Circuit Description
There should be a steady malfunction indicator lamp (MIL) with the ignition ON and the engine not running. Ignition feed voltage is supplied directly to the MIL. The powertrain control module (PCM) turns the MIL ON by grounding the MIL control circuit. No MIL with the key ON, the engine not running, and the MIL DTC set suggests an open in the MIL control circuit.
MIL Operation
The 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 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 (DTC) 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 On-Board Diagnostic (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
An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check the following items:
| • | Inspect the PCM harness and connectors for: |
| - | Improper mating |
| - | Broken locks |
| - | Improperly formed or damaged terminals |
| - | Poor terminal-to-wire connection |
| - | A damaged harness |
| • | If the engine runs OK, check for a malfunctioning MIL, an open in the MIL control circuit, or an open in the instrument cluster ignition feed. |
| • | If the engine cranks but will not run, check for an open PCM ignition, battery feed, or a poor PCM to engine ground. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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If the MIL DTC is set, the MIL Control Circuit table will better diagnose the condition.
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Using a test lamp connected to B+, probe each of the PCM ground terminals to ensure that a good ground is present.
Step | Action | Value(s) | Yes | No |
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
Does the scan tool indicate the MIL DTC set? | -- | Go to DTC P0650 Malfunction Indicator Lamp (MIL) Control Circuit | ||
3 | Inspect the PCM feed fuses. Are the fuses OK? | -- | ||
4 |
Does the test lamp illuminate? | -- | ||
5 | Probe the PCM battery feed circuit using a test lamp that is connected to a good ground. Does the test lamp illuminate? | -- | ||
Test for faulty PCM grounds or poor PCM ground connections. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | |||
7 |
Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the replacement? | -- | -- | |
8 | Locate and repair open in PCM battery feed circuit or the PCM ignition feed circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |
9 | Locate and repair short to ground in PCM ignition feed circuit or the PCM battery feed circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- |
