Fuel Gauge Inaccurate or Inoperative - Bi-Fuel/Alternative Fuels

CIRCUIT DESCRIPTION
The CNG fuel gauge 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 gauge

The Alternative Fuels ECM monitors the fuel pressure and fuel temperature in the CNG fuel tank. Since CNG is a gas it's volume varies with pressure and temperature. In order to display an accurate fuel gauge 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 - Computers and Control Systems. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in Diagnostic System Check - Alternative Fuels (Computers and Control Systems).

TEST DESCRIPTION

Steps 1-2:

Steps 3-7:

Steps 8-13:

Steps 14-23:

The number(s) below refer to the step number(s) 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. This step checks for an accurate FTT sensor. An inaccurate FTT sensor may cause an inaccurate CNG Fuel gauge 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. 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. This step checks for the proper fuel gauge reading based upon fuel pressure and temperature.
5. This step checks if the Alternative Fuels ECM and the fuel gauge relay are commanding the correct fuel gauge reading.
6. This step checks for a Fuel Tank Pressure Sensor Signal circuit that is shorted to ground.
7. This step checks the 5 volt reference circuit for proper voltage.
8. This step checks the Fuel Tank Pressure Sensor Signal circuit and the Alternative Fuels ECM for proper operation.
9. This step checks for an open FTP Sensor Ground circuit.
10. This step checks the Fuel Tank Temperature Sensor Signal circuit.
11. This step checks the Fuel Tank Temperature Sensor Signal circuit and the Alternative Fuels ECM for proper operation.