Diagnostic Instructions
Diagnostic System Check - Vehicle
DTC Descriptor
DTC P0621 : Generator L-Terminal Circuit
Circuit/System Description
The ECM monitors the state of the generator turn on signal circuit. The ECM should detect a low generator turn on signal 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 generator turn on signal circuit. The ECM performs key ON and RUN tests to determine the status of the generator turn on signal circuit.
During the key ON test, if the ECM detects a high generator turn on signal circuit voltage, DTC P0621 will set. DTC P0621 will also set if, during the RUN test, the ECM detects a low generator turn on signal circuit. When the DTC sets, the ECM will send
a serial data message to the IPC to illuminate the charge indicator or a warning message to the driver information center (DIC) if equipped. The 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. When the ECM turns ON the high side driver, the voltage regulator turns ON the field circuit. When the ECM turns OFF the high side
driver, the voltage regulator turns OFF the field circuit
Conditions for Running the DTC
Key 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. |
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
| • | The ECM will command the charge indicator and or warning message to be illuminated on the instrument panel cluster (IPC) and the driver information center (DIC), if equipped. |
| • | The ECM will not illuminate the malfunction indicator lamp (MIL). |
| • | The ECM will store conditions, which were present when the DTC set as Fail Records data only. |
Conditions for Clearing the DTC
The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
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
Engine Electrical Connector End Views
Electrical Information Reference
Scan Tool Reference
Circuit/System Testing
- Ignition OFF, test for B+ between the generator B+ output terminal (stud) to ground.
| ⇒ | If less than the specified range, test the B+ circuit for a short to ground, an open or a high resistance. |
- Ignition OFF, disconnect the harness connector at the generator.
- 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. |
- 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. |
- If the circuit test normal, replace the generator.
Repair Instructions
