Introduction to On Board Diagnostics (II)

Functionality of PowertrainControl Module (PCM) inOBD II OBD IIThe PCM also compares the signals of the components under test with acombination of information provided by other sensors, to verify thereasonableness of values provided by the components. The noise level and theperformance of each signal of the component is checked as well. Actuators aretested similarly to the way the sensors are tested for short circuit, open circuit, andrange and performance levels. The test method also includes computing a testoutput of a sensor using different engine parameters and comparing them forcompliance. This is called analytical redundancy. The actuator under certainconditions is intrusively activated and its output is measured to verify against theexpected value for proper operation. If discrepancies to the nominal values arediagnosed in any component under test , the information is stored in memorywith all the relevant supporting data, such as engine speed, MAP sensor, coolanttemperature, and others. This is called “Freeze Frame” since it gives the vehicle’sstate at the instant of failure of that component.. Thus defects that appearance orunder certain conditions can be diagnosed. If the fault occurs only once duringseveral cycles, it is deleted.