Circuit Description
The Powertrain OBD system check must be the starting point for any driveability complaint diagnosis. Before using this procedure, you should perform a careful visual/physical check of the PCM and engine grounds for being clean and tight. The Powertrain OBD system check is an organized approach to identifying a problem created by an electronic engine control system malfunction.
MIL Operation
The Malfunction Indicator Lamp (MIL) is located on the instrument panel (CHECK ENGINE) or (SERVICE ENGINE SOON). The MIL performs the following functions:
| • | It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as possible. |
| • | As a bulb check and system check, the MIL will illuminate with the ignition switch on and the engine not running. When the engine is started, the MIL will turn off. If the MIL remains on, the self-diagnostic system has detected a problem. If the problem goes away, the MIL will turn off in most cases, but a diagnostic trouble code will remain stored. |
| • | If the MIL is illuminated, then the engine stalls, the MIL will remain illuminated so long as the ignition switch is on. |
| • | If the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition switch is cycled off, then on. |
Perform the Powertrain OBD System Check first, when the following conditions are present:
| • | When the MIL does not turn on when the ignition switch is turned to the RUN position. |
| • | When the MIL remains on while the engine is running. |
| • | When you suspect a driveability problem. |
Diagnostic Aids
Inspect all related wiring and connections including the connections at the PCM. These may cause an intermittent malfunction. Check terminals for the following:
| • | Damaged |
| • | Corroded |
| • | Pushed out terminals |
Refer to Intermittents and Poor Connections Diagnosis , Repairing Connector Terminals and Wiring Repairs
Test Description
The numbers below refer to the step numbers on the Diagnostic Table:
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The MIL should be ON steady with the key ON / engine OFF. If not, the No Malfunction Indicator Lamp table should be used to isolate the malfunction.
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This test ensures that the PCM is capable transmitting Class II serial data to the DLC and that the Class II data circuit is not open or shorted. If a DLC problem exists, the Data Link Connector Diagnosis should be used to diagnose the condition.
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If the engine will not start, the Engine Cranks but Does Not Run should be used to diagnose the condition.
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The scan tool may display DTCs which are diagnosed in Automatic Transaxle Diagnosis.
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A scan tool parameter which is not within the typical range may help to isolate the area which is causing the problem.
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
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IMPORTANT: :
Is the MIL ON? | -- | |||||||||||
Does the scan tool display PCM data? | -- | |||||||||||
Attempt to start the engine. Did the engine start and continue to run? | -- | |||||||||||
Are any DTCs indicated as last test failed? | -- | Go to applicable DTCs | ||||||||||
5 |
Are any Fail Records stored? | -- | Go to applicable DTC table | |||||||||
Compare PCM data values displayed on the scan tool to the typical engine scan data values. Are the displayed values normal or close to the typical values? | -- | System OK | Go to indicated Component System Checks |
