GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 GMC C Sierra - 2WD 🢒 Engine 🢒 Engine Controls - 6.6L 🢒 DTC P0621

Circuit Description

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.

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 turn on the charge indicator in the IPC.

Conditions for Running the DTC
    • The ignition is ON.
    • The engine speed is more than 1000 RPM.

Conditions for Setting the DTC
    • The ECM detects the GEN L-Terminal active with the ignition ON.
    • The ECM detects the GEN L-Terminal inactive with the engine operating.
    • The above conditions are present for 6 seconds.

Action Taken When the DTC Sets
    • The ECM will record the operating conditions presents when the DTC set as Failure Records data only.
    • The instrument panel cluster (IPC) displays a message.
    • The malfunction indicator lamp (MIL) will not illuminate.

Conditions for Clearing the DTC
    • The Conditions for Setting the DTC are no longer present.
    • A history DTC will clear after 40 malfunction free ignition cycles.
    • The Engine Control Module (ECM) receives the clear code command from the scan tool.

Step

Action

Yes

No

Schematic Reference: Starting and Charging Schematics

1

Did you perform the Engine Electrical Diagnostic System Check?

Go to Step 2

Go to

2
  1. Install a scan tool.
  2. Start the engine.
  3. With a scant tool, monitor the DTC information for DTC P0621 in Engine Controls.

Does the scan tool indicate that DTC P0621 has passed?

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3

Test the charge indicator contol circuit for a short or open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 6

Go to Step 4

4

Inspect for poor connections at the harness connector of the ECM. Refer to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 6

Go to Step 5

5

Important: The replacement ECM must be programmed. Refer to Engine Control Module Programming Procedure in Engine Controls 6.6 L.

Replace the ECM. Refer to Engine Control Module Replacement in Engine Controls 6.6 L.

Did you compete the replacement?

Go to Step 6

--

6
  1. Review and record the scan tool Fail Records data.
  2. Clear any DTCs.
  3. Operate the vehicle within the Fail Records conditions as noted.
  4. Using a scan tool, monitor the Specific DTC info for this DTC.

Does the scan tool indicate that this DTC failed this ignition?

Go to Step 2

System OK