Jack Module
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Jack Module
Functionality of Powertrain Control Module (PCM) in OBD II Diagnostics
Powertrain Control Module (PCM) performs the following functions in relation to
OBD II Diagnostics:
Perform micro-processor based self diagnostics to ensure correct operation of the PCM and safe storage of OBD II diagnostic data in memory.
Perform On-Board diagnostics in real time and alert the driver by illuminating MIL in case of a fault Perform powertrain control functions to reduce emissions and meet OBD II regulations during open-loop operation at start-up time.
Perform powertrain control functions to reduce emissions and meet OBD II regulations in closed-loop control during normal operation.
Perform microprocessor-based self diagnostics to ensure correct operation of the PCM and safe storage of OBD II diagnostic data in memory.
The PCM performs the following self diagnostics:
- Verify the checksum of the program memory in ROM with its function and correct version.
- Perform read and write test of RAM cells for fault free memory Perform processor functions in CPU, peripheral devices including A/D converters, watchdog timers, and registers to verify that the processor is functioning properly.
- Perform checks on stored vehicle data and verify that the data is not corrupted and is within reasonable limits of vehicle operation.
- Perform On-Board diagnostics in real time and alert the driver by illuminating MIL in case of a fault, by interspersing diagnostics with vehicle control functions.
The diagnostics are classified into priority levels from 1 to 8 or 9. The highest priority level tests are done every 1 millisecond, followed by next priority level tests every 5 milliseconds, 10 milliseconds, 20 milliseconds, 50 milliseconds, 100 milliseconds, 200 milliseconds, 500 milliseconds, and 1 second. The highest priority level tests are those that effect safety and emissions to a high degree according to OBD II regulations. These include Oxygen sensor (lambda sensor), and fuel trim checks during closed loop operation of the vehicle. The next priority checks are the interrupt timers, and watchdog timers. The next priority tests are sensors, including EGO sensor, Throttle position sensor, Misfire detection, MAP sensor, Engine RPM sensor, MAF sensor, Crankshaft position sensor, and Engine coolant sensor. The next priority tests are EGR intrusive tests, Catalytic converter's secondary air, and canister purge, fuel level sensor, pedal actuator, and ignition timer. The next priority checks are periodic self tests.
The PCM performs on-board diagnostics in real time; The PCM is interrupted by the real time scheduler during the performance of its normal vehicle control functions when the on-board tests are due. At this time the PCM saves its current state of the vehicle and performs the diagnostics. This takes about 100 microseconds. Then the PCM returns to its normal vehicle control functions. This repeats for each priority level diagnostics. In this manner the PCM spends about 15-40% of its time to diagnostics and the rest to perform its normal vehicle control functions.
The method of testing each component depends on the electrical characteristics and vehicle functions performed by the device. The PCM maintains the low and high limits for each test parameter, and normal range of values and performance requirements for each component that it tests. 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, wiring harness, each sensor, actuator and control unit related to emissions control.
The PCM tests each sensor by measuring each test parameter, such as input, or output and comparing it with the expected value stored in the technical data for the sensor.
The PCM also compares the signals of the components under test with a combination of information provided by other sensors, to verify the reasonableness of values provided by the components. The noise level and the performance of each signal of the component is checked as well.
Actuators are tested similarly to the way the sensors are tested for short circuit, open circuit, and range and performance levels. The test method also includes computing a test output of a sensor using different engine parameters and comparing them for compliance. This is called analytical redundancy. The actuator under certain conditions is intrusively activated and its output is measured to verify against the expected value for proper operation. If discrepancies to the nominal values are diagnosed in any component under test, the information is stored in memory with all the relevant supporting data, such as engine speed, MAP sensor, coolant temperature, and others. This is called "Freeze Frame" since it gives the vehicle's state at the instant of failure of that component.. Thus defects that appearance or under certain conditions can be diagnosed, if the fault occurs only once during several cycles, it is deleted.
The fault persists for two cycles consecutively; it is not erased until the defect is repaired by the technician. In case of an out of range output of a sensor, the PCM substitutes a corresponding reasonable value for that vehicle condition of operation. The PCM also provides clear information to the driver by illuminating the MIL (Malfunction Indicator Light) in case of a defect without causing alarm for minor problems. All relevant data for off-board diagnostics and repair are stored by the PCM in its memory for later use. In the case of a defect that completely impairs the vehicle performance the PCM has the full capability to switch the vehicle state to a safe state of lesser capability called "Limp Home" state, in which the vehicle is brought to a safe degraded operating condition that includes a halt of the vehicle.
The PCM communicates with the OBD II scan tool and provides diagnostic data, and OBD II DTCs of all faults experienced by the vehicle so far to the external tester to facilitate off-board diagnostics, and vehicle repair. In this respect OBD II provides SAE J 1850 data link for communication of diagnostic data, SAE J 2012 provides the DTC message formats, and SAE J 1979 provides the test modes, requesting PCM for emissions related powertrain diagnostics data.
About the Author
specialist in Automotive diagnostic scanner R&D engineer
http://www.micro-tronik.com
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