Description
Important: This diagnostic procedure should only be used to diagnose a suspected inaccurate CNG fuel level reading and not a malfunctioning fuel gauge.
The fuel gauge receives a signal from the engine control unit (ECU) in order to display the CNG fuel level. The fuel sending unit (FSU) takes a temperature corrected pressure reading and outputs a voltage to the ECU. The ECU then outputs a voltage to the fuel gauge relay. During CNG operation, the relay sends the CNG fuel level signal to the fuel gauge.
Preliminary Checks
Inspect for the following conditions:
| • | Verify that the CNG fuel level is at least 1/3 full. Add fuel if needed. If fuel is added, record the fill pressure at the refueling station. |
| Important: If the tank is fast-filled, the fuel will be heated as it is compressed into the tank. This may affect any FSU voltage measurements taken for up to three hours after filling. As the fuel cools, the tank pressure may drop as much as 400 psi depending on the quantity of fuel added. |
| • | Verify that no AF Diagnostic Trouble Codes (DTCs) are set. |
| • | Verify that the fuel sending unit (FSU) electrical connector is connected. |
Note that the fuel gauge will display the dormant fuel level for ten seconds when the fuel gauge selector switch is pressed. The gauge will then return to normal operation.
Fuel Voltage Output Check
Important: Supply = 8 V to 16 V DC
Accuracy = +/- 8.0% Sample Calculation FSU supplied with 12.6 V at 14 478 kPa (2100 psi) and 5°C (40°F). FSU voltage output = 12.6 V x 0.44 = 5.54 V.- Calculate the FSU voltage output using the FSU voltage factor. Refer to Fuel Sending Unit Voltage Factor Versus Pressure at Various Temperature .
- In order to verify FSU accuracy, compare the calculated voltage output with the actual voltage output by the sensor.
