COUPLING 3D5377 - Caterpillar

This diagnostic code is recorded when the Electronic Control Module (ECM) detects a battery voltage of less than 18 DCV for 2 seconds.Determine if a battery charger or a welder is being used on the machine. Disconnect the charger or the welder if one is used. Recheck for the diagnostic code. Check the charging system if the diagnostic code is still active.System Response:The system voltage is low. The system voltage that is required in order to instruct the ECM to energize the shift solenoids is insufficient.Test Step 1. CHECK THE SYSTEM VOLTAGE AT THE ECM.
Start the engine and operate the engine at a high idle.
Measure the system voltage between all of the +battery contacts and negative battery contacts. Expected Result:The voltage is 18 DCV to 35 DCV.Results:
OK - The voltage is 18 DCV to 35 DCV.STOP
NOT OK - The voltage is NOT 18 DCV to 35 DCV.Repair: Repair the harness or replace the harness.Proceed to test step 2.Test Step 2. CHECK THE ALTERNATOR.
Run the engine at a high idle for several minutes.
Measure the voltage output at the battery posts. Expected Result:The voltage should be 27 DCV to 29 DCV.Results:
OK - The battery could have failed. Proceed to test step 3.
NOT OK - The alternator or the regulator may have failed.Repair: Repair the alternator or replace the alternator. The voltage regulator could have failed. If conditions are not resolved, proceed to test step 3.Test Step 3. CHECK THE BATTERY VOLTAGE.
Turn the key start switch to the OFF position.
Measure the battery voltage at the battery posts. Expected Result:The voltage should be 15 DCV to 35 DCV.Results:
OK - The voltage is between 15 DCV and 35 DCV. The transmission/chassis ECM could have failed.Repair: It is unlikely that the ECM has failed. Exit this procedure and perform this procedure again. Also, recheck if the diagnostic code is active. If the cause of the diagnostic code is not found, replace the ECM. See Testing and Adjusting, "Electronic Control Module (ECM) - Replace".STOP
NOT OK - The voltage is not between 15 DCV and 35 DCV. Replace the battery. If conditions are not resolved, the ECM could have failed.Repair: It is unlikely that the ECM has failed. Exit this procedure and perform this procedure again. Also, recheck if the diagnostic code is active. If the cause of the diagnostic code is not found, replace the ECM. See Testing and Adjusting, "Electronic Control Module (ECM) - Replace".STOP