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CNG Fuel Gauge Reading Inaccurate

  1. Perform FUEL GAUGE SELECTOR SWITCH/FUEL INDICATOR LIGHT  and FUEL GAUGE RELAY . Go to next step.
    2. NOTE: Fast filling CNG tank causes fuel to heat and expand. Fuel temperature may remain hotter than ambient temperature for as much as 3 hours after filling.
  2. Using scan tool, check AF ECU data. If Fuel Temperature Sensor (FTS) and Intake Air Temperature (IAT) sensor data are within 35° F (20°C), go to next step. If FTS and IAT data is not within 35° F (20°C), go to step  10.
  3. Using scan tool, monitor fuel pressure sensor data. Connect vehicle to CNG filling station. Fill tank. If FPS and filling station pressures are the same, go to next step. If pressures are not the same, go to step  6.
  4. Check CNG tank pressure and temperature. Determine percentage full. See Fig 1. If calculated percentage and fuel gauge reading are not the same, go to next step. If calculated percentage and fuel gauge reading are the same, check system. See FUEL SYSTEM DIAGNOSIS (ALTERNATIVE FUEL) .
  5. Using DVOM connected to ground, backprobe fuel gauge relay terminal No. 30 (Purple wire). Measure Pulse Width Modulated (PWM) signal. Compare expected PWM output to actual output. See FUEL LEVEL/PWM OUTPUT  table. If PWM is as expected, go to step  13. If PWM is not as expected, go to step  14.
    6. NOTE: Performing step  6 will cause a FPS DTC to set. Clear DTC after testing is complete.
  6. Turn ignition off. Remove fuel tank cover. Disconnect 3-pin Fuel Pressure Sensor (FPS) connector. Turn ignition on. Using scan tool, check FPS data. If FPS data is more than 4.9 volts, go to next step. If FPS data is 4.9 volts or less, go to step  15.
  7. Check reference voltage at FPS harness connector terminal "B" (Orange wire). If 4.7-5.2 volts exists, go to next step. If 4.7-5.2 volts does not exist, go to step  16.
  8. Remove DVOM. Using a fused jumper wire, connect FPS harness connector terminal "A" (Dark Blue wire) to terminal "C" (Gray wire). If fuse blows, repair short to power on Gray wire and recheck system. Using scan tool, check FPS data. If FPS data is.20 volt or more, go to next step. If FPS data is less than.20 volt, go to step  19.
  9. Using test light connected to battery voltage, probe FPS harness connector terminal "A" (Dark Blue wire). If test light illuminates, go to step  17. If test light does not illuminate, go to step  18.
  10. Turn ignition off. Remove fuel tank cover. Disconnect 3-pin Fuel Pressure Sensor (FPS) connector. Turn ignition on. Using scan tool, check FPS data. If FPS data is more than 4.9 volts, go to next step. If FPS data is 4.9 volts or less, go to step  15.
  11. Using a fused jumper wire, connect FPS harness connector terminal "A" (Dark Blue wire) to terminal "C" (Gray wire). If fuse blows, repair short to power on Gray wire and recheck system. Using scan tool, check FPS data. If FPS data is.20 volt or more, go to next step. If FPS data is less than.20 volt, go to step  20.
  12. Using test light connected to battery voltage, probe FPS harness connector terminal "A" (Dark Blue wire). If test light illuminates, go to step  17. If test light does not illuminate, go to step  18.
  13. Check for open, poor contact or short on Purple wire between fuel gauge relay terminal No. 30 and instrument cluster terminal No. 16.. If no problem is found, diagnose instrument cluster. See appropriate ANALOG INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT. If problem is found, repair as necessary. After repairs, go to step  22.
  14. Check for open, short or high resistance on Tan/White wire between fuel gauge relay terminal No. 87 and AF ECU connector C001 terminal No. 8. If no problem is found, go to step  21. If problem is found, repair as necessary. After repairs, go to step  22.
  15. Check for short to ground on Gray wire between FPS connector terminal "B" and AF ECU connector C001 terminal No. 24. If no problem is found, go to step  21. If problem is found, repair as necessary. After repairs, go to step  22.
  16. Check for open or short to ground (less than 4.7 volts) and short to power (more than 5.2 volts) on Orange wire between FPS harness connector terminal "B" and AF ECU connector C002 terminal No. 18. If no problem is found, go to step  21. If problem is found, repair as necessary. After repairs, go to step  22.
  17. Check for open or poor contact on Gray wire between FPS connector terminal "B" and AF ECU connector C001 terminal No. 24. If no problem is found, go to step  21. If problem is found, repair as necessary. After repairs, go to step  22.
  18. Check for open or poor contact in Dark Blue wire between FPS connector terminal "A" and AF ECU connector C002 terminal No. 19. If no problem is found, go to step  21. If problem is found, repair as necessary. After repairs, go to step  22.
  19. Replace fuel pressure sensor. After repairs, go to step  22.
  20. FTS is faulty. Replace high pressure lock-off solenoid. After repairs, go to step  22.
  21. Replace AF ECU and reprogram. After repairs, go to next step.
  22. Reconnect all components and connectors. Using scan tool, clear all DTCs. Turn ignition off for 30 seconds. Start engine. Test drive vehicle. If no problems are found, MIL operates properly and no DTCs are set, test is complete. If any problems are found, MIL does not operate properly or DTCs are set, perform OBD system check. See ON BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article.
Fig 1: CNG Pressure/Temperature To Percent Full
G00010940Courtesy of GENERAL MOTORS CORP.
FUEL LEVEL/PWM OUTPUT

Percent Fuel (1) PWM Output
10 17
20 33
30 45
40 52
50 56
60 59
70 62
80 64
90 67
100 70
(1) Pulse Width Modulated (PWM).
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