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brake07 BRAKE DIAGNOSIS.pdf

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BRAKE07 BRAKE DIAGNOSIS
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1. Conveytheimportanceofverifyingthecustomerscomplaint. 2. Describeroadtestproceduresusedtodeterminethecauseof brakevibration. 3. Usingthebrakepedal,establishpedalheight,pedalfreeplayand reservedistanceforinitialbrakesystemdiagnosis. 4. Determinetheserviceabilityofadrumorrotorusingameasuring deviceandspecificationtable. 5. Turnarotorusingtheon-carbrakelathe. 6. Measurehubrunoutandrotorrunoutusingadialindicator. 7. Usingadialindicator,phase-matcharotorandhubforminimum rotorrunout 8. Explainthecauseofrotorparallelism. 9. Usingamicrometerorverniercaliper,measurerotorparallelism. 10. Usingabrakedrummicrometer,determinebrakedrumdiameter andserviceability. Section 7 BRAKE DIAGNOSIS Lesson ObjectivesSection 7 66 LEXUS Technical Training Diagnosingaprobleminthebrakesystemissimilartodiagnosing problemsinanyothersysteminthevehicle.Theplanofactionshould includethefollowing: w verifythecustomerscomplaint. w identifythesymptoms. w isolatethecause. w repairtheproblem. w checkforproperoperation. Beginbydeterminingthesymptomsbasedonthecustomer’scomplaint recordedontheRO.Ifyourinformationisincompleteandyouproceed tofixwhatyoufindtobeaproblem,otherthanwhatthecustomer complainedabout,youbothlose.Thecustomerhastobringthecar backandyoumaynotgetpaidfortheworkyoudid. You’retheexpert,thecustomerbringsthevehicleinbecausethey perceiveaproblem.Whenyouservicethevehicleanddon’ttakecareof theproblem,youlookbadandsodoesthedealer.Worstofall,the customermaynotreturn.Sogetmoreinformationwhenindoubt. Ifyoucan’tverifythecustomer’scomplaint,itmaybenecessarytogo foratestdrivewiththecustomersothathecanpointoutthe symptom.Theproblemmaynotbebrakesatall.Itcouldbesomething assimpleasabowlingballinthetrunkthatmakesabangingnoise whenthebrakesarehithard,oritcouldbealoosesuspension componentorwornbushings. Attempttofindthesourceofthecustomer’scomplaint:thenany additionalthingsyoufindtobeimpropercanbebroughttothe customer’sattentionfortheirapprovaltorepair. Chancesareyouhavedoneapreviousrepairforasimilarsymptom. Thatgivesyouanadvantageinyourdiagnosis. Beforeyoutakethecustomer’scaroutforatestdrive,makesureit’s safe.Ifthepedalgoestothefloorsometimesbutseemsfinenow,do somepreliminarychecksbeforeyoutakeitoutontheroad. Diagnosing Brake Problems Verify Customers ComplaintBrake Diagnosis Preliminarychecksshouldestablishthattheessentialsareintactand operational.Checkfluidlevelinthemastercylinder.Eventhoughitis full,checkforleaks,thereservoirmayhavebeentoppedoffpriorto bringingitinforservice.Checkthefollowingforsignsofleakage: w brakebackingplate. w flexiblebrakehoses. w connections. w braketubing. w auxiliaryvalves. w mastercylinder. Brake System Inspection Check for leakage at the brake backing plate, flexible brake hoses, connections, brake tubing, auxiliary valves and the master cylinder. Whencheckingthebrakepedaltravel,startwiththepedalheight.It shouldbemeasuredfromtheasphaltsheet,belowthecarpet,tothetop ofthepedalpad.Pedalheightisadjustedusingthepushrodto establishthepedalposition. Preliminary Checks Pedal HeightSection 7 68 LEXUS Technical Training Brake Pedal Height Measured from the asphalt sheet, below the carpet, to the top of the pedal pad. Adjusted using the push rod and clevis. Makesurethatthefreeplayisatleast0.040“to0.120“(1-3mm). Turntheengineoffandapplythebrakesseveraltimestoreducethe vacuuminthebooster.Iffreeplayislessthanspecified,thebrakesmay belightlyappliedatalltimes,overheatingthebrakesandcausing prematurewear.Ifthereistoolittlefreeplay,checktheStopLight Switchforproperclearanceasshownbelow. Brake Pedal Freeplay Turn the engine off and apply the brakes several times to reduce the vacuum in the booster. Pedal FreeplayBrake Diagnosis Measurethedistancefromthemeltsheettothetopofthebrakepedal whileapplyingthebrakeswiththeenginerunning.Insufficientreserve distancemaybecausedby: w rotorrun-outorloosewheelbearings.Ineithercasetherotor pushesthecaliperpistonfurtherintothecylinderrequiring additionalpedaltraveltomovethebrakepadsintocontactwiththe rotor. w inoperativeautomaticadjustersreducedreservedistanceasthe shoesmusttravelfurthertocontactthedrum. w airinthelinewillalsocausethepedaltogofurthertotheflooras airinthesystemcompresses.Whenairisinthelines,thepedal willalsofeelspongy.Youmaybeabletoverifyairinthesystemby pumpingthepedalseveraltimescompressingtheair.Removethe reservoircoverandobservethebrakefluidasthebrakepedalis released.Thecompressedairwillcausethereturningfluidtoshoot abovethesideofthereservoir. Brake Pedal Reserve Distance With the engine running apply the brakes and the distance from the top of the pedal pad to the melt sheet should be as specified in the Repair Manual. Ifthepedalishardandthebrakinginefficient,suspecttheboosteror itsvacuumsource.Gothroughtheboosterdiagnosticstepsunderthe boostersectionofthistext. Pedal Reserve DistanceSection 7 70 LEXUS Technical Training Wornpadsorshoesmaybequiteobvious,butwhenyoulookclosely andcomparethewearsidetosideitmaygiveyouaclueastotheir operation.Ifpadsononesidearewornmoreseverelythantheopposite sideofthevehicle,thepistonmaybestuckinthecylinderofthe oppositecaliper.Iftheinsidepadiswornmoreseverelythanthe outsidepadofthesamebrakeassembly,thecalipermaynotbefreeto floatonthetorqueplate. Areasnotdirectlyrelatedtothebrakesystemshouldbeinspectedas theymayindirectlycausenoiseorpullwhenthebrakesareapplied. Tireconditionandinflationpressureshouldbeconsidered.The pressureandtiresizeshouldbeequalfromsidetosideonthesame axle.Tireconditionmayindicatefrontsuspensionproblems. Whenexcessivewheelandtirerun-outispresentavibrationmaybe feltonbrakeapplication.CheckthevehicleRepairManualforthe specificrun-outspecification.Vibrationmayalsobefeltatcruise speeds.Forexample,itmaycomeinat55mphandbegoneat60mph. Wornsuspensionbushingsorballjointschangefront-endgeometry andmaycauseapullordriftwhenthebrakesareapplied. Aroadtestshouldbecompletedinordertoverifythecustomer’s complaint.Becausethecustomerperceivestheproblemwhenthe brakesareapplied,henaturallyassumestheproblemtobeinthe brakes.However,thebrakesystemmaybeindirectlyrelatedtothe complaint.Itisimportanttodeterminethecorrectcauseofthe customerconcern. Aseriesofdecelerationsfrom50to20mph,whilenotingthevehicle speed,intensityandlocationofanyvibrationwillaidinfurther diagnosisofthesystem. Thefirstcheckisdonemerelybyallowingthevehicletocoastdown withoutapplyingthebrakestodetermineiftheproblemliesoutside thebrakesystem. Thiswillhelptodetermineifdriveshaftbalanceorangleiscausinga vibration.Ifthevibrationoccursatspeedsbetween40to25mph,the problemislikelythatthedriveshaftanglevariesateachendofthe driveshaft.Thiscanbeconfirmedbynotinganyvibrationonmoderate toheavyacceleration. Thesecondcheckinvolvesapplyinglighttomoderatebrakepressure. Informationfoundhereisusedinconjunctionwithparkingbrake applicationtodeterminetheareaofvibration. Ifthesymptomoccurswhentheservicebrakeisappliedandnotwhen theparkingbrakeisapplied,theproblemliesinthefrontbrakeor wheelassemblies. Brake Pad Inspection Suspension Inspection Road Test - Identify the SymptomsBrake Diagnosis Whenvibrationismostnoticeableinthesteeringwheel,checkthe frontbrakes,wheelandtirecondition.Whenvibrationismost noticeableinthebrakepedal,checkdiscrotorforparallelism.Measure thediscthicknessateightpointsaroundthecircumference,about10 mmfromtheoutsideedge. Thethirdcheckisdonebyusingtheparkingbrake.Thischeckcan onlybedonewiththosevehicleswhichsharetheserviceandparking brakeassemblies.Itdistinguishesvibrationcausedbyfrontandrear brakeassemblies. Shouldvibrationoccurwhiletheparkingbrakeisappliedatthe suspectedvehiclespeed,checkthedrumfordeformation.Usinganinside micrometerorbrakedrummicrometer,measuretheinsidediameterat severalplacestodeterminetheamountofout-of-round.Ifthedifference betweenthesmallestandthelargestmeasurementismorethan0.006“, thedrumshouldbemachinedonanoff-carlathe.Precautionsregarding off-carlathesmentionedonpage85shouldbeobserved. Drum Measurement Using an inside micrometer or brake drum micrometer, measure the inside diameter at several places to determine the amount of out-of-round. Ifthesymptomispullingwhenthebrakesareapplied,determineifthe pulliserraticorwhetheritisconsistent.Thecrownoftheroadwill contributetothesesymptoms,soperformseveralbrakeapplicationson roadswithdifferentcrownsurfaces. Whenapulliserratic,itwillcausethevehicletopulleitherleftorright withnoconsistency.Whenthisoccurs,checkthewheelalignmentand suspensionbushings.Excessivewearofthebushingsandballjointswill changethesuspensiongeometrywhilebraking.Wearinthestrutbar bushingallowsthelowercontrolarmtomoverearwardwhenthebrakes Isolate the CauseSection 7 72 LEXUS Technical Training areapplied,inducingcasterchangewhichcausesapull.Casterwill causeavehicletopulltothesideoftheleastpositivecaster. Asteeringgearthatisoutofadjustmentorlooseinitsmountingmay alsocontributetoanerraticpull. Whenthevehiclepullsconsistentlyinonedirection,theproblemis usuallyinthebrakesystem.Whenthepullistotheleftforexample, checkboththeleftandrightbrakeassemblies.Thecauseofagreater amountofbrakingononesidemaybeaconditiononeitherbrake assembly.Inspectionofthebrakesystemshouldstartwiththe conditionofthebrakedrumordiscsurface.Thesurfacecondition shouldbethesameonbothrotors. Liningthatissoakedwithbrakefluidorgearoilwillcauseapulland shouldbereplacedasanaxlesetafterrepairingthesourceoftheleak. Thebrakeassemblycreatingthegreatestheatconversionwilldothe greatestbraking.Sowhenacaliperisfrozenonthetorqueplateora pistonisfrozeninthecaliper,thevehiclewillpulltotheoppositeside. Brakenoiseiscausedbyfrictionbetweenthepadsanddrumorrotor whenthebrakesareapplied.Occasionalsquealisnormal,andnota functionalproblemanddoesnotindicatelossofbrakingeffectiveness. Whenthebrakenoiseoccursallthetime,checktheliningcondition. Liningthatisglazedshouldbereplacedorcleanedusingemerycloth. Whensandingtheliningtoremovetheglazedsurface,besuretocover theentiresurfaceevenly.(Seeprecautionsunderasbestosinthe referencesectionofthisbook).Alsocheckthedrumordiscforaglazed conditionandcleanupwithemeryclothorturnonabrakelatheifthe drumdiameterordiscthicknesspermits. Squealcanalsobecausedbyaweakorbrokenholddownspringor returnspringaswellasmissing,damagedorimproperlyinstalled anti-squealshims.Toinspectreturnsprings,checkforspacebetween thespringcoilsandnicksinthespringwirediameter.Nickscausedby toolsduringinstallationandremovalmaycausespringstobreak. Damagedspringsshouldbereplacedasaset. Anti-squealshimshelptodampenthevibrationwhichoccurswhenthe padscontactthedisc.Makesurethattheappropriateshimsarein place.Anti-rattlespringsareusedtopositionandholdthepadas rigidlyaspossibletoreducepadmovementinthecaliperassemblyand therebyreducenoisecausedbyvibration.Makesurethattheyare positionedproperlysotheyaremosteffective. Brake NoiseBrake Diagnosis Whenassemblingbrakepadstothecaliperassemblyinspecttheshims andfittingcomponents.Anti-squealspringsandsupportplatesmaybe reusedifingoodcondition.Inspectthemforproperrebound, deformation,burrs,cracks,wear,orrust.Cleantheshimsasnecessary andlubricateallsidesexceptthesidecontactingthecaliperpiston withathinlayerofshimgrease.Shimgreasecanbeordered separatelyunderpartnumber08887-80409.Inaddition,removeany rustfromthecalipergroovesintowhichtheearsofthebrakepadrest andcoatwithathinlayerofshimgrease. Shim Inspection and Lubrication Clean the shims as necessary and lubricate all sides except the side contacting the caliper piston with a thin layer of shim grease. Whenbrakenoiseoccursononlythefirstfewbrakeapplications,check forcorrosiononthediscrotors.Cleantherotorswithemeryclothorturn therotorsifitfallswithinthespecifiedrequirementofrotorthickness. Whenbrakenoiseoccursjustbeforethevehiclestops,checkforglazed lining,damagedanti-squealshimsorfluidsoakedlining.Correctthe conditionsascoveredpreviously. Groanorcreep-groan,whichoccurswhenthebrakepedalisreleased slowlywhiletheengineidlesinforwardgears,iscausedbythepad allowingthedisctoslip.Slightlyincreasingordecreasingthepedal effortwilleliminatethenoise.Itdoesnotadverselyaffectthebraking systemorbrakingperformance.Section 7 74 LEXUS Technical Training Brakevibrationisasymptomwhichoccursduringbrakingandisnot accompaniedbysound.Withbrakesappliedathighspeeds,the vibrationistransmittedtothesuspensionsystem,thesteeringwheel, instrumentpanelandbrakepedal.Inadvancedstages,vibrationmay alsooccuratlowerspeeds. Ifthevibrationcausesthesteeringwheeltooscillatesidetoside,the causeislikelythefrontbrakeassemblies.Therearparkingbrakecan beusedtoisolatethevibrationbyapplyingtheparkingbrakeatthe speedatwhichthevibrationoccurs.Ifthevibrationdoesnotoccur,itis likelythatthefrontbrakesarethecause.(Thisprocedurewillnot workiftheparkingbrakeisanexclusivedesignfoundonreardisc brakeswithadrumtypeparkingbrake.) Isolating Brake Vibration With brakes applied at high speeds. The vibration is transmitted to the suspension system, the steering wheel, instrument panel and brake pedal. Brake Vibration IsolationBrake Diagnosis Ifthevibrationcannotbeisolatedtoeitherthefrontorrearbrakes,
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