On Board Diagnostics - OBD2

Published: 21st March 2011
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Powertrain Command Module (PCM) performs the subsequent features in relation to:

OBD II Diagnostics:

Conduct micro-processor based mostly self diagnostics to assure appropriate operation of the PCM and protected storage of OBD II tools data in memory.

Conduct On-Board diagnostics in genuine time and alert the driver by illuminating MIL in scenario of a fault conduct powertrain management capabilities to decrease emissions and meet OBD II rules in the course of open-loop operation at start-up time.

Complete powertrain management functions to greatly reduce emissions and meet OBD II laws in closed-loop control throughout ordinary operation.

Complete microprocessor-primarily based self diagnostics to make certain accurate operation of the PCM and secure storage of OBD II diagnostic information in memory.

The PCM performs the subsequent self diagnostics:

· Verify the checksum of the plan memory in ROM with its operate and appropriate version.

· Conduct read through and publish check of RAM cells for fault zero cost memory Carry out processor features in CPU, peripheral gadgets which includes A/D converters, watchdog timers, and registers to verify that the processor is operating appropriately.

· Conduct checks on stored automobile information and confirm that the information is not corrupted and is inside fair limits of auto operation.

· Carry out On-Board diagnostics in genuine time and alert the driver by illuminating MIL in situation of a fault, by interspersing diagnostics with automobile management functions.

The diagnostics are classified into priority levels from 1 to eight or nine. The greatest priority degree assessments are performed any 1 millisecond, followed by next priority stage exams just about every five milliseconds, 10 milliseconds, twenty milliseconds, fifty milliseconds, one hundred milliseconds, 200 milliseconds, 500 milliseconds, and one 2nd. The highest priority degree assessments are those that impact security and emissions to a higher diploma in accordance to OBD II regulations. These involve Oxygen sensor (lambda sensor), and fuel trim checks during closed loop operation of the auto. The upcoming priority checks are the interrupt timers, and watchdog timers. The following priority checks are sensors, including EGO sensor, Throttle placement sensor, Misfire detection, MAP sensor, Engine RPM sensor, MAF sensor, Crankshaft place sensor, and Engine coolant sensor. The subsequent priority assessments are EGR intrusive tests, Catalytic converter's secondary air, and canister purge, fuel degree sensor, pedal actuator, and ignition timer. The following priority checks are periodic self exams.

The PCM performs on-board diagnostics in genuine time The PCM is interrupted by the authentic time scheduler throughout the overall performance of its typical motor vehicle command features when the on-board exams are due. At this time the PCM saves its present state of the motor vehicle and performs the diagnostics. This normally requires about a hundred microseconds. Then the PCM returns to its standard automobile manage capabilities. This repeats for every priority degree diagnostics. In this method the PCM spends about fifteen-40% of its time to diagnostics and the relaxation to complete its common automobile manage capabilities.

The technique of testing every single part is dependent on the electrical qualities and automobile functions carried out by the gadget. The PCM maintains the reduced and large limits for each and every test parameter, and typical selection of values and effectiveness demands for each and every part that it exams. The PCM also has ample hardware test capacity to discover a brief circuit, or open circuit, or the noise level of a signal, which includes battery, energy provide, wiring harness, every sensor, actuator and management unit connected to emissions handle.

The PCM assessments every single sensor by measuring just about every test parameter, these as input, or output and evaluating it with the anticipated price stored in the technical data for the sensor.

The PCM also compares the signals of the parts below test with a blend of specifics provided by other sensors, to confirm the reasonableness of values supplied by the elements. The noise degree and the efficiency of each and every signal of the component is checked as properly.

Actuators are tested similarly to the way the sensors are examined for brief circuit, open circuit, and array and overall performance ranges. The test approach also features computing a test output of a sensor applying distinctive engine parameters and evaluating them for compliance. This is referred to as analytical redundancy. The actuator below specified situations is intrusively activated and its output is measured to confirm versus the expected worth for correct operation. If discrepancies to the nominal values are diagnosed in any component below check, the info is saved in memory with all the relevant supporting information, such as engine pace, MAP sensor, coolant temperature, and people. This is named "Freeze Frame" considering that it provides the vehicle's state at the quick of failure of that element.. Thus defects that look or underneath certain disorders can be diagnosed, if the fault occurs only when for the duration of a variety of cycles, it is deleted.

The fault persists for two cycles consecutively it is not erased right until the defect is repaired by the technician. In case of an out of collection output of a sensor, the PCM substitutes a corresponding sensible price for that automobile affliction of operation. The PCM also gives you clear details to the driver by illuminating the MIL (Malfunction Indicator Light) in situation of a defect not having producing alarm for small complications. All applicable information for off-board diagnostics and repair are stored by the PCM in its memory for later use. In the scenario of a defect that entirely impairs the motor vehicle functionality the PCM has the total ability to switch the motor vehicle state to a secure state of lesser capacity identified as "Limp Home" state, in which the motor vehicle is brought to a risk-free degraded working ailment that consists of a halt of the auto.

The PCM communicates with the OBD II scan instrument and supplies diagnostic information, and OBD II DTCs of all faults seasoned by the automobile so significantly to the exterior tester to facilitate off-board diagnostics, and car fix. In this respect OBD II supplies SAE J 1850 information link for communication of diagnostic info, SAE J 2012 delivers the DTC concept formats, and SAE J 1979 presents the check modes, requesting PCM for emissions associated powertrain diagnostics info.

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