Alternator Control Circuit Troubleshooting

Connect the Honda Diagnostic System (HDS) to the data link connector (DLC), and check for DTCs (see HOW TO USE THE HDS (HONDA DIAGNOSTIC SYSTEM) ). If a DTC is present, diagnose and repair the cause before continuing with this test.
Disconnect the alternator 4P connector.
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 4P Connector Terminal No. 2 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.
With the ignition switch ON (II), connect the Honda Diagnostic System (HDS) to the data link connector (DLC) (see step 2 in HOW TO USE THE HDS (HONDA DIAGNOSTIC SYSTEM) ), and go into any of the live data screens.
NOTE: Steps 6 through 8 must be done to protect the powertrain control module (PCM) from damage.
Turn the ignition switch OFF.
Select "SCS" mode using the HDS.
Disconnect PCM connector A (73P).
Check for continuity between PCM connector A terminal No. 23 and body ground.

Fig 2: Checking Continuity Between PCM Connector A Terminal No. 23 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 PCM.
2. NO - Update the PCM if it does not have the latest software (see PCM UPDATING AND SUBSTITUTION FOR TESTING ), or substitute a known-good PCM (see HOW TO SUBSTITUTE THE PCM ), then recheck. If the prescribed voltage is now available, and the PCM was updated, troubleshooting is complete. If the PCM was substituted, replace the original PCM (see PCM REPLACEMENT ).
Turn off the headlights.
With the ignition switch ON (II), connect the HDS to the DLC (see step 2 in HOW TO USE THE HDS (HONDA DIAGNOSTIC SYSTEM) ), and go into any of the live screens.
NOTE: Steps 12 through 14 must be done to protect the PCM from damage.
Turn the ignition switch OFF.
Turn the HDS off and then back on. Select "SCS" mode using the HDS.
Disconnect PCM connector A (73P).
Check for continuity between PCM connector A terminal No. 23 and alternator 4P connector terminal No. 2.

Fig 3: Checking Continuity Between PCM Connector A Terminal No. 23 And Alternator 4P Connector Terminal No. 2
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 PCM.