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PeterF.Meiertechnicaleditor,MotorAgeMagazine
08Fall
EssentialScopeTechniques
[ C o m p a n y A d d r e s s ]
EngineMechanical:RelativeCompressionUsingStarterDraw
Performingatraditionalcompressiontestcanbetimeconsuming,andcostlytothecustomer.Usingtechniqueslikethisonecan,ataminimum,identifyweakperformingcylindersandjustifytheneedformorelaborintensivetesting.Besuretodisablethefuelsystemtopreventfuelfromwashingdownthecylinderwalls.Patternnotes:
• Lookforpeaksthatarelowerthantheothers.Mostmodernscopescandetectaslittleasa10%lossbetweencylinders.
• Triggeringoffofanignitionsource(primaryorsecondary)allowsyoutodotwothings.One,youcanusefiringordertodeterminewhichcylinderanylowpeaksareassociatewithandtwo,youcanchecktimingbylookingforthefiringlinetopassnearthecenterofitsassociatedpeak.
• Ifvalvetimingisoff,thepatternwilllooknormalbuttheamplitudeoftheindividualpeakswillbeless.Valvetiminglowerscompressionequallyacrossallcylindersthatsharetheoutoftimecam.Keepthatinmindwhendealingwithmultiplecamengines!
EngineMechanical:Relativecompression(knownbad)
Additionalnotes
EngineMechanical:Relativecompressionatexhaust
Rememberthe“dollarbill”trickfortestinganenginewithasuspectexhaustvalve?Thedollarbillwouldbesuckedintothetailpipeeverytimethatcylinderhititsexhauststroke.TheFirstLookpressuresensorallowstheuserto“see”thosepressurepulsesastheyhappen.Patternnotes:
• Thenumberofcylindersandexhaustlayoutwillaffecttheactualshapeofthispattern.Don’tfocusontheminorvariations.Insteadlookforthe“oddmanout”.Focusontheforest,notthetrees!
• Identificationoftheweakcylindercanstillbedonewiththeignitiontrigger,butyouhavetoadjustforthetiming.Remember,you’relookingattheexhauststroke,notthecompressionstroke,onthiswaveform.
Additionalnotes:
CheckingcrankcasepressuretoisolatecompressionlossOnegreatadvantageofusingapressuresensorinsteadoftraditionaltestingisthespeedatwhichyoucanquicklyisolatethecauseoftheproblem.Ifyourstartercurrenttestortailpipetestindicatesaweakcylinder,movethesensortotheoildipsticktubeandrunthetestagain.Patternnotes:
• Thistestmeasurescrankcasepressurefluctuation.Asthepistonsmoveupanddown,theycreatechangesinpressure.Ifapiston’sringsarenotsealing,itwillshowuphere.Astablepatternindicatesthelossofcompressionhastobecomingfromthetopend!
Enginemechanical:Intakemanifoldvacuumwithapressuretransducer
Bothpressuresensorsandpressuretransducerscanbeusedtotestintakemanifoldvacuum.Transducersmeasuretheactualpressurerelativetozero,whilesensorsreacttopressurechangeonly.Lookatthefluctuationsbycylinderandrememberwhatstrokeyou’reon!Thisone,usinganinjectorasareference,canmakedeterminingthecylinderyou’relookingforalittlemorechallenging.Next,imaginewhatatraditionalmanifoldvacuumgaugewouldbedoingasthesepressurechangeswereoccurring.(I’veincludedareferenceforyouruseinthishandout).Isthisagoodpattern?Whyorwhynot?Whatdoesthepressurereadingequateto?Why?
Whichiscorrect?(Triggeron#2injector,firingorder1-5-3-6-2-4)
Additionalnotes:
Enginemechanical:In-cylindercrankingcompression
Theuseofanin-cylinderpressuretransducerhaslotsofuses.Ittakesjustasmuchtimeastheuseofatraditional,mechanicalcompressiongaugebutitallowsyoutoseesomuchmore.Insteadofsimplyreadingapeakpressure,youcanobservethepressurechangesintheengineovertheentire720̊ofthefour-strokecycle.Patternnotes:
• Crankingcompressionpressureslessthan90psiisinsufficienttosupportcombustion.
• Formostengines,alinedrawnthroughthepeakwillsplitthepatternintosymmetricalhalves.Ifnot,andthepatternisaccompaniedbyadeeperthannormalexhaustpocket,thereisaleak.
Therearesoftwareapplicationsthatcanmakerotationaldegreehighlightingquickandeasy.Ifyoudon’thaveone,youcanuseWindow’s“Paint”programtomarkthemoffyourself.Simplypastethescreencaptureinto“Paint”andplaceyourmarkingcursoratthetopofonepeak,andthentheother.Atthebottomofthe“Paint”window,you’llseeapixelcoordinatelike100,210.Thefirstnumberisthehorizontalelementandthesecondistheverticalelement.Notethefirstnumber,andthenmoveyourcursortothesecondpeak.Subtractthefirstfromthesecondandyou’llgetatotalpixelspreadforyourcapture.Dividebyfourtodividethespaceinto180̊segments,byeightfor90̊,andsoon.
UsingWindows“Paint”notes:Knownbadexample
Additionalnotes:
EngineMechanical:In-cylinderrunningpressuretest
Therunningin-cylinderpressuretestisavaluablediagnostictool.Notonlycanyouidentifycylindersealingissuesandcauses,youcanusethismethodtotesttheoperationofvariablevalvetiming,verifycamandignitiontiming,andcatchintermittentlysealingvalvesintheact.Patternnotes:(Usethissectiontowritedownwhatcaughtyourattentionaboutthisdynamictestingmethod)Tolearnmore,readtheseriesonIn-CylinderPressureTestingauthoredbythemanwhodevelopedthetechnique,BernieThompson.YoucanaccessallofBernie'sarticlesonMotorAgeat:https://www.searchautoparts.com/motorage/author/bernie-thompson
In-cylinderrunningpressurepatternelements
A:TDCofthecompressionstroke TDClinetypicallysplitspeakintosymmetricalhalves.B:BDCofthepowerstroke ExhaustvalveopensjustbeforeBDC.LookforBDCtointersectrampinareamarkedby“G”.C:TDCoftheexhauststroke Theplateaubetween“B”and“C”ispressureintheexhaustmanifold.H:Theintakevalveopenswhiletheexhaustisstillopen(overlap).Pressuredropisresultofexhaustclosingandcylinderexposedtointakemanifoldabsolutepressure.Lookforramptointersectinareamarkedby“H”.D:BDCoftheintakestroke. Thispressureshouldbenearidenticaltothatintheexhaustpocket.Noteline“F”.E:BacktobeginningofpatternatTDCcompression.
Additionalnotesonin-cylinderrunningpressurepattern:
Additionalnotes:
Enginemechanical:Cam/cranksensors
Comparingknowngoodsynchrelationshipstotheonescapturedonaproblemcarisanotherwaytocheckcamtiming.Findingtheknowngoodpatternsiseasierthanever,withresourcesavailablefromsiteslikeiATN,Wavehook.com,Autonerdz,andothers.Butthatisn’ttheonlyvalueinthesecaptures.Tieininjectorand/orignitioneventstoseeifthereareanyproblemswithECMcontroloftiming.Usethezoomfeaturestocheckthecircuitsforvoltagedropjustasyouwoulduseyourvoltmeter.Patternnotes:
Knownbad
Created
Fuelsystemtesting
Manypressuretransducersallowyoutomonitorfluidpressuresaswellasairpressure.Thisopensupallnewdoorsfortroubleshootingsystemslikefueldelivery.Theuseoffuelpumpcurrentpatternsasatroubleshootingtoolisanexampleofhowinventivetechscreatediagnosticstrategiesandthensharethemwithothers.Nowitisconsideredastandarddrivabilitytroubleshootingtechnique.Let’smakesureyoucantakefulladvantageofthemboth.Patternnotes:
• Currentpatternscantellatechalotabouttheelectricalandmechanicalhealthofthefuelpump.Eachpeakisthecurrentreadingofonesetofcommutators,onewindinginthepumpmotor,asitpassesthroughthebrushes.Low,deformedpeaksindicatebrushandcommutatorwear.
• Currentcanalsotellusthespeedofthepump.Thisknowledge,whenbackedbycurrentdraw,canpointatechinthedirectionofrestrictedfuelflow(cloggedfilters)orevenemptygastanks!
• Pressuresensorscanallowatechtoperformaninjectordroptestinminutes,withouttheneedforascantool,manualinjectordriver,ortraditionalfuelpressuregauge.
• Pressuretransducerscandothesame,butwiththeaddedadvantageofprovidinganaccuratepressuremeasurementandmonitoringofthatpressureoveranytimebaseourscopeiscapableof.
Fuelsystemtesting:Fuelpumpcurrent
Fuelpumpcurrentpatterncantellyouabouttheelectricalandmechanicalhealthofthefuelpump,andtheabilityoffueltomovethroughthesystem.Highpumpspeedandlowcurrentdrawcanindicate:Lowpumpspeedandhighcurrentdrawcanindicate:Normalpumpspeedandlowcurrentdrawcanindicate:
Additionalnotes:
Fuelsystemtesting:Fuelpressurewithtransducer
Usingapressuretransducerinplaceofatraditionalpressuregaugewillnotonlyallowyoutomeasurepressure,butseethepressuredropacrosstheinjectors.
Fuelsystemtesting:Fuelpressureusingapressuresensor
TheFirstLooksensorusedhereisjustlikeitstailpipecousin.Itreactstopressurechange,butdoesnotmeasureactualpressureinthesystem.
Additionalnotes:
Fuelsystemtesting:Fuelinjectorvoltageandcurrent
Injectorpatternscanhelplocatestickinginjectors,ornaildownintermittentissues.CAUTION:Besuretoknowtheinputvoltagelimitsofyourscope.Injectorkickbackcanexceedthecapacityofmostscopes,requiringsomeformofattenuation.Markwheretheinjectorpintleopensinthiscapture.Markwheretheinjectorpintleclosesinthiscapture.Wherewouldyoumeasurecommandedontime?Actualopentime?
Whatiswrongwiththiscapture?
Whataboutthisone?
Howcouldyoucatchanintermittentproblemusinginjectorcurrent?
Isthecalculationofresistanceaccurate?Whatothersystem(s)doyouthinkyoucanapplythisto?
Ignitionsystem:Secondaryignitionpatterns
Isthisanormalpattern?Firingvoltageisaffectedby:Sparkvoltageisaffectedby:Whydoesthesecondcurrentrampnotstartat“0”amps?
Sparkdurationisaffectedby:HighsparkkVwithanupwardslopingburnlinecanindicate:LowsparkkVwithadownwardslopingburnlinecanindicate:Aburnlinethatdisplayspeaksandvalleysmayindicate:MasterignitionwaveformanalysiswiththehelpofJimMorton'svideo!https://youtu.be/yCchGNQEMdE
Additionalnotes:
Ignitionsystem:Primaryignitionpattern
Modernignitionsystemscanmakecapturingasecondaryignitioneventdifficult.Theprimaryignitionpatternmaybethealternativeyouneedtocapturethewaveform.CAUTION:Primaryvoltagecanexceedthemaximuminputvoltagecapabilityofmostscopes.Capturingthispatternmayrequiretheuseofsomeformofattenuation.Whatisthesameaboutthispatternwhencomparedtothesecondary?Whatisdifferent?Canthispatternbecapturedonallvehicles?
Currentcutoff
Samplerateexample
78,130samplespersecond(oneevery12.8microseconds)allowyoutozoominwithdetail.Verticalresolutionmakescurrentslopesmoreaccurate.
Battery-starting-chargingsystemtesting
Howwouldyouliketobeabletoperformacompletesystemtestinlessthan5minutes?Knownbad
Battery-starting-chargingsystem:Batteryvoltage
Opencircuitvoltageis:In-rushvoltageis:Loadedvoltageis:Chargingvoltageis:
Battery-starting-chargingsystem:Startercurrent
Howisthistriggerset?Whatisthepeakvoltage?Whyisitsohigh?Canyouidentifyonthepatternwheretheenginestartedrunningonitsown?Whatdothetwopeaksthatoccuraftertheinitialspikemean?Whatisconsideredanacceptablecurrentreadingafterthevehiclehasbeenrunningfor3-5minutes?Whatdoesahigherthannormalreadingindicate?
Additionalnotes:
Backtothebadpattern
Thisiscausedby:Knowngood:
Resources:AES http://www.AESWave.comAutomotiveTestSolutions http://www.AutomotiveTestSolutions.comAutonerdz http://www.Autonerdz.comInternationalAutomotiveTechniciansNetwork(iATN) http://www.iATN.netPico(UK) http://www.PicoAuto.comDiagnosticNetwork http://www.diag.net/