Component Tests and General Diagnostics

CIRCUIT DESCRIPTION
The fuel gauge relay coil is supplied a ground path all the time. The coil power is supplied by the Alternative Fuels Engine control Unit (AF ECU). When the vehicle is operating in the CNG mode the AF ECU energizes the fuel gauge relay allowing the CNG Fuel Level Output to be monitored by the PCM. When the vehicle is operating in the gasoline mode the AF ECU does NOT energize the fuel gauge relay allowing the gasoline Fuel Level Output to be monitored by the PCM. The PCM sends the fuel level information to the instrument panel using the Class 2 serial data circuit.

Regardless of the fuel system in operation the AF ECU monitors the Fuel Gauge Select Switch Input circuit. When the vehicle operator depresses the switch the AF ECU will energize (when operating on gasoline) or de-energize (when operating on CNG) the fuel gauge relay causing the IP to display the DORMANT (non-operating) fuel system level. The dormant fuel level will be displayed for about 10 seconds.

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 1-9:

Steps 10-19:

Steps 20-27:

Steps 28-32:

The numbers below refer to the step numbers on the diagnostic table.
2. If the CNG fuse opens when the switch is depressed, test for a shorted to ground fuel gauge select switch input circuit or an internally shorted AF ECU. In addition a short may have damaged the fuel gauge select switch.
This step tests the AF ECU's ability to monitor the fuel gauge select switch input circuit.
13. This step tests for proper communication between the PCM and the instrument panel. Without communication the instrument panel may default the fuel gauge to empty.
14. With the vehicle operating on CNG this step inspects for PCM DTCs which may have detected a circuit fault on the CNG fuel level output or the PCM fuel level input circuits.
15. With the vehicle operating on gasoline this step inspects for PCM DTCs which may have detected a circuit fault on the fuel level sending unit (gasoline) circuits.