GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The primary fuel level sensor and the secondary fuel level sensor change resistance in response to the fuel level. The engine control module (ECM) monitors the signal circuits of the primary fuel level sensor and the secondary fuel level sensor in order to determine the fuel level. When the fuel tanks are full, the resistances of both fuel level sensors are high, and the ECM senses a high signal voltage on both the signal circuits of the primary fuel level sensor and the secondary fuel level sensor. When the fuel tanks are empty, the resistances of the fuel level sensors are low, and the ECM senses a low signal voltage. The ECM uses the signal circuits of the primary fuel level sensor and the secondary fuel level sensor in order to calculate the percentage of remaining fuel in the tank. The ECM sends the fuel level percentage via the class 2 serial data circuit to the instrument panel cluster (IPC) in order to control the fuel gage.

This diagnostic tests for a higher than normal secondary fuel level sensor signal.

Conditions for Running the DTC

    • The ignition is ON.
    • The engine is running (LG4).

Conditions for Setting the DTC

    • The secondary fuel level signal is greater than 4.5 volts (LB7 and LLY).
    • The secondary fuel level signal is greater than 98 percent (LG4).
    • The above condition is present for greater than 20 seconds.

Action Taken When the DTC Sets

    • The fuel gage defaults to empty.
    • The ECM records the operating conditions at the time that the diagnostic test fails. The ECM displays the failure information in the Failure Records on the scan tool.

Conditions for Clearing the DTC

    • The DTC becomes history when the conditions for setting the DTC are no longer present.
    • The history DTC clears after 40 malfunction-free warm-up cycles.
    • The ECM receives the clear code command from the scan tool.

Diagnostic Aids

    • Use the Freeze Frame/Failure Records data in order to locate an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame/Failure Records data may help in determining the number of miles since the DTC set. The Fail Counter and Pass Counter can also help in determining the number of ignition cycles that the diagnostic test reported a pass and/or fail. Operate the vehicle within the same Freeze Frame conditions, including those for RPM, engine load, vehicle speed, temperature, and others. This will isolate at what point the DTC failed.
        Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
    • When the primary fuel level sensor signal is higher than normal, the Fuel Level Sensor % parameter in the ECM Fuel System data list on the scan tool indicates a value of about 100 percent.

Test Description

The number below refers to the step number on the diagnostic table.

  1. This step tests for the proper operation of the circuit in the low voltage range.

Step

Action

Value(s)

Yes

No

Schematic Reference: Instrument Cluster Schematics

Connector End View Reference: Master Electrical Component List in Wiring Systems

1

Did you perform the Diagnostic System Check - Instrument Cluster?

--

Go to Step 2

Go to Diagnostic System Check - Instrument Cluster

2

  1. Install a scan tool.
  2. Turn the ignition ON, with the engine OFF.
  3. With a scan tool, observe one of the following fuel level sensor parameters:
  4. • Fuel Level Sensor Rear parameter in the ECM Fuel System data list - LB7 and LLY only
    • Fuel Level Sensor % parameter in the ECM Fuel System data list - LG4 only

Does the scan tool indicate that the fuel level sensor parameter is greater than the specified value?

4.5 V (LB7 and LLY only)

98% (LG4 only)

Go to Step 3

Go to Diagnostic Aids

3

  1. Turn the ignition OFF.
  2. Disconnect the secondary fuel level sensor.
  3. Connect a 3-amp fused jumper wire between the signal circuit of the secondary fuel level sensor and the low reference circuit of the secondary fuel level sensor on the female terminal side of the connector.
  4. Turn the ignition ON, with the engine OFF.
  5. With a scan tool, observe one of the following fuel level sensor parameters:
  6. • Fuel Level Sensor Rear parameter in the ECM Fuel System data list - LB7 and LLY only
    • Fuel Level Sensor % parameter in the ECM Fuel System data list - LG4 only

Does the scan tool indicate that the fuel level sensor parameter is less than the specified value?

0.5 V (LB7 and LLY only)

50% (LG4 only)

Go to Step 6

Go to Step 4

4

Test the signal circuit of the secondary fuel level sensor for an open, for a high resistance, or for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 10

Go to Step 5

5

Test the low reference circuit of the secondary fuel level sensor for an open, for a high resistance, or for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 10

Go to Step 7

6

Inspect for poor connections at the harness connector of the secondary fuel level sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

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 in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 10

Go to Step 9

8

Replace the secondary fuel level sensor. Refer to:

    •  Fuel Sender Assembly Replacement in Engine Controls - 6.6L (LB7).
    •  Fuel Sender Assembly Replacement in Engine Controls - 6.6L (LLY).
    •  Fuel Sender Assembly Replacement in Engine Controls - 7.8L.

Did you complete the replacement?

--

Go to Step 10

--

9

Important: Program the replacement ECM.

Replace the ECM. Refer to:

    •  Engine Control Module Replacement in Engine Controls - 6.6L (LB7).
    •  Engine Control Module Replacement in Engine Controls - 6.6L (LLY).
    •  Engine Control Module Replacement/Restore Fuel Rate Data in Engine Controls - 7.8L.

Did you complete the replacement?

--

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.

Does the DTC reset?

--

Go to Step 2

System OK