Introduction to On Board Diagnostics (II)

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Sensors and ActuatorsEmployed in OBD IIDiagnosticsIdle air control (IAC) valve Principle of operationThe valve is an electronicallycontrolled throttle bypass valve which allows air to flow around the throttle plate(which is closed due to low engine RPM and vehicle being stationary) and produces thesame effect as if the throttle is slightly opened. A stepper motor opens the pintle (valveallowing a limited amount of air to bypass the closed throttle plate. The steppermotor controls the pintle movement accurately thus controlling the amount ofbypass opening into the intake manifold. The duty cycle of the stepper motoris controlled by the PCM which monitors the pintle position and commands thestepper motor to move back the pintle to open the bypass by the calculatedamount and move the pintle forward to close the bypass at the end of the duty cycle.DiagnosticsOBD II DTCsThe duty cycle of the stepper motor , and the amount of bypass by thepintle valve are correlated periodically by OBD IIdiagnostics. The initial position and the final position of the pintlevalve are continuously checked.The failure modes of idle air flow are diagnosed by OBD II. DTCs for thesefaults are 505-507.
Secondary air valvesSensors and ActuatorsEmployed in OBD IIDiagnosticsPrinciple of operation:The secondary air is controlled by twosolenoid valves similar to the EGR valve. One valve switches airflow to the exhaustsystem or to outside air cleaner. The other valve switches air flow to theexhaust manifold or to the second chamber of the three-way catalytic converter.The air routing is done by the PCM based on engine coolant temperature, andA/F ratio. During cold start the secondary air goes to exhaust manifold, and duringclosed loop operation, secondary air goes into catalytic converter. During heavyloads and during severe deceleration, secondary air is directed to air cleaner whereit has no effect on exhaust temperature.Diagnostics The duty cycle of the current pulse that energizes the solenoid ,and the secondary air flow are correlated periodically by OBD IIdiagnostics.OBD II DTCsThe failure modes of secondary air flow are diagnosed by OBD II.DTCs for these faults are 410-419.
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OBD II monitors more components and
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Powertrain and EmissionControls in
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Powertrain and EmissionControl in P
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OBD II for L & MD VehiclesSTD Manua
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OBD II for L & MD VehiclesDiagnosti
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Fundamentals of PowertrainControl s
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The Freeze Frame can also be used f
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Evaporative System Monitoring: Hydr
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OBD(II)In addition the electrical c
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Introduction to On Board Diagnostics (II)

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