Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSHONDA2004S2000 L4-2.2LREPAIR AND DIAGNOSISSTARTING AND CHARGINGCHARGING SYSTEMALTERNATORTESTING AND INSPECTION
2004 Honda S2000 L4-2.2L
Alternator: Testing and Inspection
2004 Honda S2000 L4-2.2LSECTION Testing and Inspection
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 13.
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.
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 13.
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.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.