GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2007 🢒 Lucerne 🢒 DTC 🢒 DTC P1668

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC P1668: Generator L-Terminal Control Circuit

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 ON 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 voltage 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 key ON and RUN tests to determine the status of the generator turn ON signal 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.
    • The engine is running.

Conditions for Setting the DTC

During the Key ON test the ECM detects a high signal voltage on the generator turn ON signal circuit for at least 5 seconds or during the RUN test the ECM detects a low signal voltage on the generator turn ON signal 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 ECM will command the message OFF after one trip in which the diagnostic test has been run and passed.
    • The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

Reference Information

Schematic Reference

Starting and Charging Schematics

Connector End View Reference

Engine Electrical Connector End Views

Description and Operation

Charging System Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Data Definitions

Circuit/System Testing
  1. Ignition OFF, disconnect the harness connector at the generator.
  2. Ignition ON, test for less than 1 volt between the generator turn on signal and ground.
    3. ⇒If greater than the specified range, test the generator turn on signal circuit for a short to voltage. If the circuit tests normal, replace the ECM.
  3. Engine Running, test for greater than 3.5 volts between the generator turn on signal and ground.
    4. ⇒ If less than the specified range, test the generator turn on signal circuit for a short to ground, an open or a high resistance. If the circuit tests normal, replace the ECM.
  4. If all circuits test normal replace the generator.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

    •  Control Module References for ECM replacement, setup, and programming
    •  Generator Replacement