GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The engine control module (ECM) uses the generator turn-on signal circuit to control the voltage output of the generator. When a different voltage is desired, the ECM will change the pulse width modulation (PWM) signal to the voltage regulator via the generator turn-on signal circuit. This causes the voltage regulator to the voltage output of the generator. The ECM has fault detection circuitry which monitors the state of the generator turn-on signal circuit. If the fault detection circuit senses a voltage other than what is expected, this DTC will set. The voltage regulator also contains fault detection circuitry. If the regulator detects a problem, the regulator will ground the generator turn-on signal circuit, pulling the voltage low. This also causes the ECM to set the DTC.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P1668 Generator L - Terminal Control Circuit

Conditions for Running the DTC

Ignition ON Test

    • The ignition is in the ON position for 5 seconds.
    • The engine is OFF.

Engine Run Test

    • The engine is running.
    • The engine speed is less than 3,000 RPM.

Conditions for Setting the DTC

Ignition ON Test

    • The ECM detects a high signal voltage on the generator turn on signal circuit for at least 5 seconds.
    • The engine must be started.

During the RUN test, the ECM detects a low signal voltage on the generator turn on signal circuit for at least 5 seconds.

Action Taken When the DTC Sets

    • The ECM sends a class 2 message to the instrument panel cluster (IPC) to illuminate the charge indicator and/or turn on a message.
    • The ECM will not illuminate the malfunction indicator lamp (MIL).
    • The ECM will store the conditions present when the DTC set as Fail Records data only.

Conditions for Clearing the MIL/DTC

    • The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared by using the scan tool Clear DTC Information function.

Step

Action

Value

Yes

No

Schematic Reference: Engine Controls Schematics or Starting and Charging Schematics

Connector End View Reference: Engine Control Module Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

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Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Wait 5 seconds.
  4. Start the engine.
  5. With a scan tool, monitor the DTC information for DTC P1668.

Does the scan tool indicate that DTC P1668 has passed?

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Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3

  1. Turn OFF the engine.
  2. Disconnect the generator harness connector.
  3. Start the engine.
  4. With a DMM measure the pulse width modulation (PWM) signal between the generator turn on signal circuit and ground.

Is the PWM signal within the specified value?

15-87%

Go to Step 5

Go to Step 4

4

Test the generator turn on signal circuit for a short or open. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 10

Go to Step 7

5

Test the generator battery positive voltage output circuit for a high resistance or open. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 10

Go to Step 6

6

Inspect for poor connections at the harness connector of the generator. Refer to Connector Repairs .

Did you find and correct the condition?

--

Go to Step 10

Go to Step 8

7

Inspect for poor connections at the harness connector of the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

--

Go to Step 10

Go to Step 9

8

Replace the generator. Refer to Generator Replacement .

Did you complete the replacement?

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Go to Step 10

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9

Replace the ECM. Refer to Control Module References for replacement, setup, and programming.

Did you complete the replacement?

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Go to Step 10

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10

  1. Use the scan tool in order to clear the DTCs .
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

--

Go to Step 3

System OK