P0621
DTC P0621
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptor
DTC P0621
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Generator L-Terminal Circuit
Diagnostic Fault Information
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 Ignition ON and RUN tests to determine the status of the generator turn on signal circuit.
Conditions for Running the DTC
Ignition ON Test
* No generator, crankshaft position (CKP) sensors, or camshaft position (CMP) sensor DTCs are set.
* The ignition is in RUN position.
* The engine is not running.
Run Test
* No generator, CKP sensors, CMP sensor DTCs are set.
* The engine is running.
Conditions for Setting the DTC
* During the Ignition ON test, the ECM detects high voltage on the generator turn on signal circuit for 5 seconds.
* During the RUN test, the ECM detects low voltage on the generator turn on signal circuit for 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 [1][2]Electrical Diagrams
Connector End View Reference
Component Connector End Views 5 Bow Lowered Switch (CM7)
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Control Module References Control Module References for scan tool information
Circuit/System Testing
LS7 Engine
1. Verify that a test lamp illuminates between the G13 generator B+ terminal 1 X2 and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Ignition OFF.
2. Test for less than 2 ohms in the B+ circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, verify the fuse is not open and there is voltage at the fuse.
• If the test lamp does not illuminate and the circuit fuse is open
1. Ignition OFF.
2. Test for greater than 100 ohms between the B+ circuit and ground.
• If 100 ohms or less, repair the short to ground on the circuit.
• If greater than 100 ohms, replace the generator.
• If the test lamp illuminates
2. Ignition OFF, disconnect the X1 harness connector at the generator, ignition ON.
3. Test for 3.5V or greater between the control circuit terminal B X1 and ground.
• If less than 3.5V
1. Ignition OFF, disconnect the harness connectors at the ECM.
2. Test for infinite resistance between the control circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 ohms in the control circuit end to end.
• If greater than 2 ohms, repair the open/high resistance in the circuit.
• If less than 2 ohms, replace the ECM.
• If 3.5V or greater
1. Ignition OFF, disconnect the harness connectors at the ECM.
2. Test for less than 1 V between the control circuit and ground.
• If less than 1 V, replace the Generator.
• If 1 V or greater, repair the short to voltage on the circuit.
LS3 Engine
1. Verify that a test lamp illuminates between the G13 generator B+ terminal 1 X2 and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Ignition OFF.
2. Test for less than 2 ohms in the B+ circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, verify the fuse is not open and there is voltage at the fuse.
• If the test lamp does not illuminate and the circuit fuse is open
1. Ignition OFF.
2. Test for greater than 100 ohms between the B+ circuit and ground.
• If 100 ohms or less, repair the short to ground on the circuit.
• If greater than 100 ohms, replace the generator.
• If the test lamp illuminates
2. Ignition OFF, disconnect the X1 harness connector at the generator, ignition ON.
3. Test for 4.5-5.5 V between the control circuit terminal B X1 and ground.
• If less than 4.5 V
1. Ignition OFF, disconnect the harness connectors at the ECM.
2. Test for infinite resistance between the control circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 ohms in the control circuit end to end.
• If greater than 2 ohms, repair the open/high resistance in the circuit.
• If less than 2 ohms, replace the ECM.
• If greater than 5.5 V
1. Ignition OFF, disconnect the harness connectors at the ECM, ignition ON.
2. Test for less than 1 V between the control circuit and ground.
• If greater than 1 V, repair the short to voltage on the circuit.
• If less than 1 V, replace the ECM.
• If between 4.5-5.5 V
4. Test or replace the generator.
LS9 Engine
1. Verify that a test lamp illuminates between the G13 generator B+ terminal 1 X2 and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Ignition OFF.
2. Test for less than 2 ohms in the B+ circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, verify the fuse is not open and there is voltage at the fuse.
• If the test lamp does not illuminate and the circuit fuse is open
1. Ignition OFF.
2. Test for greater than 100 ohms between the B+ circuit and ground.
• If 100 ohms or less, repair the short to ground on the circuit.
• If greater than 100 ohms, replace the generator.
• If the test lamp illuminates
2. Ignition OFF, disconnect the X1 harness connector at the generator, ignition ON.
3. Test for 4.5-5.5 V between the control circuit terminal 1 X1 and ground.
• If less than 4.5 V
1. Ignition OFF, disconnect the harness connectors at the ECM.
2. Test for infinite resistance between the control circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 ohms in the control circuit end to end.
• If greater than 2 ohms, repair the open/high resistance in the circuit.
• If less than 2 ohms, replace the ECM.
• If greater than 5.5 V
1. Ignition OFF, disconnect the harness connectors at the ECM, ignition ON.
2. Test for less than 1 V between the control circuit and ground.
• If greater than 1 V, repair the short to voltage on the circuit.
• If less than 1 V, replace the ECM.
• If between 4.5-5.5 V
4. Test or replace the generator.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
* Generator Replacement (LS3 or LS7) Generator ReplacementGenerator Replacement (LS9) Generator Replacement
* Control Module References Control Module References for ECM replacement, programming and setup