Alternator FR Signal Circuit Troubleshooting

WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2005 Honda S2000, 2004 Honda S2000, 2003 Honda S2000, 2002 Honda S2000, and 2001 Honda S2000. However, it is still accessible from the selected car via links, so may be relevant.

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

Fig 1: Measuring Voltage Between ECM Connector Terminals B20 And 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 speed at 3,000 RPM without load (in neutral) until the radiator fan comes on, then let it idle.
Measure voltage between ECM connector terminals B20 and C5.
Does the voltage decrease when the headlights and rear window defogger are turned on?
1. YES - The alternator FR signal is OK.
2. NO - Go to step 8.
Turn the ignition switch OFF.
Disconnect ECM connector C (31P).
Disconnect 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 ECM connector terminal C5.

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

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

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

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