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P codepowertrainMedium severity

P0625Generator Field/F Terminal Circuit Low

The voltage on the generator (alternator) field control circuit is stuck below the expected range, so the PCM cannot properly regulate charging output.

Definition

The PCM regulates alternator output by pulse-width modulating the field (F) terminal circuit and monitoring its voltage or duty-cycle feedback. P0625 sets when that circuit reads continuously low — typically from a short to ground, an open feed, or a failed regulator holding the line down. Depending on the design the alternator may stop charging entirely or default to a fixed setpoint, producing a discharged battery, dimming lights, and a battery warning lamp; the MIL usually illuminates. Continued driving on battery alone will eventually cause a stall and no-start.

Common causes

  • Field circuit wiring shorted to ground (chafed harness near the engine)
  • Failed voltage regulator or field winding inside the alternator
  • Corroded or damaged terminals at the alternator connector
  • Open in the field circuit feed causing the monitored line to float low
  • PCM field driver stuck low (least common)

Common misdiagnoses

  • Replacing the battery for what is a field-control no-charge condition
  • Replacing the alternator when a grounded field wire in the harness is the fault
  • Replacing the belt or tensioner for an electrical regulation failure
  • Replacing the PCM before isolating the short with the connector unplugged

Troubleshooting steps

  1. 1.Measure charging performance

    Check battery voltage with the engine running; below about 13.2 V at fast idle with loads off indicates the field is not being driven.

  2. 2.Compare commanded and actual field duty

    On the scan tool, load the electrical system and watch generator field duty; an actual value pinned at or near 0% while desired rises confirms the low circuit.

  3. 3.Isolate the circuit

    Unplug the alternator connector with key on and measure the field control wire; if it remains near 0 V when it should show bias voltage, check the wire for a short to ground and the feed for an open.

  4. 4.Inspect the harness and connector

    Trace the field wire for chafe-through points against brackets or the block, and verify the alternator connector terminals grip firmly and are corrosion-free.

  5. 5.Test or replace the alternator

    If the circuit is intact and properly biased with the alternator connected, the internal regulator or field winding is holding the line low — bench-test and replace the alternator.

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