Alternator FR Signal Circuit Troubleshooting

Disconnect the alternator 4P connector.
Turn the ignition switch ON (II).
Measure voltage between PCM connector terminals B20 and C5.

Fig 1: Measuring Voltage Between PCM Connector Terminals B20 & C5
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there about 5 V?
1. YES -Go to step 4.
2. NO -Go to step 13.
Turn the ignition switch OFF.
Reconnect the alternator 4P connector.
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.
Measure voltage between 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.
Turn the ignition switch OFF.
Disconnect PCM connector C (31P).
Disconnect the alternator 4P connector.
Connect 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 A Jumper Wire
Courtesy of AMERICAN HONDA MOTOR CO., INC.
Check for continuity between body ground and PCM connector terminal C5.

Fig 3: Checking For Continuity Between Body Ground & PCM Connector Terminal C5
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there continuity?
1. YES -Test the alternator (see CHARGING CIRCUIT ).
2. NO -Repair open in the wire between the PCM (C5) and the alternator.
Turn the ignition switch OFF.
Disconnect PCM connector C (31P).
Check for continuity between body ground and PCM connector terminal C5.

Fig 4: Checking For Continuity Between Body Ground & PCM Connector Terminal C5
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there continuity?
1. YES -Repair short in the wire between the PCM (C5) and the alternator.
2. NO -Substitute a known-good PCM and recheck (see HOW TO SUBSTITUTE THE PCM FOR TESTING ). If voltage is normal, replace the original PCM.