Introduction to On Board Diagnostics (II)

Functionality of PowertrainControl Module (PCM) inOBD II OBD IIThe PCM is interrupted by the real time scheduler during the performance ofits normal vehicle control functions when the on-board tests are due. At this time thePCM saves its current state of the vehicle and performs the diagnostics. Thistakes about 100 microseconds. Then the PCM returns to its normal vehiclecontrol functions. This repeats for each priority level diagnostics. In thismanner the PCM spends about 15- 40% of its time to diagnostics and the rest toperform its normal vehicle control functions. The method of testing each componentdepends on the electrical characteristics and vehicle functions performed by thedevice. The PCM maintains the low and high limits for each test parameter, andnormal range of values and performance requirements for each component that ittests. The PCM also has adequate hardware test capability to find a short circuit,or open circuit, or the noise level of a signal, including battery, power supply, wiringharness, each sensor, actuator and control unit related to emissions control.The PCM tests each sensor by measuring each test parameter, such as input, or outputand comparing it with the expected value stored in the technical data for the sensor.