Circuit Description
The CNG fuel gage system consists of the following components:
| • | The fuel tank pressure (FTP) sensor |
| • | The fuel tank temperature (FTT) sensor |
| • | The dash mounted fuel gage select switch |
| • | The dash mounted fuel gage |
The powertrain control module (PCM) monitors the CNG FTP and FTT sensors. The volume of CNG stored in the tank varies with pressure and temperature. In order to display an accurate fuel gage reading, the PCM performs a calculation of based upon the CNG tank fuel pressure and temperature. Refer to Compressed Natural Gas (CNG) Temperature Versus Percent Full .
The fuel gage select switch allows the vehicle operator to observe the fuel level for the non-operating system. When the switch is applied, the PCM will display the fuel level of the non-operating fuel system for about 10 seconds.
The PCM sends the fuel level information to the instrument panel fuel gage on the Class 2 serial data circuit.
Diagnostic Aids
If the vehicle operators concern is uniquely related to fast filling of the CNG fuel tank review the following with the vehicle operator:
| • | 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. |
| • | The fuel tank pressure may decrease by up to 800 psi within 3 hours of fast-filling. |
The CNG fuel tank will not completely fill during a fast-fill, due to the expansion of the fuel. The fuel gage will display full as long as the fuel remains hot. The fuel gage display will decrease as the fuel cools.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Alternative Fuels (Engine Controls) |
2 |
Important: Verify that the customers concern is not a normal operating condition. Refer to Diagnostic Aids. Is the customers concern a normal operating condition? | -- | System OK | Go to Step 3 |
3 |
Does the parameter indicate the switch is operating as commanded? | -- | Go to Step 4 | Go to Step 9 |
4 | Command the fuel gage between full and empty with the scan tool Instrument Panel Special Functions. Does the fuel gage operate as commanded? | -- | Go to Step 5 | Go to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gages and Console in the J Platform Service Manual |
5 |
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.
Does each sensor indicate the same temperature plus or minus the specified value? | 20°C (35°F) | Go to Step 6 | Go to Step 18 |
6 | Observe the Fuel Tank Pressure Sensor voltage data parameter with the scan tool. Does the parameter indicate a voltage less than the specified value? | 4.8 V | Go to Step 7 | Go to Step 14 |
7 |
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 three hours of fast filling.
Does the FTP sensor accurately indicate a full tank? | -- | Go to Step 8 | Go to Step 19 |
8 |
The fuel pressure should slowly and smoothly drop until the vehicle switches to gasoline (KL6) or stalls. Did the fuel pressure drop smoothly and consistently? | -- | System OK | Go to Step 19 |
9 | Does the scan tool always indicate released? | -- | Go to Step 11 | Go to Step 10 |
10 |
Does the parameter indicate released? | -- | Go to Step 20 | Go to Step 13 |
11 |
Does the parameter indicate applied? | -- | Go to Step 16 | Go to Step 12 |
12 | Test the Fuel Gage Select Switch Signal circuit for an open condition. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the J Platform Service Manual. Did you find and correct the condition? | -- | Go to Step 22 | Go to Step 17 |
13 | Test the Fuel Gage Select Switch Signal circuit for a short to B+ condition. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the J Platform Service Manual. Did you find and correct the condition? | -- | Go to Step 22 | Go to Step 21 |
14 |
Is the voltage more than the specified value? | 0.2 V | Go to Step 15 | Go to Step 19 |
15 | Test the fuel tank pressure sensor ground circuit for an open condition. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the J Platform Service Manual. Did you find and correct the condition? | -- | Go to Step 22 | Go to Step 17 |
16 | Inspect for proper terminal contact at the fuel gage switch connector. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems in the J Platform Service Manual. Did you find and correct the condition? | -- | Go to Step 22 | Go to Step 20 |
17 | Inspect for proper terminal contact at the PCM connector. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems in the J Platform Service Manual. Did you find and correct the condition? | -- | Go to Step 22 | Go to Step 21 |
18 | Replace the fuel tank temperature sensor. The temperature sensor is not Serviceable separately from the HPL. Refer to High Pressure Lock-Off Solenoid Replacement in Engine Controls. Did you complete the replacement? | -- | Go to Step 22 | -- |
19 | Replace the fuel tank pressure sensor. Refer to Fuel Tank Pressure Sensor Replacement in Engine Controls. Did you complete the replacement? | -- | Go to Step 22 | -- |
20 | Replace the fuel gage select switch. Refer to Alternative Fuel Gage Selector Switch/Fuel Indicator Lamp Replacement . Did you complete the replacement? | -- | Go to Step 22 | -- |
21 | Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls in the J Platform Service Manual. Did you complete the replacement? | -- | Go to Step 22 | -- |
22 |
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) |
