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Alternator FR Signal Circuit Troubleshooting

  1. Disconnect the 4P connector from the alternator.
  2. Turn the ignition switch ON (II).
  3. Measure voltage between ECM/PCM connector terminals B20 and C5.
    Fig 1: Measuring Voltage Between ECM/PCM Connector Terminals B20 & C5
    G02557375Courtesy of AMERICAN HONDA MOTOR CO., INC.

    Is there approx. 5V? 

    1. YES  - Go to step 4.
    2. NO  - Go to step  14.
  4. Turn the ignition switch OFF.
  5. Reconnect the alternator 4P connector.
  6. Start the engine. Hold the engine at 3,000 rpm with no load (in Park or neutral) until the radiator fan comes on, then let it idle.
  7. Measure voltage between ECM/PCM connector terminals B20 and C5.

    Does the voltage decrease when the headlights and rear window defogger are turned on? 

    1. YES  - The ALT FR signal is OK.
    2. NO  - Go to step 8.
  8. Turn the ignition switch OFF.
  9. Disconnect the negative cable from the battery.
  10. Disconnect ECM/PCM connector C (31P).
  11. Disconnect the ALT 4P connector.
  12. Connect the alternator 4P connector terminal No. 4 to body ground with a jumper wire.
    Fig 2: Connecting Alternator 4P Connector Terminal No. 4 To Body Ground With Jumper Wire
    G02557376Courtesy of AMERICAN HONDA MOTOR CO., INC.
  13. Check for continuity between body ground and ECM/PCM connector terminal C5.
    Fig 3: Checking Continuity Between Body Ground & ECM/PCM Connector Terminal C5
    G02557377Courtesy of AMERICAN HONDA MOTOR CO., INC.

    Is there continuity? 

    1. YES  - Troubleshoot the charging circuit (see GENERATORS & REGULATORS ).
    2. NO  - Repair open in the wire between the ECM/PCM (C5) and the alternator.
  14. Turn the ignition switch OFF.
  15. Disconnect the negative cable from the battery.
  16. Disconnect ECM/PCM connector C (31P).
  17. Check for continuity between body ground and ECM/PCM connector terminal C5.
    Fig 4: Checking Continuity Between Body Ground & ECM/PCM Connector Terminal C5
    G02557378Courtesy of AMERICAN HONDA MOTOR CO., INC.

    Is there continuity? 

    1. YES  - Repair short in the wire between the ECM/PCM (C5) and the alternator.
    2. NO  - Substitute a known-good ECM/PCM and recheck (see HOW TO SUBSTITUTE THE ECM/PCM FOR TESTING PURPOSES ). If the prescribed voltage is now available, replace the original ECM/PCM.
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