Inspection Procedure 19: Battery Run Down

OPERATION

The exciting currents of the alternator field coil are input into the engine-ECU (terminal No. 51) from the alternator (terminal No. 3).
The electric generating capacity of the alternator (terminal No. 1) is restricted according to the signal from the engine-ECU (terminal No. 53)

COMMENT ON TROUBLE SYMPTOM

Failure is possibly caused by failed alternator, failed generation control circuit or other faults.

PROBABLE CAUSES

Failed battery
Open/short circuit in alternator L terminal circuit, or connector damage.
Shorted alternator G terminal circuit, or connector damage
Failed alternator
Failed engine-ECU

DIAGNOSIS PROCEDURE

STEP 1. Check battery voltage.

Measure battery voltage during cranking.

OK: 8 V or more

Q. Is the check result normal?

	Go to Step 2 .

	Check battery (Refer to GROUP 54A - Battery - On-vehicle Service - Battery Test ).

STEP 2. Connector check: A-127 alternator connector

Q. Is the check result normal?

Go to Step 3 .

Repair or replace the connector.

STEP 3. Perform voltage measurement at A-127 alternator connector.

Disconnect connector, and measure at harness side.
Ignition switch: ON
Voltage between terminal No. 3 and earth.

OK: System voltage

Q. Is the check result normal?

Go to Step 7 .

Go to Step 4 .

STEP 4. Connector check: A-08 engine-ECU connector

Q. Is the check result normal?

Go to Step 5 .

Repair.

STEP 5. Check harness between A-08 (terminal No. 51) engine-ECU connector and A-127 (terminal No. 3) alternator connector.

Check power supply line for open/short circuit.

Q. Is the check result normal?

Go to Step 6 .

Repair the damaged harness wire.

STEP 6. Check the trouble symptoms.

Q. Does trouble symptom persist?

	Replace the engine-ECU then perform the variant coding (Refer to GROUP 00 - Precautions Before Service - How to Perform Variant Coding ).

	Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).

STEP 7. Connector check: A-08 engine-ECU connector

Q. Is the check result normal?

Go to Step 8 .

Repair or replace the connector.

STEP 8. Perform voltage measurement at A-08 engine-ECU connector.

Disconnect connector, and measure at harness side.
Ignition switch: ON
Voltage between terminal No. 53 and earth.

OK: System voltage

Q. Is the check result normal?

Go to Step 10 .

Go to Step 9 .

STEP 9. Check harness between A-08 (terminal No. 53) engine-ECU connector and A-127 (terminal No. 1) alternator connector.

Check output line for short circuit.

Q. Is the check result normal?

Replace the alternator.

Repair the damaged harness wire.

STEP 10. Perform voltage measurement at A-08 engine-ECU connector.

Disconnect engine-ECU connector, and connect special tool power plant ECU check harness (MB991987), and then measure the voltage on the check connector.
Engine: Idling after warm-up
Transmission: Neutral
Radiator fan: Inactive
Voltage between terminal No. 53 and earth.

OK: Switching the headlamps to ON from OFF causes the voltage to increase.

Q. Is the check result normal?

Go to Step 6 .