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 ECM. When the vehicle is operating in the CNG mode the Alternative Fuels ECM energizes the fuel gauge relay allowing the CNG Fuel Level Output to be monitored by the Instrument Panel (IP). When the vehicle is operating in the gasoline mode the Alternative Fuels ECM does NOT energize the fuel gauge relay allowing the gasoline Fuel Level Output to be monitored by the IP.

Regardless of the fuel system in operation the Alternative Fuels ECM monitors the Fuel Gauge Select Switch Input circuit. When the vehicle operator depresses the switch the Alternative Fuels ECM 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.

TEST DESCRIPTION

Steps 1-5:

Steps 6-13:

Steps 14-21:

Steps 22-31:

The number(s) below refer to the step number(s) 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 Alternative Fuels ECM. In addition a short may have damaged the fuel gauge select switch.

This step tests the Alternative Fuels ECM's ability to monitor the fuel gauge select switch input circuit.