Curriculum Training TF S Brake Systems

Transcription

1 Technica Service Training Goba Fundamentas Curricuum Training TF0005S Brake Systems Student Information FCS-30-REF CG797/S 04/00

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3 Introduction Preface Goba fundamentas training overview The goa of the Goba Fundamentas Training is to provide students with a common knowedge base of the theory and operation of automotive systems and components. The Goba Fundamentas Training Curricuum (FCS-303-REF) consists of nine sef-study books. A brief isting of the topics covered in each of the sef-study books appears beow. Shop Practices (FCS-30-REF) expains how to prepare for work and describes procedures for ifting materias and vehices, handing substances safey, and performing potentiay hazardous activities (such as weding). Understanding hazard abes, using protective equipment, the importance of environmenta poicy, and using technica resources are aso covered. Brake Systems (FCS-30-REF) describes the function and operation of drum brakes, disc brakes, master cyinder and brake ines, power-assist brakes, and anti-ock braking systems. Steering and Suspension Systems (FCS-396-REF) describes the function and operation of the powerassisted steering system, tires and whees, the suspension system, and steering aignment. Cimate Contro (FCS-398-REF) expains the theories behind cimate contro systems, such as heat transfer and the reationship of temperature to pressure. The sef-study aso describes the function and operation of the refrigeration systems, the air distribution system, the ventiation system, and the eectrica contro system. Eectrica Systems (FCS-397-REF) expains the theories reated to eectricity, incuding the characteristics of eectricity and basic circuits. The sef-study aso describes the function and operation of common automotive eectrica and eectronic devices. Manua Transmission and Drivetrain (FCS-399-REF) expains the theory and operation of gears. The sef-study aso describes the function and operation of the drivetrain, the cutch, manua transmissions and transaxes, the driveshaft, the rear axe and differentia, the transfer case, and the 4x4 system. Automatic Transmissions (FCS-300-REF) expains the function and operation of the transmission and transaxe, the mechanica system, the hydrauic contro system, the eectronic contro system, and the transaxe fina drive. The sef-study aso describes the theory behind automatic transmissions incuding mechanica powerfow and eectro-hydrauic operation. Engine Operation (FCS-395-REF) expains the four-stroke process and the function and operation of the engine bock assemby and the vave train. Aso described are the ubrication system, the intake air system, the exhaust system, and the cooing system. Diese engine function and operation are covered aso. Engine Performance (FCS-394-REF) expains the combustion process and the resuting emissions. The sef-study book aso describes the function and operation of the powertrain contro system, the fue injection system, the ignition system, emissions contro devices, the forced induction systems, and diese engine fue injection. Read Engine Operation before competing Engine Performance. To order curricuum or individua sef-study books, contact Hem Inc. To Free: (8:00 am 6:00 pm EST) Mai: 430 Hamiton Ave., Highand Park, MI 4803 USA Internet: (4 hours a day, 7 days a week) Service Training

4 Contents Introduction Introduction... Preface... Goba fundamentas training overview... Contents... Lesson Braking system... 4 Genera... 4 Objectives... 4 At a gance... 5 Brake system... 5 Theory... 6 Energy... 6 Power... 7 Friction... 7 Traction... 8 Weight and baance... 8 Mechanica everage... 9 Hydrauic principes... 0 Lesson Drum brakes... Genera... Objectives... At a gance... 3 Drum brakes... 3 Components... 4 Brake drum... 4 Parking brakes... 7 Lesson 3 Disc brakes... 8 Genera... 8 Objectives... 8 At a gance... 9 Disc brakes... 9 Components... 0 Disc brakes (continued)... 0 Lesson 4 Master cyinders... 5 Genera... 5 Objectives... 5 At a gance... 6 Master cyinder... 6 Operation... 8 Spit front and rear brake system... 8 Diagonay spit brake system... 8 Components... 9 Brake ines... 9 Brake fuid Braking force contro vaves... 3 Service Training

5 Introduction Contents Lesson 5 Power brakes Genera Objectives At a gance Power-assisted brakes Components Singe-diaphragm booster Dua-diaphragm booster Hydroboost power assisted brakes... 4 Lesson 6 Anti-ock brakes... 4 Genera... 4 Objectives... 4 At a gance Anti-ock brake system (ABS) Components Anti-ock braking system (ABS) contro modue Whee speed sensors Hydrauic contro unit (HCU) Operation Anti-ock brakes (ABS) operation Lesson 7 Diagnostic process Genera Objective At a gance Symptom-to-system-to-component-to-cause diagnostic procedure diagnosis Workshop manua List of abbreviations... 5 Service Training 3

6 Genera Lesson Braking system Objectives Upon competion of this esson you wi be abe to: Expain the purpose and function of a braking system. Identify the brake components. Describe brakes and identify brake types. Expain the theory and operation of braking. 4 Service Training

7 Lesson Braking system At a gance Brake system BRK006-A/VF Brake system components Parking brake assemby Drum and brake shoe assemby 3 Brake peda 4 Disc brake caiper 5 Master cyinder The purpose of a brake system is to aow the driver to stop the vehice safey in the shortest distance possibe on a types of road surfaces and conditions. The brakes reduce the vehice s speed through the appication of friction. 6 Brake fuid reservoir 7 Brake disc and pad assemby 8 Combination vave 9 Brake ines 0 Power brake assist assemby When peope traveed in wagons, brake systems did not need to be powerfu or sophisticated. A wooden bock, attached to a ever and appied to a whee, produced enough friction to stop a wagon. However, this brake system was not powerfu enough for faster, heavier, motorized vehices. Today s sophisticated systems use mechanica, hydrauic, and eectronic components to contro braking. Service Training 5

8 Theory Lesson Braking system Energy Energy is the abiity to do work. Energy can be converted from one form to another but can never be destroyed. Moving a vehice requires the use of energy. Energy is never ost, it just changes forms. When you put fue in a car it is a iquid, but it contains potentia energy or the abiity to do work. When the gasoine is injected into the engine and ignited, the potentia energy of the iquid fue changes states and becomes heat energy. The engine converts heat energy into motion, and then, using different components of the vehice, transfers motion to the whees. The whees rotate, using this motion to move the vehice. Finay, the onger the vehice is driven the warmer the tires become. Energy is reeased in the form of heat. 6 Service Training

9 Lesson Braking system Theory Power Automotive engines change the potentia energy of gasoine or diese fue into heat energy. The rate at which an engine can perform this change can be considered the power of an engine. Thus an engine that causes energy to change states quicky is said to be powerfu. But what about the power of the brake system? If power can be defined as the rate a device can change the state of energy, then the brakes must be capabe of deivering much more power than the engine. BRK007-A/VF On a modern fue-efficient vehice, acceerating from zero to 00 kph (6 mph) in 0 seconds is an acceptabe eve of acceeration. But if it takes a brake system 0 seconds to stop a vehice from 00 kph (6 mph), it is considered weak at best. A propery functioning brake system shoud stop that vehice in 3 to 4 seconds from 00 kph (6 mph). If we use a device s abiity to change the state of energy as the measure of its power, then the brake system of a vehice must be two to three times as powerfu as the engine. The faster and heavier a vehice is, the arger and more efficient the brake system must be. Braking power Vehice acceerating from 0 to kph (6 mph) in 0 seconds Vehice stopping in 3-4 seconds from 96 kph (60 mph) Friction Friction is the resistance to motion between two objects in contact with each other. By rubbing two surfaces together, a rotating energy is changed into heat energy. This change occurs because of the friction between the two surfaces. Brake systems use friction to sow down, stop, and hod the whees of a vehice. To stop a vehice, friction has to be made to convert the energy to heat. Brake pads and shoes appy friction to the brake drums or discs to convert motion energy to heat energy. Service Training 7

10 Theory Lesson Braking system Traction Traction and friction work together to et the tires grip the road. Just as the brake shoes and drums require friction to sow or stop the rotation of the whees, the tires require friction to sow or stop the momentum of the vehice. The abiity of the tires to suppy friction is caed traction. No matter how we the brakes stop the rotation of the whees, if the tires do not suppy traction the vehice does not stop. The amount of traction avaiabe to stop a vehice depends on many conditions. If the vehice is used on ice or snow, the traction of the tires is reduced. Tire tread pattern must be correct to match the conditions of the road. When stopping, it is actuay the friction between the tires and the road that stops the vehice. BRK05-A/VF Road surface condition affects stopping Good traction Poor traction Weight and baance Weight and baance are two important factors in safey stopping a vehice. There must be a baance between the brake force sent to the whees from side to side, and from front to rear. If brake force is not baanced, it coud cause a whee to ock up. When a vehice s whee ocks up or does not turn because of the brakes hoding the whee from turning, traction is ost between the tire and the road. Loss of traction can cause poor stopping, skidding and oss of contro. Braking weight ratio is the comparison of front whee and rear whee braking effort. When a vehice brakes, its weight tends to transfer to the front whees. The front whees are pressed against the road with greater force. At the same time, the rear whees ose some of their grip on the road. As a resut, the front brakes do more braking than the rear brakes. BRK009-A/VF Braking and vehice weight Norma brake appication Hard brake appication, front of vehice pushed down because of weight transfer. Rear brakes of vehice doing itte braking 8 Service Training

11 Lesson Braking system Theory Mechanica everage Mechanica evers are used in the brake system to increase braking power. The brake peda arm or brake peda inkage are simpe evers. When the driver steps on the brake peda, the force appied by the driver s foot is increased to the braking system because the peda arm pivots or moves from a fixed point. The onger the ever is the more force that can be appied. A onger ever produces more force but must trave farther. A shorter ever appies ess force but traves a shorter distance. A ong ever can produce a ot of mechanica force from a short distance from the pivot point. 3 BRK045-A/VF Mechanica evers Short trave, powerfu movement Pivot point 3 Long ever, ong trave Service Training 9

12 Theory Lesson Braking system Hydrauic principes BRK0-A/VF Hydrauic force Hydrauics are used on modern brake systems to add additiona force to mechanica everage. Hydrauics combined with mechanica everage and brake components sow and stop a vehice. Hydrauic theory is based on the fact that a iquid does not compress. A stee spring compresses when a weight or force is paced on it. A iquid in a container does not compress when the same force is appied to the surface of the iquid. The force or pressure is appied equay to a surfaces of the container. If two pistons of equa size are in a contained hydrauic system, any force appied to one of the pistons is transferred to the other, moving the pistons an equa distance. 0 Service Training

13 Lesson Braking system Theory BRK0-A/VF Force and surface areas Pistons having a arger surface area create more force using the same pressure as a piston with a smaer surface area. The area of a piston determines how much force a piston exerts on an object. A piston of a greater surface area moves a shorter distance using more fuid, but has more force to move an object. A smaer piston area size moves a onger distance with the same amount of fuid but has much ess force because of the smaer surface area. Because air compresses, there must be no air in the system. If there is air in the brake system, the air compresses and the fuid motion is reduced or even competey stopped. Service Training

14 Genera Lesson Drum brakes Objectives Upon competion of this esson you wi be abe to: Expain the purpose and function of drum brakes. Describe drum brakes. Identify drum brake components. Expain the theory and operation of drum brakes. Expain the theory and operation of the parking brake. Service Training

15 Lesson Drum brakes At a gance Drum brakes BRK03-A/VF Drum brake components Backing pate Brake shoe retainer 3 Brake shoe 4 Brake springs 5 Brake drum 6 Whee cyinder Drum brakes are the odest type of brake and are sti widey used. The drum rotates with the whee and the brake shoes rub against the drum. Drum brakes convert the moving motion of the vehice into heat energy to sow or stop the vehice. Drum brake shoes are contained inside the brake drum and press outward against the sides of the brake drum when pressure is appied to the brake peda. Spring and reainers hod the brake shoes onto the backing pate. The backing pate is boted to the axe assemby. The brake shoes are mounted to a backing pate. The backing pate is a fat round stee disk that is boted to the axe. The backing pate anchors the brake shoes with springs and retainers. Hydrauic pressure forces a pair of pistons in the whee cyinders outward, pushing the brake shoes outward so they contact the rotating brake drum. The friction between the brake shoes and the rotating brake drum cause the rotation of the whee to sow or stop. The brake springs in the drum assemby hod the brake shoes in pace. The brake springs aso return the brake shoes to the rest position when the driver removes the pressure from the brake peda. Service Training 3

16 Components Lesson Drum brakes Brake drum The brake drum is made from stee and is attached to the axe and rotates with the whee assemby. The brake drum has a machined inner surface that serves as the braking surface. It is against this machined surface that the brake shoes make contact with the brake drum. Brake drums must be capabe of reeasing arge amounts of converted energy or heat back to the surrounding air. BRK00-A/VF Drum brake operation Friction materia Brake drum 4 Service Training

17 Lesson Drum brakes Components Whee cyinders 3 BRK04-A/VF 4 Whee cyinder components Brake shoe return springs Whee cyinder dust boots 3 Whee cyinder pistons 4 Brake shoes Whee cyinders of drum brakes are mounted to the backing pate with retainers. The whee cyinder receives hydrauic pressure when the driver pushes on the brake peda. The hydrauic pressure acts on the whee cyinders, pushing the pistons outward and into the brake shoes. The brake shoes are then forced into the rotating brake drum, sowing the vehice. The whee cyinders have dust boots to keep dirt and water from entering the whee cyinder. Service Training 5

18 Components Lesson Drum brakes Brake drum (continued) Brake shoe assemby At one time brake shoes used asbestos fibers as friction materia. Asbestos has been found to cause ung cancer. Today s vehices use asbestos free materias for brake systems. The materia now used varies with the manufacturer. Some companies use the man-made fiber Kevar. Others use a combination of stee and minera fiber. The friction materia is riveted or gued to a stee shoe or pad. The brake friction or ining materia must be changed once the ining reaches a minimum thickness. If the minimum ining thickness is exceeded, the stee shoe that the friction materia is attached to rubs the stee brake drum causing noise and damage BRK047-A/VF Parts of the brake shoe assemby Brake ining Stee brake shoe 3 Brake ining thickness 3 6 Service Training

19 Lesson Drum brakes Components Parking brakes The parking brakes use cabes and a hande or foot peda to mechanicay appy the brakes. The parking brake system is independent of the hydrauic braking system so in case of hydrauic brake system faiure the vehice can be stopped. The parking brake system uses disc or drum type brakes to make up the rest of the parking brake system. BRK040-A/VF 3 Parking brakes components Brake hande Parking brake cabes 3 Rear brakes Service Training 7

20 Genera Lesson 3 Disc brakes Objectives Upon competion of this esson you wi be abe to: Expain the purpose and function of disc brakes. Describe disc brakes and identify types of disc brakes. Identify disc brake components. Expain the theory and operation of the disc brakes. 8 Service Training

21 Lesson 3 Disc brakes At a gance Disc brakes Disc brakes have more stopping power than most drum brakes. Many new vehices use disc brakes because of higher vehice speeds and vehice weight. Brake pads rubbing against the surface of the disc stop the rotating disc. 3 4 Disc brakes aso convert vehice motion or speed into heat energy using friction. Disc brakes were deveoped as a more efficient means of stopping a vehice. Athough drum brakes perform we, they retain much of the heat and dust generated during braking. Disc brakes operate at cooer temperatures because disc brakes are exposed to the passing airfow of the vehice. Disc brakes are aso sef-ceaning and give greater durabiity and onger braking ife. Disc brakes use two friction pads pushing against a rotating disc to sow and stop a vehice. Operation of disc brakes The pressurization of the brake fuid in the hydrauic brake system forces the piston out of the caiper assemby. The inboard brake pad is against the piston. The brake pad is driven into contact with the brake rotor as the piston moves. Reaction pressure from contact with the rotor sides the brake caiper in the opposite direction. This motion brings the outboard brake pad into contact with the opposite side of the brake rotor. Now the brake rotor is sandwiched between the two brake pads. BRK09-A/VF Disc brake components Brake caiper Brake disc 3 Caiper piston 4 Brake pads As braking pressure is increased, the brake disc is pinched tighter and the rotationa speed of the disc is decreased. As a resut, the vehice s speed is reduced. As with drum brakes, the pads must be occasionay repaced and the brake rotors may need to be machined. Service Training 9

22 Components Lesson 3 Disc brakes Disc brakes (continued) Brake caiper The brake caiper is mounted to the whee axe and does not rotate with the whee assemby. The brake caiper ooks and works much ike a C-camp. The brake caiper may use one or more hydrauic pistons, and contains seas to retain the hydrauic fuid and to keep dirt out. Caipers have one or more beed screws used to remove trapped air from the hydrauic system. 4 5 Brake caipers may use one or two pistons to camp the brake pads against the brake rotor. The brake pads are hed in pace by the brake caiper. Brake caipers are fastened to the axe assemby using a variety of retainers depending on the manufacturer. There are three types of brake caipers: fixed, foating and siding. Fixed caiper brake The fixed caiper brake is an oder stye of disc brake. Fixed caiper disc brakes make use of two pistons using hydrauic pressure to push the brake friction materia or pads against both sides of a rotating brake rotor. The stationary caiper contains the pistons that push the pads inward to contact the brake rotor. 3 BRK048-A/VF Fixed caiper brake components Caiper pistons Brake disc 3 Caiper mount 4 Brake fuid 5 Brake pads 0 Service Training

23 Lesson 3 Disc brakes Components Foating brake caiper The foating brake caiper uses hydrauic pressure and one piston or more to press the inner brake pad against the rotor. The caiper is designed to move on pins, and because the caiper is not fixed the caiper can foat or move. The foating action aows the caiper to move in the opposite direction of the brake caiper piston and draws the outer brake pad against the brake disc, at the same time camping the pads against the brake disc. The foating brake caiper is used when there is itte room between the caiper and the whee assemby BRK050-A/VF Foating brake caiper components Brake caiper Brake disc 3 Brake pads 4 Caiper mount 5 Caiper side pins 6 Caiper piston Service Training

24 Components Lesson 3 Disc brakes Disc brakes (continued) Siding brake caiper The siding brake caiper works much ike the foating brake caiper. The siding brake caiper makes use of a arger piston(s) and arger brake pads to increase stopping power. Some siding brake caipers may use two opposing pistons to further increase stopping power BRK049-A/VF Siding brake caiper components Brake caiper Brake disc 3 Brake pad 4 Caiper mount 5 Caiper piston Service Training

25 Lesson 3 Disc brakes Components Brake discs 3 BRK0-A/VF Types of brake discs Soid brake disc Vented brake disc 3 Air gaps Brake discs are typicay smooth and made from cast iron. Most brake discs are either internay vented or soid, to dissipate heat. As air from the moving vehice passes by the disc, the heat is dissipated into the passing airfow. Internay vented discs have more surface area to dissipate heat, aowing them to transfer heat more efficienty than soid brake discs. The brake disc must be perfecty round and smooth. A brake disc that is not fat or true on the brake pad contact areas causes poor and erratic braking. To restore the brake disc to a smooth, fat surface the brake disc can be machined. As with brake drums, brake discs have a minimum thickness that cannot be exceeded. If the brake disc is machined beyond this minimum thickness, brake disc and system faiure may occur. Service Training 3

26 Components Lesson 3 Disc brakes Disc brakes (continued) Brake pad Like drum brakes, disc brakes aso must have some form of friction materia to contact the brake disc and produce friction. Since friction produces heat, the brake pads must be abe to get rid of the heat and withstand the camping force of the brake caiper. Most brake pads use a combination of metaic fibers in a resin materia to create the friction materia. The friction materia is bonded to a stee backing to form a brake pad. The brake caiper piston pushes the stee pad with the bonded friction materia into the rotating brake disc. BRK0-A/VF Brake pad Brake pad BRK03-A/VF Caiper assemby Brake caiper Brake pad 4 Service Training

27 Lesson 4 Master cyinders Genera Objectives Upon competion of this esson you wi be abe to: Identify the components of the master cyinder. Expain purpose and function of the master cyinder and brake ines. Describe a master cyinder and brake ines and identify types of brake ines. Expain the theory and operation of the master cyinder and brake ines. Expain the theory and operation of braking force contro vaves. Service Training 5

28 At a gance Lesson 4 Master cyinders Master cyinder BRK05-A/VF 5 4 Master cyinder components Reservoir cover Reservoir gasket 3 Reservoir 4 Primary piston 5 Secondary piston 6 Master cyinder housing 7 Brake fuid The master cyinder is the heart of the hydrauic brake system. The master cyinder pressurizes the fuid in the brake system when the driver depresses the brake peda. The brake ines route the pressurized brake fuid to the whee, cyinders or caipers. The master cyinder provides a separate reservoir, pistons, and piston cups for each pair of diagonay opposite or spit brake systems. The two pistons are typicay found one behind the other. The two separate pistons in the master cyinder aow the brake system to have two individua brake systems. If one brake system fais due to a eak, the other system sti aows two whees to have hydrauic brake pressure. 6 Service Training

29 Lesson 4 Master cyinders At a gance The pistons in the master cyinder push brake fuid to the individua brakes, causing the pistons in the brake cyinders/caipers to appy the brakes. As the brakes wear, fuid is added to the system from the brake fuid reservoir in the master cyinder. The reservoir s gasket and cover are designed to keep contaminants out of the fuid. A brake master cyinder contains a primary piston and a secondary piston. When the brake peda is pressed, the master cyinder push rod pushes the primary piston. The primary piston coses the return port and appies pressure to the hydrauic fuid between the primary and secondary pistons. The secondary piston is pushed forward and hydrauic pressure is transferred to the brakes controed by that piston. Once pressure on the brake peda is reeased, hydrauic pressure and a spring return the primary piston. The piston returns more quicky than the brake fuid can return from the brakes, so brake fuid from the reservoir fis the space between the two pistons. Once the primary piston has returned, a excess brake fuid returns to the reservoir. This process is simiar for the secondary piston. Service Training 7

30 Operation Lesson 4 Master cyinders Spit front and rear brake system Master cyinders use two pistons to divide the brake systems into two individua brake systems. On some vehices, the front two brakes are connected to one haf of the dua master cyinder, whie the rear two brakes are connected to the other haf. This system is known as a spit front and rear brake system. BRK06-A/VF Spit front and rear brake system Diagonay spit brake system On other vehices, the brake system may be diagonay spit. A diagonay spit brake system has one front brake connected to the opposite rear brake. A front whee brake is aways in operation, even during a partia brake faiure. Since the front brakes contribute the argest part of a vehice s braking abiity, a diagonay spit brake system can improve safety during partia brake faiure. BRK07-A/VF Diagonay spit brake system 8 Service Training

31 Lesson 4 Master cyinders Components Brake ines 3 BRK04-A/VF Brake ine system Rubber brake hoses Meta brake ines 3 Master cyinder Brake ines are a series of stee tubes that contain brake fuid. Pressure created in the hydrauic system by the master cyinder is transmitted through the fuid in the brake ines to other brake system components. Brake ines must be strong enough to withstand the great pressures exerted by the brake fuid. Stee brake ines are used on the vehice except in areas where the ine must move or fex. Specia rubber fexibe ines are used in areas that require the brake ine to move or fex. The underbody to the brakes at the whees is one exampe of when a fexibe ine is needed. Service Training 9

32 Components Lesson 4 Master cyinders Brake fuid Brake fuid maintains its performance even when extremey hot or cod. The United States of America federa government s Department of Transportation (DOT) sets minimum standards for brake fuid. Brake fuid must be noncorrosive to a brake system parts and have a very high boiing point. Chemica and physica characteristics must not change as a resut of ong storage, cooing, or heating. Brake fuid must have a ow freezing point and must have ubrication properties. Brake fuid must be abe to absorb sma amounts of moisture that accumuate in the system. Air and water must be kept out of the system. As the brake fuid ages, the brake fuid absorbs contaminants and shoud be repaced according to manufacture specifications. Brake fuid shoud never be reused and shoud be stored in a airtight container to avoid moisture contamination. Brake fuid with the correct DOT rating must be used according to the vehice manufacturer. Brake fuid ratings determine fuid boiing points and chemica compounds used in the oi. DOT fuid types shoud never be mixed and the correct fuid shoud aways be used. Some common DOT brake fuid ratings are: DOT 3 bois at 05 C (40 F) DOT 4 bois at 30 C (446 F) DOT 5 bois at 60 C (500 F) 30 Service Training

33 Lesson 4 Master cyinders Components Braking force contro vaves BRK08-A/VF Contro vave component ocations Load apportioning vave or (LAV) vave Proportioning vave 3 Combination vave 4 Metering vave 5 Pressure conscious reguator or (PCR) vave Brake contro vaves hep monitor and contro hydrauic pressure sent to each of the brakes. Since front brakes typicay do more work than rear brakes, if too much brake fuid is sent to the rear brakes, they may ock up prematurey. Premature ock-up reduces the abiity of the brakes to stop the vehice. Brake contro vaves can reduce hydrauic pressure to the rear brakes. Service Training 3

34 Components Lesson 4 Master cyinders Braking force contro vaves (continued) Metering vave The metering vave is designed to equaize braking action on vehices that combine front disc and rear drum brakes. Located in the disc brake ine, the metering vave prevents the front disc brakes from appying unti the rear brakes begin to work. The metering vave is required because disc brakes are fast-acting. The front whees woud ock up before the rear drum brakes because drum brakes must overcome spring retainer tension before they appy. When the brake peda is first appied, fu pressure is immediatey appied to the rear brakes, whie the pressure to the front brakes is imited. As the peda is depressed further, master cyinder pressure buids up. When master cyinder pressure reaches a caibrated point, the metering vave opens to aow fu pressure to be suppied to the disc front brakes. BRK09-A/VF Metering vave operation Hydrauic pressure to the front brakes Hydrauic pressure from master cyinder Proportioning vave The proportioning vave is aso used to reguate brake force on rear brake systems. Located in the rear brake ines, the proportioning vave imits the pressure to the rear brakes. On initia appication of the brake peda, fu pressure is appied to the rear brakes. After the pressure reaches a certain point, the proportioning vave reguates pressure to the rear brakes, aowing a baanced front/rear braking ratio. BRK030-A/VF Proportioning vave operation Hydrauic pressure to the rear brakes Hydrauic pressure from the master cyinder 3 Service Training

35 Lesson 4 Master cyinders Components Combination vave Some vehices combine the pressure differentia vave, metering vave, and proportioning vave into a singe vave body commony caed the combination vave. Whie combination vaves are often used in modern vehices, some master cyinders have proportioning vaves buit into them. 4 3 BRK03-A/VF Combination vave operation Pressure differentia vave Proportioning vave 3 Metering vave 4 Master cyinder Service Training 33

36 Components Lesson 4 Master cyinders Braking force contro vaves (continued) Pressure differentia vave The pressure differentia vave ets the driver know if there is a brake fuid pressure probem. The driver may notice increased brake peda trave and braking effort. The pressure differentia vave monitors the vehice s two brake systems. If either system is eaking, the pressure differentia vave activates the brake warning ight. A piston in the vave is normay centered, hed in pace by equa pressure at both ends. If the pressure becomes unequa, the piston moves to the side with ower pressure, causing the switch trigger punger to side up from its groove in the piston, turning on a warning ight. 3 BRK03-A/VF Pressure differentia vave operation Brake warning amp Brake peda 3 Trigger switch assemby Load apportioning vave (LAV) The weight at the rear of some vehices can vary greaty between when they are unoaded and oaded. Since heavier oads require more braking force at the rear, the height-sensing brake proportioning vave automaticay adjusts rear brake force. Increasing the weight pressing down on the rear of the vehice compresses the vave s inkage and opens it. This vave increases the rear brakes stopping power when the vehice is oaded. 34 Service Training

37 Lesson 4 Master cyinders Components Pressure conscious reguator (PCR) The pressure conscious reguator (PCR) is a vave that imits the brake fuid pressure going to the rear whees. Brake fuid pressure to the rear whees must be imited at times because the front brakes normay have more stopping power and do more work than the rear brakes. With the pressure conscious reguator the pressure to the rear brakes is hed beow the front brake pressure, preventing the rear whees from ocking up. The pressure conscious reguators are normay screwed into the master cyinder outet ports. The brake ines are screwed into the vave. The pressure conscious reguators are controed by a spring acting against the brake fuid pressure. BRK033-A/VF 3 Pressure conscious reguator component ocation Master cyinder Pressure conscious reguators 3 Brake ines screwed into the PCR vave Service Training 35

38 Components Lesson 4 Master cyinders Braking force contro vaves (continued) Deceeration-sensing brake pressure reducer 4 The deceeration-sensing brake pressure reducers have one vave for each brake circuit. The deceeration-sensing brake pressure reducer vave is needed because as the front of the vehice dives toward the ground on a hard brake appication, ess weight is on the rear whees. The weight transfer from the rear to the front axe may cause the rear whees to ock and oss of vehice contro may resut. 3 BRK034-A/VF The deceeration vave is mounted on the foor in front of the rear axe at a certain ange and eve with the vehice foor. With a deceeration of approximatey 0.5 gravity force or more, the vaves aow the pressure to rise more sowy to the rear whee brakes than to the front whee brakes. Deceeration vave assemby Hydrauic ines to brake assembies Deceeration-sensing brake pressure reducer vave 3 Body of vehice 4 Hydrauic ines from master cyinder The preset instaation ange determines the switching point of the vaves. As more g s or gravity force is appied, there is ess braking force sent to the rear whees. The deceeration-sensing brake pressure reducers prevent rear whee ockup during a hard brake appication. 36 Service Training

39 Lesson 5 Power brakes Genera Objectives Upon competion of this esson you wi be abe to: Expain the purpose and function of power-assisted brakes. Describe power-assisted brakes and identify types of power-assisted brakes. Identify power-assisted brake components. Expain the theory and operation of power-assisted brakes. Service Training 37

40 At a gance Lesson 5 Power brakes Power-assisted brakes Power-assisted brakes hep the driver to sow and stop a vehice by using hydrauic fuid from the power steering system or air pressure and engine vacuum to mutipy the force provided by mechanica everage to push the master cyinder pistons. Power-assisted brakes use singe and doube diaphragm vacuum boosters or hydroboost systems to accompish this task. 38 Service Training

41 Lesson 5 Power brakes Components Singe-diaphragm booster The brake peda arm is connected to the vacuum booster by the brake booster push rod. Within the vacuum booster is a power chamber, consisting of two vacuum chambers divided by a moveabe diaphragm. The diaphragm is a rubber piston-ike assemby that separates the atmospheric air pressure from vacuum inside the vacuum booster. 3 4 The brake peda moves the brake booster push rod against a vacuum vave assemby, which moves the master cyinder push rod. Atmospheric pressure is higher than vacuum pressure. The atmospheric pressure on the other side of the diaphragm forces the diaphragm towards the chamber with the increased vacuum. As the diaphragm moves, it pushes the push rod into the master cyinder, sending hydrauic fuid to the brakes. Depending on the amount of brake peda effort appied, the vave coses the vacuum suppy and opens one side of the diaphragm to atmospheric pressure. The resuting pressure difference acting on the diaphragm provides the assist force for the master cyinder push rod. When the brake peda is reeased, the reaction vave aows vacuum to immediatey buid in the booster. With vacuum on both sides of the diaphragm, booster action is hated. BRK037-A/VF Singe-diaphragm booster operation Diaphragm Reaction vave 3 Master cyinder push rod 4 Return spring Service Training 39

42 Components Lesson 5 Power brakes Dua-diaphragm booster The dua-diaphragm booster works ike the singediaphragm booster with the exception that two diaphragms are used instead of one. The duadiaphragm booster doubes the output of the booster and can be made smaer but have the same power as a singe diaphragm booster of the same size. The dua-diaphragm booster is we suited for confined areas where a arger singe diaphragm booster may not fit. BRK038-A/VF Dua-diaphragm booster operation Dua diaphragms 40 Service Training

43 Lesson 5 Power brakes Components Hydroboost power-assisted brakes 4 3 BRK039-A/VF Hydroboost system components Master cyinder Hydroboost 3 Gear box 4 Power steering pump Hydroboost is a different type of power assisted brake system. Hydroboost uses power steering pump fuid pressure or a separate hydrauic system to operate components to assist power brake appication. Hydroboost is sometimes used on diese engine powered vehices because of ack of vacuum to power a vacuum brake booster. Service Training 4

44 Genera Lesson 6 Anti-ock brakes Objectives Upon competion of the esson you wi be abe to: Identify ABS components. Expain the purpose and function of anti-ock brake systems (ABS). Describe ABS braking systems. Expain the theory and operation of ABS. 4 Service Training

45 Lesson 6 Anti-ock brakes At a gance Anti-ock brake system (ABS) BRK005-A/VF Advantages of the anti-ock braking systems Vehice with anti-ock brakes Vehice without anti-ock brakes Anti-ock brakes prevent the whees on a vehice from skidding across the surface of the road during a hard brake appication, aowing the driver to maintain steering contro of the vehice. The anti-ock brake system uses eectronics to contro the braking force appied to the whees. Service Training 43

46 Components Lesson 6 Anti-ock brakes Anti-ock braking system (ABS) contro modue 3 4 BRK053-A/VF Anti-ock system components Rear whee speed sensors ABS contro modue 3 Front whee speed sensors 4 Hydrauic contro unit Under the direction of a contro modue, the anti-ock brake system uses a number of vaves to direct brake fuid to where it is needed. Hydrauic brake pressure is decreased if the ABS contro modue determines that whee ockup is imminent. The ABS contro modue is the brain of the system. The ABS computer determines if a brake or brakes need pressure moduation to prevent whee ockup, and then acts upon its determinations. The contro modue receives eectronic signas from whee speed sensors. 44 Service Training

47 Lesson 6 Anti-ock brakes Components Whee speed sensors BRK04-A/VF Whee speed sensor operation Whee speed sensor Gear tooth Whee speed sensors monitor vehice whee speed. The whee speed sensors may be at each whee, on the differentia ring gear of some rear whee drive vehices, or a combination of both. As the whee assemby rotates, a gear with teeth passes by the whee speed sensor. The whee speed sensor detects the passing teeth and passes the whee speed information to the ABS computer. The ABS computer, using whee speed sensor information, can detect if one of the four whees of the vehice has stopped rotating. The ABS computer sends commands to other ABS components to prevent vehice skidding and oss of vehice contro. Service Training 45

48 Components Lesson 6 Anti-ock brakes Hydrauic contro unit (HCU) 4 BRK043-A/VF 3 Hydrauic contro unit components Hydrauic contro unit Accumuator The HCU takes the signas from the ABS contro modue and appies or removes hydrauic pressure to the brake assembies. Eectric soenoids in the HCU contro hydrauic vaves. The hydrauic vaves reduce the fow of pressurized brake fuid to the brakes. The pump motor turns on and restores fuid fow by pressurizing the fuid when the anti-ock brake contro modue determines that the whees are no onger about to ock up. On some systems the pump can be used both to pump fuid to reservoir when the whees are about to ockup and to pump fuid to brake units when whees are no onger about to ock-up. 3 Pump 4 Pump motor Backpressure from the pump is what the driver fees through the brake peda. If the computer determines that a whee is deceerating too quicky, it activates vaves that open and cose the hydrauic ines eading to the brake units. These vaves are ocated in the hydrauic contro unit and are not serviceabe. If a vave is defective, the hydrauic contro unit must be repaced. On some systems, the hydrauic and eectronic contro units are combined to form one assemby. The inet and outet vaves are controed by soenoids. A soenoid contains an eectrica coi caed a winding. When current is passed through this winding, a strong magnetic fied is created that activates the vave. The anti-ock brake contro modue turns the current on and off in these soenoids based on input signas it receives from the whee speed sensors. 46 Service Training

49 Lesson 6 Anti-ock brakes Operation Anti-ock brakes (ABS) operation Siding tires provide poor braking and no steering contro. The increased friction of roing tires produces much greater vehice contro. During hard braking conditions, it is possibe for the whees of a vehice to ock or stop rotating. A ocked whee causes reduced steering and braking. Eectronic whee sensors and a computer system constanty monitor whee rotation on vehices equipped with an anti-ock brake system. If one or more of the whees begins to ock, the system opens and coses eectric vaves, cycing up to 0 times per second. The brakes are appied and reeased and appied rapidy, so that the front whees aternatey steer and brake. The rear whees are aso prevented from ocking up. Anti-ock brake systems make it possibe for vehices to avoid skidding. The anti-ock brake system has an eectronic contro modue, whee speed sensors and a hydrauic contro modue to appy the correct hydrauic brake pressure to prevent whee ockup. Traction Maximum whee braking occurs just before whee ockup. When the whees skid, steering contro is ost. The point at which ockup occurs is determined by the frictiona coefficient of the road, the grip of the tires, the speed and weight of the vehice, the suspension, and more. Frictiona coefficient The frictiona force or grip between the road and the tires can be expressed as a frictiona coefficient that is dependent on the road. A dry asphat road has a greater frictiona coefficient than an icy road. Sip Ratio The difference between how fast the vehice is going, and whee speed when the brakes are appied, is caed sip. A sip ratio of zero refects no sippage. ABS significanty imits sip, improving braking effectiveness. Service Training 47

50 Genera Lesson 7 Diagnostic process Objective Upon competion of this esson you wi be abe to: Expain the Symptom-to-System-to-Component-to-Cause diagnostic procedure and provide an exampe. 48 Service Training

51 Lesson 7 Diagnostic process At a gance Symptom-to-system-to-component-to-cause diagnostic procedure diagnosis Diagnosis requires a compete knowedge of the system operation. As with a diagnosis, a technician must use symptoms and cues to determine the cause of a vehice concern. To aid the technician when diagnosing vehices, the strategies of many successfu technicians have been anayzed and incorporated into a diagnostic strategy and into many service pubications. For exampe, as the driver steps on the brakes the vehice swerves to the eft because of contaminated brake pads. The brake pads are contaminated because of a brake fuid eak at the brake caiper. Repacing the brake pads wi correct the probem for a short time unti the pads again become oi soaked. Both the brake pads and oi eak must be corrected together to repair the vehice correcty. Symptom-to-system-to-component-to-cause diagnostic method Using the Symptom-to-System-to-Component-to- Cause (SSCC) diagnostic routine provides you with a ogica method for correcting customer concerns: First, confirm the Symptom of the customer s concern. Next, determine which System on the vehice coud be causing the symptom. Once you identify the particuar system, determine which Component(s) within that system coud be the cause for the customer concern. After determining the fauty component(s) you shoud aways try to identify the cause of the faiure In some cases parts just wear out. However, in other instances something other than the faied component is responsibe for the probem. BRK05-A/VF SSCC diagram Symptom Vehice systems 3 Components 4 Causes Service Training 49

52 At a gance Lesson 7 Diagnostic process Workshop manua The vehice Workshop Manua contains information for diagnostic steps and checks such as: preiminary checks,verification of customer concern, specia driving conditions, road tests and diagnostic pinpoint tests. 50 Service Training

53 List of abbreviations Brake systems The abbreviations conform to the standard SAE J930 except those marked with an asterisk (*). ABS* Anti-ock Brake Systems L* Liter x 064 = gaons BTU* British Therma Unit x 5.99 = CAL ( ) 5 C * Cesius = x F -3 9 LAV* LB-FT* Load Apportioning Vave Pounds-Feet x.3558 = Nm CAL* Caories = = BTU MM* Miimeter x 0.04 = inches Disc* Rotor PCR* Pressure Conscious Reguator ECU* Eectronic Contro Unit ( ) 9 F * Fahrenheit = x C +3 5 Petro PSI* Gasoine Pounds per Square Inch x 6.89 = kpa Gasoine Petro SSCC Symptom-to-System-to-Componentto-Cause IN* Inches x 5.4 = centimeters Tires Tyres kpa* Kiopascas x.45 PSI Service Training 5

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