Fuel Gauge Inaccurate or Inoperative - Alternative Fuels

CIRCUIT DESCRIPTION
The CNG fuel gauge system contains the following components:
^ fuel pressure sensor (FPS)
^ fuel temperature sensor (FTS)
^ pulse width modulated (PWM) output circuit the dash mounted fuel gauge

The AF ECU 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 gauge reading, the AF ECU will perform a calculation based upon fuel pressure and temperature and determine the correct amount of fuel remaining in the fuel tank. The AF ECU 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 gauge using the Class 2 serial data circuit.

DIAGNOSTIC AIDS
Passenger vehicles have a filtered fuel level display, depress the fuel gauge select switch 30 seconds after the vehicle speed sensor indicates the vehicle is at rest in order to avoid inaccurate gauge displays

TEST DESCRIPTION

Steps 1-6:

Steps 7-13:

Steps 14-22:

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. This step checks for an accurate FTS. An inaccurate FTS 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 FPS. An inaccurate FPS 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 AF ECU and the fuel gauge relay are commanding the correct fuel gauge reading.
6. This step checks for a Fuel 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 Pressure Sensor Signal circuit and the AF ECU for proper operation.
9. This step checks for an open FPS Sensor Ground circuit.
10. This step checks the Fuel Temperature Sensor Signal circuit.
11. This step checks the Fuel Temperature Sensor Signal circuit and the AF ECU for proper operation.