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
- 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 (Electrical Diagrams)
Connector End View Reference
Component Connector End Views (Connector End Views By Name)
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 (Programming and Relearning)for scan tool information
Circuit/System Testing
LS7 Engine
1. Ignition OFF, test for B+ between the B+ terminal 1 X2 and ground.
• If less than the specified range, test the B+ circuit for a short to ground, an open/high resistance.
2. Ignition OFF, disconnect the X1 harness connector at the generator.
3. Ignition ON, test for less than 1 V between the control circuit terminal B and ground.
• If greater than the specified range, test the control circuit for a short to voltage. If the circuit tests normal, replace the ECM.
4. Engine running, test for greater than 3.5 V between the control circuit terminal B and ground.
• If less than the specified range, test the control circuit for a short to ground, an open/high resistance. If the circuit tests normal, replace the ECM.
5. If the circuit tests normal, replace the generator.
LS3 Engine
1. Verify that a test lamp illuminates between the generator B+ terminal 1 X2 and ground.
• If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
2. Ignition OFF, disconnect the X1 harness connector at the generator.
3. Ignition ON, test for 4.5-5.5 V between the control circuit terminal 1 and ground.
• If greater than the specified range, test the control circuit for a short to voltage. If the circuit tests normal, replace the ECM.
• If less than the specified range, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal replace the ECM.
4. If the circuits test normal, replace the generator.
LS9 Engine
1. Verify that a test lamp illuminates between the generator B+ terminal 1 X2 and ground.
• If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
2. Ignition OFF, disconnect the X1 harness connector at the generator.
3. Ignition ON, test for 4.5-5.5 V between the control circuit terminal 1 and ground.
• If greater than the specified range, test the control circuit for a short to voltage. If the circuit tests normal, replace the ECM.
• If less than the specified range, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal replace the ECM.
4. If the circuits test normal, replace the generator.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Generator Replacement (LS3 or LS7) (Generator Replacement)Generator Replacement (LS9) (Generator Replacement)
* Control Module References (Programming and Relearning)for ECM replacement, programming and setup