Introduction to On Board Diagnostics (II)

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Fundamentals of PowertrainControl strategies & OBD IIDiagnosticsHowever, manufacturers choosing to allow the OBD II to better isolate the fault to specificcause would not use the general fault code/message, but would use the more specificcode/message associated with the particular circuit, component, or system.For example, in diagnosing a 5 volt reference throttle position sensor, if the input signal atthe PCM is stuck at near 0 volt, the manufacturer has the choice to select either of two codes:P0120 (general malfunction), or P0122 (specific low circuit input ), depending on themanufacturer’s diagnostic procedures. The root cause of this fault can be any one of electricalor mechanical problems. Identification of the root cause is done using the diagnosticprocedures and is not implied by the DTC message, thus allowing the manufacturer theflexibility in assigning DTCs.The powertrain control strategies in performing OBD II diagnostics depend on eachmanufacturer who has considerable flexibility as to how the diagnostics are implementedprovided the above guidelines of SAE J 2012, SAE 1979, and SAE J 1930 are complied with.A typical use of OBD II procedure is given below as a generic example:The diagnostic mode is entered by switching on the ignition and then simultaneouslydepressing the OFF and Warmer buttons on the climate control system (cadillac).
Fundamentals of PowertrainControl strategies & OBD IIDiagnosticsThe fault codes are displayed by flashing the “Check Engine” light and entering thedisplay mode. Each fault code is displayed in sequence starting with the code thatchecks that all display segments are working correctly.After verifying that all display segments are working, the fault codes for all componentfailures are displayed in sequence, beginning with the lowest and proceeding to thehighest code. The mechanic notes the fault codes that are displayed , and using areference manual, identifies the failed components. The fault codes must comply withthe SAE J 2012 standard. After all fault codes are displayed, special code appearson the display indicating the end of display, and the engine control system awaitsfurther action by the mechanic.Typically the “check engine” light on the instrument panel is illuminated whenever anyfault occurs. For emissions related faults the MIL light will not go out until cleared frommemory by the mechanic. For non-emissions related faults the MIL light goes outautomatically if the malfunction clears. However the PCM stores the DTC associatedwith the detected failure until the diagnostic system is manually cleared oruntil a specified number of engine cycles (twenty) occur with no malfunction. Forsome DTCs (of lesser consequence) there is no activation of the “check engine” MIL light.
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Introduction to On BoardDiagnostics
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OBD II monitors more components and
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Page 39 and 40: OBD II for L & MD VehiclesSTD Manua
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Functionality of PowertrainControl
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Introduction to On Board Diagnostics (II)

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