Introduction to On Board Diagnostics (II)

Sensors and ActuatorsKnock sensorEmployed in OBD IIDiagnosticsPrinciple of operation The sensor measures the sudden risein cylinder pressure during combustion which commonly occurs with highmanifold pressure and excessive spark advance. The sensor consists sts ofmagnetorestrictive rods placed in a magnetic field of a coil. When excessive cylinderpressure is sensed the rods change the flux field in the coil which produces a voltagechange in the coil. The engine cylinder is mechanically resonant to the knockfrequency band, and the output signal is responsive to the first time derivative ofacceleration, also called jerk. The output signal of the sensor forms a closed loopsystem that retards the ignition to reduce the knock detected at the cylinders. Theproblem of detecting knock is complicated by the presence ofother vibrations and noise in the engine.Another version of knock sensor uses piezoelectric crystals, or the piezoresistance ofa doped silicon semiconductor.Diagnostics The electrical characteristics of the Knock sensor may deteriorate,resulting in incorrect output, out of range/performance values,stuck at low signal, stuck at high signal,, and intermittent failure.OBD II DTCs The failure modes of knock sensor are diagnosed by OBD II. DTCs for thesefaults are 325-329.