- If fuel gauge relay diagnosis has been performed, go to next step. If fuel gauge relay diagnosis has not been performed, go to FUEL GAUGE RELAY DIAGNOSIS (CNG) under FUEL SYSTEM in SYSTEM & COMPONENT TESTING - CARS - EXCEPT METRO & PRIZM article. After performing fuel gauge relay diagnosis, go to next step.
CAUTION:
Fast-filling CNG fuel tank causes fuel to heat-up and expand. Fuel temperature may remain hotter than ambient temperature for up to 3 hours after filling.
- Using scan tool, communicate with Alternate Fuel Engine Control Unit (AF ECU) and observe engine data parameters. Compare Fuel Temperature Sensor (FTS) and Intake Air Temperature (IAT) parameters. If sensor readings are within 35°F (20°C) of each other, go to next step. If sensor readings are not within 35°F (20°C) of each other, go to step 10.
CAUTION:
Fast-filling CNG fuel tank causes fuel to heat-up and expand. Fuel tank pressure may decrease by up to 800 psi within three hours of filling.
- Using scan tool, communicate with Alternate Fuel Engine Control Unit (AF ECU) and observe Fuel Pressure Sensor (FPS) data parameters. Verify amount of CNG fuel pressure in CNG fuel tank by filling CNG fuel tank. Monitor scan tool data while filling CNG fuel tank. Observe CNG dispenser pressure gauge reading. Compare CNG dispenser pressure gauge reading Fuel Pressure Sensor (FPS) parameter. If FPS parameter is approximately CNG dispenser pressure gauge reading, go to next step. If FPS parameter is not approximately CNG dispenser pressure gauge reading, go to step 6.
- Determine expected fuel gauge percentage using Compressed Natural Gas (CNG) temperature vs percent full specifications table. See Fig 1. Compare expected fuel gauge percentage to actual fuel gauge reading. If fuel gauge displays expected percentage of fuel, go to POWERTRAIN DIAGNOSTIC SYSTEM CHECK (CNG) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 2.2L & 2.4L "J" & "N" BODIES - GASOLINE & CNG article in ENGINE PERFORMANCE. If fuel gauge does not display expected percentage of fuel, go to next step.
- Using DVOM connected to ground, backprobe fuel gauge relay connector terminal No. 30 (Purple wire). Fuel gauge relay is located behind left front headlight. Measure Pulse Width Modulated (PWM) signal. Compare expected PWM signal to actual PWM signal using fuel gage PWM output specifications table. See Fig 2. If measured PWM signal matches expected PWM signal, go to step 13. If measured PWM signal does not match expected PWM signal, go to step 14.
NOTE:
An FPS DTC may set during next step. Clear this DTC after diagnosis is complete.
- Turn ignition off. Remove fuel tank cover. Disconnect FPS connector. FPS is located on left side of fuel tank in trunk. Turn ignition. Using scan tool, communicate with AF ECU and monitor FPS data parameter. If FPS voltage is more than 4.9 volts, go to next step. If FPS voltage is less than 4.9 volts, go to step 15.
- Using DVOM, measure voltage between FPS connector terminal "B" (Orange wire) and ground. If voltage is 4.7-5.2 volts, go to next step. If voltage is not 4.7-5.2 volts, go to step 16.
- Disconnect DVOM. Connect a fused jumper between FPS connector terminals "A" (Black wire) and "C" (Gray wire). Monitor FPS data on scan tool. If voltage is less than 0.20 volt, go to step 20. If voltage is more than 0.20 volt, go to next step. If fused jumper blows, repair short to voltage in Gray wire between FPS and AF ECU Blue connector C001 terminal No. 24.
- Using a test light connected to battery voltage, probe FPS connector terminal "A" (Black wire). If test light illuminates, go to step 17. If test light does not illuminate, go to step 18.
- Turn ignition off. Remove fuel tank cover. Disconnect Fuel Temperature Sensor (FTS) connector. FTS is located on left side of fuel tank. Turn ignition on. Using scan tool, communicate with AF ECU and monitor FTS data parameter. If FTS voltage is more than 4.9 volts, go to next step. If FTS voltage is less than 4.9 volts, go to step 15.
- Connect a fused jumper FTS connector terminals "A" (Black wire) and "B" (Purple wire). Monitor FTS data on scan tool. If voltage is less than 0.20 volt, go to step 20. If voltage is more than 0.20 volt, go to next step. If fused jumper blows, repair short to voltage in Purple wire between FTS and AF ECU Blue connector C001 terminal No. 23.
- Using a test light connected to battery voltage, probe FTS connector terminal "A" (Black wire). If test light illuminates, go to step 17. If test light does not illuminate, go to step 18.
- Check for high resistance, poor connections, opens or shorts in Purple wire between PCM connector C1 terminal No. 55 and fuel gauge relay connector terminal No. 30. Repair as necessary. After repairs, go to step 22. If circuit is okay, go to TEST B: FUEL GAUGE INACCURATE/INOPERATIVE .
- Check for high resistance, poor connections, opens or shorts in Tan/White wire between AF ECU Blue connector C001 terminal No. 25 and fuel gauge relay connector terminal No. 87. See Fig 3. Repair as necessary. After repairs, go to step 22. If circuit is okay, go to step 21.
- For FPS, check for short to ground or short to sensor ground in Gray wire between FPS connector terminal "C" and AF ECU Blue connector C001 terminal No. 24. See Fig 3. For FTS, check for short to ground or short to sensor ground in Purple wire between FPS connector terminal "B" and AF ECU Blue connector C001 terminal No. 23. Repair as necessary. After repairs, go to step 22. If circuit is okay, go to step 21.
- Check for short to voltage (more than 5.2 volts), open, high resistance or short to ground (less than 4.7 volts) in Orange wire between FPS connector terminal "B" and AF ECU White/Clear connector C002 terminal No. 18. See Fig 3. Repair as necessary. After repairs, go to step 22. If circuit is okay, go to step 21.
- For FPS, check for open, high resistance or poor connections in Gray wire between FPS connector terminal "C" and AF ECU Blue connector C001 terminal No. 24. See Fig 3. For FTS, check for open, high resistance or poor connections in Purple wire between FPS connector terminal "B" and AF ECU Blue connector C001 terminal No. 23. Repair as necessary. After repairs, go to step 22. If circuit is okay, go to step 21.
- Check for open, high resistance or poor connections in Black wire between FPS or FTS connector terminal "A" and AF ECU White/Clear connector C002 terminal No. 19. See Fig 3. Repair as necessary. After repairs, go to step 22. If circuit is okay, go to step 21.
- Replace FPS. See FUEL TANK PRESSURE SENSOR under FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CARS article in ENGINE PERFORMANCE. After repairs, go to step 22.
- Replace FTS. See FUEL TEMPERATURE SENSOR under FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION - CARS article in ENGINE PERFORMANCE. After repairs, go to step 22.
- Replace AF ECU. See ALTERNATIVE FUEL ENGINE CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION - CARS article in ENGINE PERFORMANCE. After repairs, go to next step.
- Remove all test equipment except scan tool. Reconnect any disconnected components. Using scan tool, clear DTCs from AF ECU and PCM. Turn ignition off and wait 30 seconds. Start engine and operate vehicle. Monitor FIL and MIL operation. Observe vehicle performance and driveability. After operating vehicle check for DTCs in AF ECU and PCM. If vehicle operates on CNG with normal driveability, no FIL/MIL illumination and no set DTCs, system is okay. If vehicle does not operate on CNG with normal driveability, or FIL/MIL are illuminated or DTCs are set, go to POWERTRAIN DIAGNOSTIC SYSTEM CHECK (CNG) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 2.2L & 2.4L "J" & "N" BODIES - GASOLINE & CNG article in ENGINE PERFORMANCE.