Introduction to On Board Diagnostics (II)

Mass Air Flow (MAF) sensorSensors and ActuatorsEmployed in OBD IIDiagnosticsPrinciple of operation:The sensor consists of a hot film element (resistor) which is electrically heated to a constant temperature,that is measured by a temperature sensor. This element is incorporated in a whetstone bridge withpower supply from the output of an amplifier whose input is the differential voltage, of the bridgeresistors, which is balanced when there is no air flow over the hot film at constant temperature.When air flows over the film, the film cools and the resistance of the film element drops, causing bridgeunbalance thereby producing an input voltage to the amplifier. The output of the amplifier is connectedto the bridge circuit and provides power for the circuit. The amplifier voltage changes the resistance insuch a way as to maintain a fixed hot film temperature relative to the inlet temperature.The output voltage of the amplifier is a measure of the additional current required to heat the wire backto its original temperature. The additional current required is a measure of the heat transfer andtherefore of air mass flow rate. The second arm of the bridge is a similar self-heated wire, placed in stillair which provides compensation for changes in air temperature. and amplifier output voltage. Thisvoltage is converted to frequency which is measured by PCM using a counter. The counter value isproportional to the air flow rate (volume) from which the mass is computed by multiplying the volumeby the air density at that temperature.Diagnostics:The electrical characteristics of the Mass Air Flow sensor may deteriorate, resulting in incorrect output, outof range/performance values, stuck at low signal, stuck at high signal,, and intermittent failure..OBD II DTCs : The failure modes of crankshaft position sensor are diagnosed by OBD II. DTCs for these faults are 100-104.