GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2002 🢒 Express (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel Gauge Inaccurate or Inoperative - Alternative Fuels

Circuit Description

The CNG fuel gage system contains the following components:

    • Fuel tank pressure (FTP) sensor
    • Fuel tank temperature (FTT) sensor
    • Pulse width modulated (PWM) output circuit
    • The dash mounted fuel gage

The Alternative Fuels ECM monitors the fuel pressure and fuel temperature in the CNG fuel tank. Since CNG is a gas its volume varies with pressure and temperature. In order to display an accurate fuel gage reading the Alternative Fuels ECM will perform a calculation based upon fuel pressure and temperature and determine the correct amount of fuel remaining in the fuel tank. The Alternative Fuels ECM converts the calculated fuel level into a PWM signal that is monitored by the instrument panel.

Diagnostic Aids

Intermittent Condition

A fault not present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to the Intermittent Conditions diagnostic information located in Symptoms - Engine Controls. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in Diagnostic System Check - Alternative Fuels (Engine Controls) .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. The other diagnostic procedure MUST be performed before proceeding with this diagnostic table. Failure to perform the diagnosis in the proper order will result in misdiagnosis.

    2.

  2. This step checks for an accurate FTT sensor. An inaccurate FTT sensor may cause an inaccurate CNG Fuel gage reading.

    If a large difference between temperatures is observed allow the underhood and fuel tank temperatures to stabilize by not operating the vehicle for several hours with the hood raised. Repeat this step after temperatures have stabilized before proceeding with the diagnosis.

    3.

  3. This step checks for an accurate FTP sensor. An inaccurate FTP sensor may indicate pressure when the tank is empty or an empty tank when there is sufficient pressure. Either condition would cause an Unwanted Switch to Gasoline.

    4.

  4. This step checks for the proper fuel gage reading based upon fuel pressure and temperature.

    5.

  5. This step checks if the Alternative Fuels ECM and the fuel gage relay are commanding the correct fuel gage reading.

    6.

  6. This step checks for a Fuel Tank Pressure Sensor Signal circuit that is shorted to ground.

    7.

  7. This step checks the 5 volt reference circuit for proper voltage.

    8.

  8. This step checks the Fuel Tank Pressure Sensor Signal circuit and the Alternative Fuels ECM for proper operation.

    9.

  9. This step checks for an open FTP Sensor Ground circuit.

    10.

  10. This step checks the Fuel Tank Temperature Sensor Signal circuit.

    11.

  11. This step checks the Fuel Tank Temperature Sensor Signal circuit and the Alternative Fuels ECM for proper operation.

Step

Action

Value(s)

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Fuel Gauge Relay Diagnosis - Alternative Fuels before starting this diagnostic procedure?

--

Go to Step 2

Go to Fuel Gauge Relay Diagnosis - Alternative Fuels

2

Important: Fast-filling the CNG fuel tank causes the fuel to heat-up and expand.

The fuel temperature may remain hotter than the ambient (IAT) temperature for up to 3 hours after filling.

  1. Communicate with the Alternative Fuels ECM using a scan tool and observe the engine data parameters.
  2. Compare the Fuel Tank Temperature (FTT) sensor scan tool parameter value to the Intake Air Temperature (IAT) scan tool parameter value.

Does each sensor indicate the same temperature plus or minus the range specified?

20°C (35°F)

Go to Step 3

Go to Step 10

3

Important: Fast-filling the CNG fuel tank causes the fuel to heat-up and expand.

The fuel tank pressure may decrease by up to 5516 kPa (800 psi) within 3 hours of filling.

  1. Communicate with the Alternative Fuels ECM using a scan tool and observe the Fuel Pressure Sensor data parameter.
  2. Verify the amount of CNG fuel pressure in the fuel tank by filling the CNG fuel tank at a CNG dispensing station equipped with a dispensor pressure gage.

    3. Observe the scan tool data parameter while filling the CNG fuel tank.

  3. Observe the CNG dispenser pressure gage reading.
  4. Compare the dispenser pressure gage reading to the Fuel Pressure sensor scan tool parameter value.

Is the fuel tank pressure sensor accurately indicating fuel tank pressure?

--

Go to Step 4

Go to Step 6

4

  1. Using the Compressed Natural Gas (CNG) Temperature Versus Percent Full specifications table determine the expected fuel gage percentage.
  2. Compare the expected fuel gage percentage to the actual fuel gage reading.

Is the fuel gage displaying the expected percentage of fuel?

--

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

Go to Step 5

5

  1. Disconnect the fuel gage relay.
  2. Connect the unpowered test lamp between the Alternative Fuels ECM CNG fuel level output circuit at the fuel gage relay terminal (87) and battery positive.
  3. Command the Fuel gage Output from full to empty using the scan tool Alt. Fuel Special Functions.
  4. Observe the test lamp while commanding the Fuel gage Output from full to empty.

Does the test lamp blink and/or increase and decrease in brightness as the Fuel Gage Output is commanded?

--

Go to Step 13

Go to Step 14

6

  1. Turn the ignition OFF.
  2. Remove the fuel tank cover. Refer to Fuel Tank Shield Replacement .
  3. Disconnect the fuel tank pressure (FTP) sensor connector.
  4. Turn the ignition ON leaving the engine OFF. An FTP DTC may set. Clear this DTC after diagnosis is complete.
  5. Communicate with the Alternative Fuels ECM using a scan tool.
  6. Observe the Fuel Pressure Sensor data parameter on the scan tool.

Is the voltage greater than the specified voltage?

4.9 V

Go to Step 7

Go to Step 15

7

  1. Connect the J 39200 DMM between the FTP sensor 5 volt reference circuit terminal (B) and a known good ground.
  2. Measure the voltage of the 5 volt reference circuit.

Is the voltage WITHIN the specified voltage range?

4.7-5.2 V

Go to Step 8

Go to Step 16

8

  1. Remove the DMM.
  2. Using a fused jumper connect the Fuel Tank Pressure Sensor Signal circuit terminal (C) to the Fuel Tank Pressure Sensor Ground circuit terminal (A).

    3. If the jumper fuse opens repair the short to voltage on the signal circuit.

  3. Observe the Fuel Tank Pressure Sensor data parameter on the scan tool.

Is the Fuel Tank Pressure Sensor voltage displayed on the scan tool LESS than the specified voltage?

0.20 V

Go to Step 19

Go to Step 9

9

Probe the Sensor Ground circuit terminal (A) using the J 35616-200 Un-powered Test Lamp connected to BATTERY POSITIVE.

Is the test lamp illuminated?

--

Go to Step 17

Go to Step 18

10

  1. Turn the ignition OFF.
  2. Remove the fuel tank cover. Refer to Fuel Tank Shield Replacement .
  3. Disconnect the fuel tank temperature (FTT) sensor connector.
  4. Turn the ignition ON leaving the engine OFF.
  5. Communicate with the Alternative Fuels ECM using a scan tool.
  6. Observe the Fuel Tank Temperature Sensor data parameter on the scan tool.

Is the Fuel Tank Temperature Sensor voltage GREATER than the specified voltage?

4.9 V

Go to Step 11

Go to Step 15

11

  1. Using a fused jumper connect the Fuel Tank Temperature Sensor Signal circuit terminal (B) to the Sensor Ground circuit terminal (A).

    2. If the jumper fuse opens repair the short to voltage on the signal circuit.

  2. Observe the Fuel Tank Temperature Sensor data parameter on the scan tool.

Is the Fuel Tank Temperature Sensor voltage displayed on the scan tool LESS than the specified voltage?

0.20 V

Go to Step 20

Go to Step 12

12

Probe the Sensor Ground circuit terminal (A) using the J 35616-200 connected to BATTERY POSITIVE.

Is the test lamp illuminated?

--

Go to Step 17

Go to Step 18

13
  1. Check the Fuel Gage circuit between the relay connector terminal and the instrument panel connector terminal for the following:
    2. • High resistance
    • Poor terminal contact
    • Opens
    • Shorts
  2. Repair circuit or terminals as necessary.

Was a circuit or terminal repair made?

--

Go to Step 23

Go to Fuel Gage Inaccurate or Inoperative in Instrument Panel, Gages, and Console in the G-Van Service Manual

14
  1. Test the CNG Fuel Level Output circuit between the relay connector terminal and the Alternative Fuels ECM connector terminal for the following:
    2. • High resistance
    • Ensure the relay contact resistance is checked with the relay energized.
    • Poor terminal contact
    • Opens
    • Shorts
  2. Repair circuit or terminals as necessary.

Was a circuit or terminal repair made?

--

Go to Step 23

Go to Step 22

15
  1. Test the signal circuit for a short to ground.
  2. Repair circuit as necessary.

Was a circuit repair made?

--

Go to Step 23

Go to Step 22

16
  1. Test the 5 volt reference circuit for the following:
    2. • A short to voltage- greater than 5.2 volts
    • An open or high resistance- less than 4.7 volts
    • A short to ground- less than 4.7 volts
  2. Repair circuit as necessary.

Was a circuit repair made?

--

Go to Step 23

Go to Step 22

17
  1. Test the signal circuit for an open circuit, high resistance or poor terminal contact.
  2. Repair circuit or terminals as necessary.

Was a circuit or terminal repair made?

--

Go to Step 23

Go to Step 22

18
  1. Test the sensor ground circuit for an open circuit, high resistance or poor terminal contact.
  2. Repair circuit or terminals as necessary.

Was a circuit or terminal repair made?

--

Go to Step 23

Go to Step 22

19

Replace the fuel tank pressure sensor. Refer to Fuel Tank Pressure Sensor Replacement .

Is the replacement complete?

--

Go to Step 23

--

20
  1. Test the signal circuit for a shorted to 5 volts condition.
  2. Repair the circuit as necessary.

Was a circuit repair made?

--

Go to Step 23

Go to Step 21

21

Replace the fuel tank temperature sensor. The fuel tank temperature sensor is NOT serviceable separately from the high pressure lock-off. Refer to High Pressure Lock-Off Solenoid Replacement .

Is the replacement complete?

--

Go to Step 23

--

22

Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .

Is the replacement complete?

--

Go to Step 23

--

23
  1. Remove all test equipment except the scan tool.
  2. Connect any disconnected components, fuses, etc.
  3. Using a scan tool clear DTC Information from the Alternative Fuels ECM and the gasoline control module (VCM/PCM or ECM).
  4. Turn the ignition OFF and wait 30 seconds.
  5. Start the engine and operate the vehicle.
  6. Observe the fuel indicator lamp and the MIL.
  7. Observe vehicle performance and driveability.
  8. After operating the vehicle inspect BOTH control modules for stored DTCs.

Does the vehicle operate on CNG with normal driveability, no FIL/MIL illumination and no stored DTCs?

--

System OK

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)