Alternator Control Circuit Troubleshooting

Check for proper operation of the electrical load detector (ELD) by checking for a DTC. If a DTC is present, diagnose and repair the cause before continuing with this test.
NOTE: The ELD DTC does not turn on the MIL.
Disconnect the alternator 4P connector from the alternator.
Start the engine, and turn on the headlights to high beam.
Measure the voltage between alternator 4P connector terminal No. 2 and the positive terminal of the battery.

Fig 1: Measuring Voltage Between Alternator Connector Terminal And Positive Terminal Of Battery
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there 1 V or less?
1. YES - Go to step 10.
2. NO - Go to step 5.
Turn off the headlights.
Turn the ignition switch OFF.
Connect the Honda Diagnostic System (HDS) to the data link connector (DLC) (see step 2 on HOW TO USE THE HDS (HONDA DIAGNOSTIC SYSTEM) ). Turn the ignition switch ON (II), and jump the SCS line with the HDS, then turn the ignition switch OFF.
NOTE: This step must be done to protect the engine control module (ECM)/powertrain control module (PCM) from damage.
Disconnect ECM/PCM connector B (24P).
Check for continuity between ECM/PCM connector terminal B18 and body ground.

Fig 2: Checking Continuity Between ECM/PCM Connector Terminal And Body Ground
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there continuity?
1. YES - Repair a short to ground in the wire between the alternator and the ECM/PCM (B18).
2. NO - Update the ECM/PCM if it does not have the latest software (see ECM/PCM UPDATING AND SUBSTITUTION FOR TESTING ), or substitute a known-good ECM/PCM (see HOW TO SUBSTITUTE THE ECM/PCM ), then recheck. If the symptom/indication goes away with a known-good ECM/PCM, replace the original ECM/ PCM (see ECM/PCM REPLACEMENT ).
Connect the HDS to the DLC. Turn the ignition switch ON (II), and jump the SCS line with the HDS, then turn the ignition switch OFF.
NOTE: This step must be done to protect the ECM/ PCM from damage.
Disconnect ECM/PCM connector B (24P).
Check for continuity between ECM/PCM connector terminal B18 and alternator 4P connector terminal No. 2.

Fig 3: Checking Continuity Between ECM/PCM Connector Terminal And Alternator Connector Terminal
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there continuity?
1. YES - Replace the alternator (see ALTERNATOR REMOVAL AND INSTALLATION ), or repair the alternator (see ALTERNATOR OVERHAUL ).
2. NO - Repair an open in the wire between the alternator and the ECM/PCM (B18).