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Primary And Secondary Fuel Level Sensor Circuit Fault Diagnostics

NOTE:

There are multiple test methods listed below. There can be advantages to using each method. All of the methods do not need to be performed. Pick the best test method based on accessibility, ease of testing or availability of test equipment. In some cases it may be beneficial to use parts of each method to test for different failure modes.

CIRCUIT DIAGNOSTICS USING THE ELECTRICAL TEST LEAD KIT:  Check the BCM signal and return circuits together to determine if the Fuel Level Sensor is faulty. Disconnect the Fuel Pump harness connector. Connect the (special tool #2064100081, Kit, Electrical Test Lead) to the Fuel Level Signal and Return Signal circuits in the Fuel Pump harness connector. Move the switch on the Electrical Test Lead Kit between the open and closed positions while monitoring the scan tool. If monitoring for faults, the circuit high fault should set when switched to the open position and the circuit low fault should set when switched to the closed position. If the sensor voltage is available, monitor the voltage readings on the scan tool. It should be 5.0 volts when switched to the open position and 0.0 volts when switched to the closed position. Move the switch to adjust and use the tool to vary the resistance between the Full Stop and Empty Stop resistance ranges while monitoring the BCM signal voltage on the scan tool with the ignition on.

NOTE:

Even though the actual signal will read 12.0 volts with the switch in the open position, the scan tool will register the signal as 5.0 volts

  1. If the faults set as described and the signal voltage is 5.0 volts and 0.0 volts on the scan tool  - the BCM and Fuel Level Sensor circuits are testing good. The Fuel Level Sensor is most likely faulty or has a poor connection at the harness connector. Check the Fuel Pump harness connector for pushed out, spread, corroded or dirty terminals before condemning the sensor. The internal resistance of the sensor can be measured and compared to the table (if available) to verify a faulty sensor.
  2. If the signal voltage is stuck low or only the circuit low fault sets  - the most likely cause is the Fuel Level Sensor Signal circuit shorted to chassis ground or the Fuel Level Sensor Return circuit. Check the Fuel Level Sensor Signal circuit for a short to ground or the Sensor Return circuit. If the signal circuit is not shorted, use the wiring information to check all related harness connectors in the system for pushed out, spread, corroded or dirty terminals before condemning a BCM. If no issues are found, the BCM is faulty.
  3. If the signal voltage is stuck high or only the circuit high fault sets  - the most likely cause is an open or very high resistance in the Fuel Level Sensor Signal or Return circuit. Continue testing in step 4.
  4. If the signal reading on the scan tool shows voltage present when switched to closed  - remove the test lead from the sensor return circuit and connect it to a good chassis ground while observing the voltage on the scan tool. If the voltage drops to 0.0 volts the resistance is in the sensor return circuit. If the voltage is still present the resistance is in the sensor signal circuit. Check the appropriate circuit for high resistance. Typically should be less than 3.0 Ohms. If the circuits check good, continue testing in step 5.
  5. If the circuits are checked and do not have excessive resistance  - use the wiring information to check all related harness connectors for pushed out, spread, corroded or dirty terminals before condemning BCM. If no issues are found, the BCM is faulty.

CIRCUIT DIAGNOSTICS USING CONVENTIONAL VOLTAGE TESTING WITH A DVOM: 

  1. Verify the Fuel Pump and BCM harness connectors are connected and locked properly.
  2. Turn the ignition on. Back Probe and measure the voltage on the Fuel Level Sensor Signal circuit at the Fuel Pump harness connector.
    1. If the signal voltage is reading between 0.5 and 4.5 volts  - the sensor signal is giving a legitimate reading. If the circuit fault is active and the voltage reading on the scan tool does not match the voltage reading, the BCM is likely faulty.
    2. If the signal has a constant 0.0 volt reading  - this indicates that the signal circuit is shorted to ground or has an open between the BCM and Fuel Level Sensor. If the circuit high fault is set, the most likely failure is an open in the signal circuit. If the circuit low fault is set, the most likely failure is short to ground in the signal circuit. Disconnect the Fuel Pump and BCM harness connectors. Check the signal circuit for high resistance and short to ground. If the signal circuit checks good, check at all connectors in the system for pushed out, spread, corroded or dirty terminals before condemning a BCM.
    3. If the signal reading has a constant 12.0 volts  - this indicates an open further downstream in the circuitry. Most likely the Fuel Level Sensor or the Fuel Level Sensor Return circuit.
  3. To check the Fuel Level Sensor, move the test lead over and back probe the Fuel Level Sensor Return circuit at the Fuel Pump harness connector.
    1. If the signal changes to 0.0 volts  - this would indicate an open in, or short across the Fuel Level Sensor resistance card. The internal resistance of the Fuel Level Sensor can be measured and compared to the table (if available) to verify a faulty sensor.
    2. If the signal is still at 12.0 volts  - this indicates an open in the Fuel Level Sensor Return circuit path. Disconnect the Fuel Pump and BCM harness connectors. Check the return circuit for high resistance. If the return circuit checks good, use the wiring information to check all related harness connectors in the system for pushed out, spread, corroded or dirty terminals before condemning a BCM. If no issues are found, the BCM is faulty.

CIRCUIT DIAGNOSTICS USING CONVENTIONAL RESISTANCE TESTING WITH A DVOM: 

  1. Verify the Fuel Pump and BCM harness connectors are connected and locked properly.
  2. Disconnect and check the Fuel Pump and BCM harness connectors for pushed out, spread, corroded or dirty terminals.
  3. Check the Fuel Level Sensor Signal circuit for an open/high resistance, short to the return circuit or short to ground.
  4. Check the Fuel Level Sensor Return circuit for an open/high resistance.
  5. Check for the Fuel Level Sensor resistance being above or below the minimum and maximum specifications.
  6. Reconnect the BCM harness connector leaving the Fuel Pump harness connector disconnected. Turn the ignition on and monitor the scan tool for faults. The circuit high fault should set with the Fuel Pump harness connector disconnected. Connect a fused jumper between the Fuel Level Sensor Signal circuit and the Fuel Level Sensor Return at the Fuel Pump harness connector. The circuit low fault should set with the jumper in place. If the faults set as described, replace the Fuel Level Sensor. If the faults do not set as described, the BCM is faulty.
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