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For 1990-2009 cars only
Makes 🢒 Isuzu 🢒 2000 🢒 S/T Isuzu - 2WD 🢒 DTC 🢒 DTC P0621 Generator L-Terminal Circuit

Circuit Description

The L-terminal circuit from the generator is a discrete circuit (a discrete circuit has no splices and only one source and destination) into the PCM. The PCM applies ignition voltage to the generator L-terminal circuit. A small amount of current flows from this circuit through the generator windings to ground to create a magnetic field which starts the generator process. When the generator is at operating speed and producing voltage, a solid state switch for the L-terminal circuit in the generator opens and the PCM detects that the initial startup current flow has stopped.

The PCM expects to detect low voltage on the L-terminal circuit prior to the generator rotating at operating speed and conversely expects the circuit to be at ignition voltage potential when the generator is operational. When the PCM detects a fault (circuit shorted to ground, or circuit shorted to voltage), the Generator Lamp may also illuminate.

Conditions for Running the DTC

The ignition switch in the ON position or the engine is operating.

Conditions for Setting the DTC
    • The PCM detects the L terminal as Active with the key ON and the engine not running or the PCM detects the L-terminal as Inactive with the engine running.
    • All conditions met for 6 seconds.

Action Taken When the DTC Sets
    • The PCM stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Failure Records.
    • A message is displayed on the Drivers Information Center.

Conditions for Clearing the DTC
    • A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
    • A history DTC will clear after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
    • Use a scan tool in order to clear the DTC.

Diagnostic Aids

Important: 

   • Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing/replacing the module. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion into the PCM.
   • For any test that requires probing the PCM or a component harness connector, use the Connector Test Adapter Kit J 35616 . Using this kit prevents damage to the harness/component terminals.

    • Use a scan tool in order to diagnosis the generator circuits. The scan tool should display Inactive for the L terminal and 10-40 percent for the F terminal when the ignition is ON. When the engine is operating, the display should indicate that the L terminal is Active and the F terminal is greater than 5 percent.
    • A generator fault, such as a shorted output diode, malfunctioning regulator, open or shorted rotor, or open sense lead or battery feed may cause a low battery charge. Refer to Engine Electrical for further information and generator diagnosis.

Test Description

The numbers below refer to the step numbers on the Diagnostic Table.

  1. This step verifies the fault is current.

    2.

  2. Inspect the circuit for an open or a short to ground if the test lamp did not illuminate.

    3.

  3. If the scan tool displays Active, this indicates the circuit is open.

    4.

  4. This step tests the F terminal for being shorted to a voltage. With the ignition ON, the F terminal should display between 10-40 percent. If the F terminal displays greater than 40 percent, inspect the circuit for a short to voltage.

    5.

  5. When the engine is started the L terminal should be active.

    6.

  6. This step is testing the L terminal for an open. If the L terminal displays Active when the circuit is grounded, this indicates the circuit is open.

    7.

  7. This step is testing the F terminal circuit for an open or a short to ground. If the F terminal parameter goes to 99 percent the condition is within the generator.

DTC P0621 Generator L Terminal

Step

Action

Values

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

Important: The following tests assumes that the generator belt is installed correctly and in good condition. The battery is adequately charged.

  1. Install a scan tool.
  2. Turn ON the ignition leaving the engine OFF.
  3. Monitor the DTC Information using a scan tool.

Did P0621 set?

--

Go to Step 4

Go to Step 3

3
  1. Start the engine.
  2. Monitor the DTC Information using a scan tool.

Did DTC P0621 fail this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4

  1. Turn OFF the engine.
  2. Disconnect the generator 4-way electrical connector.
  3. Turn ON the ignition with the engine OFF.
  4. Probe the generator sense circuit (terminal S on generator) with the test lamp connected to ground and then probe the generator battery feed circuit (mounted to rear of generator) with the test lamp connected to ground.

Does the test lamp illuminate for both circuits?

--

Go to Step 5

Go to Charging System Check in Engine Electrical

5

  1. Turn OFF the ignition.
  2. Reconnect the generator 4-way electrical connector.
  3. Turn ON the ignition with the engine OFF.
  4. Monitor the L terminal parameter using a scan tool.

Does the scan tool display L terminal as inactive?

--

Go to Step 6

Go to Step 8

6

Monitor the F terminal parameter with a scan tool in the Engine 3 Data list.

Is the F terminal percentage less than the specified value?

40%

Go to Step 7

Go to Step 13

7

  1. Start the engine.
  2. Monitor the L terminal parameter.

Does the scan tool display L terminal as inactive?

--

Go to Step 14

Go to Step 10

8

  1. Turn OFF the engine.
  2. Disconnect the generator 4-way electrical connector.
  3. Jumper the L terminal at the Generator harness connector to battery ground.
  4. Monitor the Generator L terminal using a scan tool.

Does the scan tool display L terminal as inactive?

--

Go to Step 20

Go to Step 9

9
  1. Turn OFF the ignition.
  2. Disconnect the PCM connector C2 located on the opposite side of the manufacturer's logo.
  3. Test the continuity of the L terminal circuit from the PCM harness connector to the generator harness connector with the DMM.

Does the DMM indicate continuity within the specified range?

0-2ohms

Go to Step 18

Go to Step 15

10

  1. Disconnect the generator 4-way electrical connector.
  2. Probe the F terminal at the Generator harness connector with the test lamp connected to B+ .
  3. Monitor the Generator F terminal using a scan tool while probing the circuit with a test lamp to B+.

Did the Generator F terminal parameter change from 0 to 99 percent?

--

Go to Step 20

Go to Step 11

11

Test the continuity of the F terminal circuit from the generator harness connector to battery ground with the DMM.

Does the DMM indicate continuity within the specified range?

0-10ohms

Go to Step 16

Go to Step 12

12
  1. Turn OFF the ignition.
  2. Disconnect the PCM connector C2 located on the opposite side of the manufacturer's logo.
  3. Test the continuity of the F terminal circuit from the PCM harness connector to the generator harness connector with the DMM.

Does the DMM indicate continuity within the specified range?

0-2ohms

Go to Step 18

Go to Step 15

13
  1. Disconnect the generator 4-way electrical connector.
  2. Monitor the Generator F terminal with a scan tool.

Is the F terminal percentage at the specified value?

0%

Go to Step 21

Go to Step 17

14
  1. Disconnect the generator 4-way electrical connector.
  2. Monitor the Generator L terminal with a scan tool.

Does the scan tool display L terminal as inactive?

--

Go to Step 16

Go to Step 21

15

Repair the open/high resistance in the generator circuit. Refer to Wiring Repairs in Wiring Systems.

Is the action complete?

--

Go to Step 22

--

16

Repair the short to ground in the generator circuit. Refer to Wiring Repairs in Wiring Systems.

Is the action complete?

--

Go to Step 22

--

17

Repair the short to voltage on the F terminal circuit. Refer to Wiring Repairs in Wiring Systems.

Is the action complete?

--

Go to Step 22

--

18
  1. Inspect for a poor connection at the PCM harness connector. Refer to Connector Repairs in Wiring Systems.
  2. If you find a poor connection, repair the connection as necessary. Refer to Connector Repairs in Wiring Systems.

Is the action complete?

--

Go to Step 22

Go to Step 19

19

Important: Program the replacement PCM.

Replace the PCM. Refer to Powertrain Control Module Replacement/Programming .

Is the action complete?

--

Go to Step 22

--

20
  1. Inspect for poor connections at the generator electrical connector. Refer to Connector Repairs in Wiring Systems.
  2. If you find a poor connection, repair the connection as necessary. Refer to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 21

21

Replace the generator. Refer to Generator Replacement in Engine Electrical.

Is the action complete?

--

Go to Step 22

--

22
  1. Select the Diagnostic Trouble Code (DTC) option and the Clear DTC Information option using the scan tool.
  2. Start the engine and idle at the normal operating temperature.
  3. Select the Diagnostic Trouble Code (DTC) option and the Specific DTC option, then enter the DTC number using the scan tool.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text, if applicable.

Does the scan tool indicate that this diagnostic ran and passed?

--

Go to Step 23

Go to Step 2

23

Select the Capture Info option and the Review Info option using the scan tool.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to the applicable DTC table

System OK