GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2002 🢒 Cavalier (CNG) 🢒 Engine 🢒 Engine Controls - 2.2L (LN2) 🢒 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 Engine Control Module (AF ECM) monitors the fuel pressure and fuel temperature in the CNG fuel tank. The volume of CNG varies with pressure and temperature. In order to display an accurate fuel gage reading, the AF ECM will perform a calculation based upon fuel pressure and temperature and determine the correct amount of fuel remaining in the fuel tank. The AF ECM converts the calculated fuel level into a PWM signal that is monitored by the PCM. The PCM sends the fuel level information to the instrument panel fuel gage using the Class 2 serial data circuit.

Diagnostic Aids

The PCM filters the fuel level display during vehicle movement, depress the fuel gage select switch 30 seconds after the vehicle speed sensor indicates the vehicle is at rest in order to avoid inaccurate gage displays.

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 mis-diagnosis.

    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 AF 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 AF ECM for proper operation.

    9.

  9. This step checks for an open Fuel Tank Pressure 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 AF 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 AF 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 800 psi within 3 hours of filling.

  1. Communicate with the AF ECM using a scan tool and observe the Fuel Tank Pressure (FTP) sensor data parameters.
  2. Verify the amount of CNG fuel pressure in the fuel tank by filling the CNG fuel tank at a CNG dispensing station.

    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 Tank Pressure (FTP) sensor scan tool parameter value.

Is the FTP 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. Backprobe the Fuel gage circuit at the fuel gage relay connector terminal (30) with the J 39200 DMM connected to ground.
  2. Measure the Pulse Width Modulated (PWM) signal.
  3. Using the Fuel Gage PWM Output specifications table determine the expected fuel gage PWM signal.
  4. Compare the expected PWM signal to the actual PWM signal.

Is the measured PWM signal the same as the expected PWM signal?

--

Go to Step 13

Go to Step 14

6

  1. Turn the ignition OFF.
  2. Remove the fuel tank cover. Refer to Fuel Tank Cover Replacement .
  3. Disconnect the FTP sensor connector (C452).
  4. Turn the ignition ON leaving the engine OFF. An FTP sensor DTC may set. Clear this DTC after diagnosis is complete.
  5. Communicate with the AF ECM using a scan tool.
  6. Observe the Fuel Tank Pressure (FTP) 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 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 (FTP) Sensor Signal circuit terminal (C) to the Fuel Tank Pressure (FTP) 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 12-V Unpowered 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 Cover Replacement .
  3. Disconnect the FTT sensor connector (C451).
  4. Turn the ignition ON leaving the engine OFF.
  5. Communicate with the AF 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 PCM 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 22

Go to Fuel Gage Inaccurate or Inoperative in Instrument Panel, gages, and Console in the J Platform Service Manual

14
  1. Check the Fuel gage circuit between the relay connector terminal and the AF 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 22

Go to Step 21

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

Was a circuit repair made?

--

Go to Step 22

Go to Step 21

16
  1. Check 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 22

Go to Step 21

17
  1. Check 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 22

Go to Step 21

18
  1. Check 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 22

Go to Step 21

19

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

Is the replacement complete?

--

Go to Step 22

--

20

Replace the fuel tank temperature sensor. The fuel tank temperature sensor is NOT serviceable separately from the High Pressure Lock-off (HPL) solenoid. Refer to High Pressure Lock-Off Solenoid Replacement in Engine Controls.

Is the replacement complete?

--

Go to Step 22

--

21
  1. Replace the AF ECM. Refer to Alternative Fuels Engine Control Module Replacement in Engine Controls.
  2. Program-flash-the replacement control module with the correct vehicle software and calibration.

Is the replacement complete?

--

Go to Step 22

--

22
  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 AF 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)