Alternator FR Signal Circuit Troubleshooting

1. Disconnect the alternator 4P connector.
2. Turn the ignition switch ON (II).




3. Measure voltage between ECM connector terminals B20 and C5.

Is there about 5 V?

YES - Go to step 4.

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 neutral) until the radiator fan comes on, then let it idle.
7. Measure voltage between ECM connector terminals B20 and C5.

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

YES - The alternator FR signal is OK.

NO - Go to step 8.

8. Turn the ignition switch OFF.
9. Disconnect ECM connector C (31P).
10. Disconnect the alternator 4P connector.




11. Connect alternator 4P connector terminal No.4 to body ground with a jumper wire.




12. Check for continuity between body ground and ECM connector terminal C5.

Is there continuity?

YES - Test the alternator.

NO - Repair open in the wire between the ECM (C5) and the alternator.

13. Turn the ignition switch OFF.
14. Disconnect ECM connector C (31P).




15. Check for continuity between body ground and ECM connector terminal C5.

Is there continuity?

YES - Repair short in the wire between the ECM (C5) and the alternator.

NO - Substitute a known-good ECM and recheck. If prescribed voltage is now normal, replace the original ECM.