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1、Lesson21AutomobileSystemDiagnosisWithcomputerizedengine-controlsystems,aservicemanualthatappliesspecificallytothevehiclebeingservicedisamust.Suchamanualwillprovideyouwithaccurateinformationonthefollowing:locationofengineandemissioncontrolparts;systemwiringandblockdiagrams;descriptionofthevariouscomp
2、onents;operationofthevarioussubsystemsforfuel,spark,andemissioncontrol;safetyprecautionsandrecommendedserviceprocedures;specificationsforsensorsandactuators;accessingofself-diagnosiscodes;interpretationofdiagnostictroublecodes;toolsandinstrumentsnecessaryfordiagnosisandrepair.Almosteveryautomobilema
3、nufacturerusessometypeofAssemblyLineCommunicationLink(ALCL)orAssemblyLineDataLink(ALDL)totestthesystem’soperationasitleavestheassemblyline.Specialautomotivediagnosticscannersareavailableforaccessinginformationfromcomputerizedvehiclesthroughconnectionstothisserialdatalink.Informationoninputsandoutput
4、stothecomputer,suchasfaultcodes,coolanttemperature,RPM,timingretard/advance,vacuum,manifoldabsolutepressure,andoxygensensoroutputcanberetrievedusingthesedevices.Thisinformationcanbedisplayedonascreen,shownasadigitalreadout,orprintedonapieceofpaperandusedfortroubleshootingthesystem.Adiagnosticbreakou
5、tboxcanbeconnectedtoprovideaneasywaytoaccessallelectricalcircuitconnectionstotheenginecomputer.Whenusedwithadigitalmultimeter,resistanceandvoltagemeasurementsareeasilyaccessiblefromallcircuitsofthecomputerthroughtheremotelylocatedbox.Thedatafromthetestsarecomparedwithavailablediagnosticandvoltagecha
6、rtstoindicatetheproblem.Thesearethesametestsyouwouldbeperformingwithoutabreakoutbox,onlynowthetestsareeasytodo.Insomecases,diagnosisofthefuelinjectionsystemmayleadyoutosuspectthatthereissomethingwrongwiththefuelinjectors.Forexample,iftheenginemissesorlackspower,itcouldmeanthatoneormoreofthefuelinjec
7、torshasbecomecloggedwithatarlikeresidue.Generally,thisconditionresultsfromtheuseoffuelthatdoesnothavesufficientdetergentadditivesorfromfrequentcitydriving.Mostdiagnostictroublecodesrelatetothecomputer