Introduction to On Board Diagnostics (II)

Sensors and ActuatorsEmployed in OBD IIDiagnostics :Manifold Absolute Pressure (MAP) sensor: Principle of operation: The sensor measures the theManifold Absolute Pressure (MAP) sensor:The sensor measures the thedisplacement of a diaphragm which is deflected by the manifold absolute apressure. There are two versions. Instrain gauge MAP sensor, the silicon diaphragm is sealed to a pyrex plate under vacuum. A set of sensingresistors is formed around the edge of this vacuum. The resistors s are formed by diffusing a “dopingimpurity” into the silicon. Manifold pressure applied to the diaphragm cause it to deflect which changes theresistance due to piezoresistivityproportional to the pressure. An electrical signal voltage, proportional ortional to the manifold pressure is obtained byconnecting the resistors in a Wheatstone bridge. In the second version vof MAP sensor, a film electrodeis deposited on the inside face of two alumina plates forming a capacitor. The capacitor capsule is placed in asealed housing which is connected to manifold pressure by a small l diameter tube. The deflection of theseplates when pressure is applied to them , causes their capacitance ce to change proportional to the appliedpressure. The capacitor is placed in an oscillator circuit. the frequency of oscillation is proportional to intaketemperature.Diagnostics The electrical characteristics of the strain gauge MAP sensor may deteriorate, resulting inincorrect output, stuck at low signal, stuck at high signal,, and intermittent failure.OBD II DTCsThe failure modes of MAP sensor are diagnosed by OBD II.DTCs for these faults are 105-109.