DTC P0621 6.6L/ 8.1L

Diagnostic Instructions

DTC Descriptor

DTC P0621 : Generator L-Terminal Circuit

Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
Generator L-Terminal Circuit	P0621	P0621	P0621	--

Circuit/System Description

The engine control module (ECM) uses the generator turn ON signal circuit to control the load of the generator on the engine. A high side driver in the ECM applies a voltage to the voltage regulator. This signals the voltage regulator to turn the field circuit ON and OFF. The ECM monitors the state of the generator turn ON signal circuit. The ECM should detect low voltage on generator turn on signal circuit when the ignition is ON and the engine is OFF, or when the charging system malfunctions. With the engine running, the ECM should detect high voltage on the generator turn on signal circuit. The ECM performs the Ignition ON and RUN tests to determine the status of the generator turn on signal circuit.

Conditions for Running the DTC

Conditions for Setting the DTC

Action Taken When the DTC Sets

DTC P0621 is a C type DTC.

Conditions for Clearing the DTC

DTC P0621 is a C type DTC.

Diagnostic Aids

On vehicles equipped with a sense circuit connected to the voltage regulator.This circuit must have voltage for the charging system to operate properly.This circuit is used as a reference to control system voltage. All circuits must be tested at the generator before replacing any components. refer to system schematic to confirm if the vehicle being serviced uses the sense circuit as part of the charging system.

Reference Information

Schematic Reference

Starting and Charging Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Charging System Description and Operation

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Testing

1. Ignition OFF, test for B+ between the generator B+ output terminal (stud) to ground.

⇒	If less than the specified range, test the circuit for a short to ground, an open or a high resistance.

2. Ignition OFF, disconnect the harness connector at the generator.
3. Ignition OFF, test for B+ between the battery positive voltage circuit terminal  D and ground.

⇒	If less than the specified range, test the circuit for a short to ground, an open or a high resistance.

4. Ignition ON, test for less than 1  volt between the generator turn on signal circuit terminal B and ground.

⇒	If greater than the specified range, test the generator turn on signal circuit for a short to voltage. If the circuit tests normal, replace the ECM.

5. Engine running, test for greater than 3.5 volts between the generator turn on signal circuit terminal 1 and ground.

⇒	If less than the specified range, test the generator turn on signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the ECM.

6. If the circuit tests normal, test or replace the generator.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

DTC P0621 7.2L / 7.8L

Diagnostic Instructions

DTC Descriptor

DTC P0621 : Charge Indicator Control Circuit

Circuit/System Description

The ECM monitors the state of the charge indicator control circuit. The ECM should detect a low circuit voltage when the key is ON and the engine is OFF, or when the charging system malfunctions. With the engine running, the ECM should detect a high circuit voltage. The ECM performs key ON and RUN tests to determine the status of the charge indicator control circuit.

During the key ON test, if the ECM detects a high circuit voltage, DTC P0621 will set. DTC P0621 will also set if, during the RUN test, the ECM detects a low circuit voltage. When the DTC sets, the IPC will illuminate the charge indicator or a warning message will be displayed in the driver information center (DIC) if equipped.

Conditions for Running the DTC

Key ON Test

RUN Test

Conditions for Setting the DTC

During the key ON test, the ECM detects high voltage on the charge indicator control circuit for at least 5 seconds. OR during the RUN test the ECM detects low voltage on the charge indicator control circuit for at least 15 seconds.

Action Taken When the DTC Sets

DTC P0621 is a C type DTC.

Conditions for Clearing the DTC

DTC P0621 is a C type DTC.

Diagnostic Aids

On vehicles equipped with a sense circuit connected to the voltage regulator. This circuit must have voltage for the charging system to operate properly. This circuit is used as a reference to control system voltage. All circuits must be tested at the generator before replacing any components. Refer to system schematic to confirm if the vehicle being serviced uses the sense circuit as part of the charging system.

Reference Information

Schematic Reference

Starting and Charging Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Testing

1. Ignition OFF, test for B+ between the generator B+ output terminal (stud) to ground.

⇒	If less than the specified range, test the circuit for a short to ground, an open or a high resistance.

2. Ignition OFF, disconnect the harness connector at the generator.
3. Ignition ON, test for B+ between the Ignition voltage circuit terminal D and ground.

⇒	If less than the specified range, test the circuit for a short to ground, an open or a high resistance.

4. Ignition ON, test for greater than 3.5 volts between the charge indicator control circuit terminal B and ground.

⇒	If less than the specified range, test the circuit for a short to ground, an open or a high resistance. If the circuit tests normal, replace the ECM.

5. If the circuit test normal, replace the generator.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.