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OBD-II Trouble Shooting Got a code reader? Find out what your GM car's Check Engine light is trying to tell you (and then turn Check Engine light off).

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Old 11-25-2009, 08:11 PM   #1
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Generator Performance
The power control module (PCM) output (L) terminal to the alternator determines when the alternator will be turned off. The alternator is on under all normal vehicle operating conditions except during the low RPM conditions. The alternator is turned off during low RPM conditions in order to remove the load from the engine.

The PCM input field (F) terminal from the alternator provides a 400 Hz pulse-width modulated (PWM) signal to the PCM. The duty cycle of the F terminal mimics the duty cycle of the field coils in the alternator, thus, the F terminal duty cycle indicates the relative electrical load on the alternator.

The alternator telltale lamp will illuminate any time the ignition is on and the engine is not running. The telltale lamp will turn off after the engine is running and the alternator is putting out the desired amperage.


  • The PCM will not illuminate the malfunction indicator lamp (MIL).
  • The PCM will store the conditions, which were present when the DTC set as Failure Records Data only. This information will not be stored as Freeze Frame data.
  • The L terminal voltage is low while the PCM is commanding the generator on.
  • The above condition is present for more than 30 seconds.
  • The F terminal PWM is less than 5 percent.
  • The engine RPM is less than 2500
  • The above conditions are present for more than 30 seconds.
Check for the following conditions:
  • A faulty connection at the PCM inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections.
  • Inspect the wiring harness for damage.
  • Several steps prior to below require a Scan tool to command the generator on and to verify voltage which may indicate a problem with the starting and charging engine electrical system.
  • Test the L circuit for a short to voltage. Were problems found?
    • YES Repair as necessary and clear DTC
    • NO - Test the F circuit for an open or a short to ground. Were problems found?
      • YES Repair as necessary and clear DTC
    • NO - Test the F circuit for a short to voltage. Were problems found?
      • YES Repair as necessary and clear DTC
      • NO Check for a faulty connection at the PCM. Were problems found?
        • YES - Repair as necessary and clear DTC
        • NO Replace the PCM.
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