and reactivate the ESC system under certain situations, such as driving above a specified speed or when a spin is detected while the brakes are being applied.

Some ESC systems incorporate a switch that allows the driver to select one or more varying levels of assist from the ESC, such as “touring,” “track,” or “sport” (FIGURE 12-4). This option allows the driver to experience differing levels of wheel slip by being able to push the vehicle closer to the edge of control than when ESC is fully activated, while still having the ESC system available as a backup, but with limited assistance. When driving on a racetrack, for example, the driver may want full control of the vehicle instead of being limited by the ESC system.

In the continuous search for new bells and whistles to impress FIGURE 12-3 The EBCM is a computer that takes inputs from various braking sensors and then uses those readings to operate the ABS. customers and enhance safety, manufacturers have designed other features into ESC systems, such as the following: ■■

■■

■■

■■

■■

Hill assist—holds the brake pressure until the throttle is depressed and the vehicle starts to move forward.

All-wheel-drive traction control—applies brake pressure as needed to any of the four individual wheels that may be slipping to maintain power to the wheels with the most traction.

Engine braking control—increases the engine torque if the ESC system detects wheel slippage during deceleration.

Panic-stop assist—detects a driver’s rapid throttle release and lightly applies the brakes to dry the rotors and prepare the brakes for a panic stop.

Accident avoidance—works in conjunction with adaptive cruise control to monitor objects in front of the vehicle.

If the ESC system detects an imminent collision, it can apply the brakes or boost the brake pressure above driver pressure.

FIGURE 12-4 The switch to select the desired level of ESC assist.

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Chapter 12 Electronic Stability Control Systems ■■

▶▶TECHNICIAN TIP Many ESC-equipped vehicles monitor signals from other sensors as well, to help prevent a loss of control of the vehicle; these sensors include the throttle position sensor, vehicle speed sensor, and brake pedal position sensor. When diagnosing an ESC system fault, research the sensors monitored by the EBCM.

12-2 Explain TCS operation.

■■

■■

Hill descent control—works in conjunction with the ESC system to control the speed of the vehicle when going down loose, rough, or slippery slopes.

Trailer sway control—detects trailer sway and uses the ESC system to keep it under control.

Optimized hydraulic braking—monitors brake pressure in each brake circuit and increases it above boosted pressure if deemed necessary.

▶▶ Traction

Control System Operation

Although a basic ABS can prevent skidding by holding or releasing individual brake circuit pressure, it has no ability to apply the brakes, apart from the driver-created hydraulic pressure. This system works fine as long as the tire slippage is a result of the driver applying the brakes. However, tires also slip because the engine torque accelerating them exceeds their traction with the road surface; in this scenario, they can slip, spin, or break loose, causing a loss of control of the vehicle. The TCS was developed to prevent the drive wheels from slipping while the vehicle is being accelerated. It is active up to a manufacturer-specified speed.

Above that speed, traction control is deactivated by the EBCM because further acceleration is unlikely to cause the wheels to lose traction.

To obtain traction control capabilities, manufacturers have added a few design features to the basic ABS, one of which is the high-pressure pump and accumulator that was discussed earlier. This pressure is used to activate brake units on the drive wheels independently of the driver. The sensors are the same as in the ABS, but the ability to apply the individual drive wheel brakes is needed; thus, two to four extra solenoid valves, called boost valves, are added to the hydraulic control unit (HCU) (FIGURE 12-5). These

Dump Valve Isolation Valve Boost Valve Control Unit

To RHF Brake

To LHR Brake

IN (from master cylinder)

Inlet Check Valve ABS Pump Element

High-Pressure Switch

Pump Drive Eccentric (driven by ABS motor)

High-Pressure Accumulator Valve

IN (from master cylinder)

Suction Accumulator Valve

To RHR Brake

To LHF Brake

Control Unit FIGURE 12-5 An HCU with boost valves.

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boost valves direct hydraulic pressure from the accumulator to the ABS solenoid valves so that individual wheel brake units can be applied independently. Additional programming is added to the EBCM to control the high-pressure pump and extra HCU valves and to decide when each of them needs to be activated.

Operation of the TCS When the TCS is active, the EBCM monitors the speed of the individual drive and non-drive wheels, along with the vehicle speed from the vehicle speed sensor. If the driven wheels are accelerating at different speeds from each other or the non-driven wheels, the EBCM can identify which wheel or wheels are slipping. It will then take action to reduce the torque to the appropriate wheels by first applying the brake to any wheels that are slipping. It does this by activating the isolation valve to close off the supply port from the FIGURE 12-6 A switch for deactivating the traction control system. master cylinder. It then activates the boost valve to pressurize the brake circuit on the spinning wheel to slow it down. If that is not enough to prevent the slippage, the EBCM will request reduced power from the engine. This can be accomplished by reducing the throttle plate opening, shutting down one or more fuel injectors, reducing the engine timing, or selecting a higher gear in the transmission. Once the wheel speeds return to proper parameters, the EBCM will return the TCS to normal and continue to monitor the wheels for slippage. ▶▶TECHNICIAN TIP Some TCSs can be temporarily deactivated by a TCS function switch located on the Manufacturers use various strategies dash or center console (FIGURE 12-6). If the driver deactivates the TCS, the system will in their TCSs, and not all of them apply not intervene during wheel slip. Drivers will disable the TCS for a variety of reasons. the brakes as a first step. Some of them They might be climbing a long hill on a rough gravel road, which would continuously reduce engine power first. Even so, activate traction control, overheating the brakes. Or they might want to show off by EBCMs today operate very fast, so there “roasting” the tires or to experience driving without traction control as they would on a may only be a few milliseconds between racetrack. The TCS will automatically default back to On during the next ignition switch each action. cycle. In most cases, if the TCS is deactivated, the ABS will still be active.

▶▶ Electronic

Stability System with Other Driver Safety Systems

With the integration of the vehicle’s systems onto a computer network, the speed at which sensors can cause ECMs to react to the present conditions are increasing every day. The purpose of stability systems is to allow the driver to maintain control of the vehicle at all times, regardless of what situation the vehicle is in. With the driver in control, the potential for a collision or accident is lessened. Although it cannot eliminate the possibility of a collision, it can lessen the chances. This section will introduce some systems that can help with keeping the driver in control and minimizing the possibility of a collision or accident.

Collision Avoidance Systems Collision avoidance systems (CASs) are an integrated structure that alerts the driver and may provide further assistance to help the driver avoid a collision. Using radar, GPS, cameras, and other sensors, the collision avoidance computer (CAC) determines whether the vehicle is getting close to another object where there may be an impact (FIGURE 12-7). When it is determined that a collision is eminent, the CAC will

12-3 Connect an ESC to other driver safety systems.

Collision Warning with Brake Support

Brake Su pport Collissio on

Warrning

Collision Warning Using radar to detect moving vehicles ahead. the system warns the driver of a collision risk with an alarm and warning light.

Brake Support If the risk of a collision increases despite the warning, the brake support is activated, Brake support enables harder, quicker decleration to help drivers stop or reduce speed and lessen the impact of a collision.

FIGURE 12-7 The CAS uses multiple inputs to calculate whether the vehicle will

impact a given obstacle. Each system is different, so verifying the capability of the vehicle is the first step in diagnosing it.

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alert the driver. On some ­models, it will actuate the braking system through the ABS ­module and direct the steering module to steer the vehicle clear of the hindrance. The system is susceptible to all forms of precipitation, which means rain and snow can affect the accuracy of the system. Inclement weather should always be taken seriously because it can cause the system to malfunction. This is a gateway to autonomous vehicle operation. The calculations being performed to determine how to avoid collisions are complex, and that complexity requires the technician to be current on the latest technology.

The National Highway Safety Administration (NHTSA) is making the inclusion of a CAS as standard equipment on all vehicles sold in the United States, starting in the year 2022. The use of the CAS will increase to the point that it will be on every vehicle produced for the U.S. market, which means every technician will need to understand the operation of the system.

Lane Departure System and Lane Keeping Assist Systems To keep the driver aware of where their vehicle is at any given moment the vehicle manufactures have developed a lane departure and lane keeping assist systems (LKAS). These systems use a camera that is usually mounted on the front of the vehicle, a lot of the time at the top of the windshield. As the driver drives down the road, the camera recognizes the lines on the road and when the vehicle crosses those lines, it alerts the diver to pay attention to maintain their lane. The alert could be a visual light coming on that the driver can see without taking their eyes off of the road, or it could be a vibrating seat (­ FIGURE 12-8). The whole point of this system is to make the driver pay attention to where the vehicle is without taking the ­driver’s attention away from driving.

In a lane keeping assist system, the vehicle uses the steering system to help guide the vehicle away from the road line markings (FIGURE 12-9). This system is close to autonomous driving system since it takes over for the diver to correct the possibility of a vehicle collision or the vehicle running off of the road. The integration of both of these systems increases the possibility of the driver keeping control of the vehicle and keeping the vehicle inside the lane that they are currently supposed to be in.

Adaptive Cruise Control

FIGURE 12-8 Lane departure helps keep the driver aware at all times

of the position of the vehicle on the road. If the diver does deviate from the lane, it will alert the diver to correct this action.

Adaptive cruise control (ACC) is a version of cruise control that maintains the pace of the vehicle that is in front of the vehicle ahead of the driver. Using radar and other sensors, the vehicle calculates the distance that the vehicle in front of it is in relation to its own position and accelerates or decelerates itself to maintain that distance (FIGURE 12-10). In concert with the EBCM, powertrain

Depends on vehicle speed and angle of CORRECTION divergence

WARNIG

FIGURE 12-9 Lane keeping assist turns the vehicle’s steering wheel electrically to recenter the vehicle between the road lane markings.

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Wrap-Up

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DISTRONIC PLUS

300

200

100

Oft

R N P D

FIGURE 12-10 The adaptive cruise

control is a feature that keeps the vehicle a set distance away from the vehicle in front of it. This allows the diver to focus on keeping the vehicle on the road.

control module (PCM), ABS, and various other modules, the ACC allows for complete speed control of the vehicle. The ACC is adjustable for the comfort of the driver, so the driver can adjust the gap between their vehicle and the one in front of them. This customizable feature helps with putting the driver at ease by allowing them to participate in customizing the cruise control settings.

▶▶Wrap-Up Ready for Review ▶▶ ▶▶ ▶▶ ▶▶

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The ESC system is used to maintain control of the ­vehicle.

The ESC system uses the ABS and various other sensors to calculate what is happening in the vehicle.

By using TCS, the vehicle can maintain controlled ­operation.

The ESC system works in concert with the crash ­avoidance, lane departure, and adaptive cruise control (ACC) systems.

Collision avoidance systems (CASs) use radar to sense obstacles in front of the vehicle.

Lane departure and lane keeping assist systems use radar and cameras to verify that the vehicle is still in its lane.

Alerting the driver is the purpose of the collision ­avoidance and lane departure systems.

ACC systems are used to maintain the distance between the vehicle and the vehicle in front of it.

Key Terms adaptive cruise control (ACC) A system that allows for maintaining the distance between the vehicle and the vehicle in front of it. boost valves Valves that direct hydraulic pressure from the accumulator to the ABS solenoid valves so that individual wheel brake units can be applied independently.

collision avoidance systems (CASs) Systems that use radar and other sensors to alert the driver to a situation that could potentially be a collision. It helps the driver to avoid a collision. electronic stability control (ESC) An integral part of maintaining control of a potentially out-of-control automobile by using the ABS and other systems to allow the driver to regain control of the vehicle. lane departure A system that alerts the driver when the vehicle wanders out of the lane of traffic that it is currently supposed to be in. lane keeping assist systems (LKAS) A system that redirects the vehicle back to the center of the lane if it wanders out of the lane of traffic. lateral acceleration sensor A sensor used to measure the lateral acceleration acting on the car, to help calculate its actual position. roll-rate sensor A sensor that tells the computer the rate of roll and the amount of roll that the vehicle is experiencing. steering angle sensor A sensor that tells the computer what the driver’s directional intent is. traction control system (TCS) A system that uses the ABS, PCM, and (electronic stability control) ESC to help the driver maintain control of the vehicle throughout a loss-of-traction event. yaw sensor A sensor that measures the amount of directional rotation of the vehicle on its vertical axis.

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Chapter 12 Electronic Stability Control Systems

Review Questions 1. The ESC system is used to __________. a. control the stability of the vehicle b. control the engine temperature of the vehicle c. control the charging system d. control the type of fluid in the braking system 2. The traction control system helps maintain _________. a. the driver’s ability to engage in high-speed driving b. control of the vehicle c. equal tire wear d. transmission slippage 3. The yaw sensor measures how much the vehicle is _________. a. driving b. turning c. crashing d. braking 4. A vehicle comes in with a vehicle steering angle sensor fault. The technician must diagnosis why the traction ­control/ESC light is on. What could be the potential reason that those systems are not in operation? a. A bad cruise control switch. b. A bad steering angle sensor. c. A bad ABS module. d. Nothing, because this is normal operation.

5. The traction control system operates because of wheel lockup. True or false? a. True. b. False.

6. The purpose of the traction control system (TCS) is so that the driver can maintain control of the vehicle at all times under all conditions. When the ABS has a fault, how does that affect the TCS? a. It does not affect it at all. b. It causes the system to become more crucial. c. It disables the system. d. It increases the effectiveness of the TCS. 7. Adaptive cruise control (ACC) uses what features to operate effectively? a. PCM control. b. ABS control. c. Radar-sensing control. d. All of the above.

8. When a lane keeping assist system senses that the vehicle is moving in the wrong direction, what does the vehicle do when it operates the steering? a. It maintains course. b. It directs the steering toward the center of the lane. c. It allows the driver to maintain control. d. It stops the vehicle.

9. The features of the ESC system require communication and sometimes control of other systems in the vehicle. If a communication wire gets shorted or becomes open, will the ESC work correctly? a. Yes. b. No.

c. Part of the system will operate. d. It will switch to Wi-Fi mode.

0. Using accumulator pressure, the TCS uses boost valves 1 to_____________. a. actuate the braking components on the affected wheel b. operate the abs c. boost the speed of the wheel d. maintain the state of the wheel

ASE Technician A/Technician B Style Questions 1. Technician A says that a collision avoidance system (CAS) alerts the driver to a potential collision. Technician B says that the stability of the vehicle is not very important as long as the braking system works. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that in a lane departure warning system, the PCM steers the vehicle toward the center of the lane. Technician B says that ACC maintains the distance between the front of the vehicle and the one that is in front of it when the cruise is engaged. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that a vehicle with collision avoidance is not affected by rain or snow. Technician B says that lane keeping assist systems (LKAS) operate the steering gear to recenter the vehicle to the middle of the lane. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that a vehicle with ABS has a TCS. Technician B says that a lateral acceleration sensor monitors how the vehicle is moving, to determine whether it is in a spin.

Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says that an electronic braking system has sensors that monitor wheel speed. Technician B says that understeer is generally easier to recover from than oversteer.

Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. Technician A says that traction control can reduce the power output of the engine to increase traction. Technician B says

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Wrap-Up

that electronic stability control (ESC) increases the risk of rollover. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says that a yaw sensor tells the computer the vehicle’s actual direction. Technician B says that raising a vehicle’s curb height has no effect on the ESC system. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 8. Technician A says that the TCS uses the ABS information to make decisions on how to apply the brakes in an event.

Technician B says that a vehicle without an ESC will fail.

Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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9. Technician A says that using a scan tool on a modern automobile with a TCS is a must to diagnosis an issue.

Technician B says that some vehicles with ACC have an option to allow for the driver to input how far they want the vehicle to follow behind the one in front of them.

Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that a network problem in the vehicle can cause the ACC, TCS, ESC, and the CAS to fail. Technician B says that proper diagnosis of any of the ESCs requires the technician to understand how the system operates. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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