Alternator FR Signal Circuit Troubleshooting

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




3. Measure voltage between PCM connector terminals B20 and C5.
Is there approx. 5 V?
YES - Go to step 4.
NO - Go to step 14.
4. Turn the ignition switch OFF.
5. Reconnect the ALT 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 PCM connector terminals B20 and C5.
Does the voltage decrease when the headlights and rear defogger are turned on?
YES - The ALT FR signal is OK.
NO - Go to step 8.
8. Turn the ignition switch OFF.
9. Disconnect the negative battery cable from the battery.
10. Disconnect the PCM connector C (31P).
11. Disconnect the ALT 4P connector.




12. Connect the ALT 4P connector terminal No.4 to body ground with a jumper wire.




13. Check for continuity between body ground and PCM connector terminal C5.
Is there continuity?
YES - Test the ALT.
NO - Repair open in the wire between the PCM (C5) and the ALT.
14. Turn the ignition switch OFF.
15. Disconnect the negative battery cable from the battery.
16. Disconnect the PCM connector C (31P)




17. Check for continuity between body ground and PCM connector terminal C5.
Is there continuity?
YES - Repair short in the wire between the PCM (C5) and the ALT.
NO - Substitute a known-good PCM and recheck. If voltage is normal, replace the original PCM.
Diagnostic Strategies