Bi-Fuel - CNG/Gasoline Models

The PCM controls fuel delivery and determines which fuel system the engine is operating on. The PCM monitors various engine and vehicle functions to provide the correct amount of CNG or gasoline fuel under all operating conditions. This provides excellent driveability and fuel economy while maintaining reduced emission levels. When operating on CNG, the PCM grounds the AFO enable circuit. When the AFO enable circuit is grounded, the Fuel Injector Control Module (FICM) will sequentially operate the CNG fuel injectors. The PCM is not capable of operating the high current CNG fuel injectors. The PCM injector pulse width signals are received by the FICM and duplicate pulse width signals are generated by the FICM to operate the CNG injectors.

The PCM monitors a Pulse Width Modulation (PWM) signal from the FICM to determine the diagnostic status of the FICM. The PCM controls the Low Pressure Lock-Off (LPL) solenoid, and the High Pressure Lock-Off (HPL) solenoid. The PCM commands the HPL solenoid open for one second at every ignition ON, to prime the CNG fuel system. The PCM commands the HPL, and the LPL open when the engine is cranking or running on CNG.

When operating on gasoline, the PCM will turn the fuel indicator light ON. The Fuel Indicator Light (FIL) is located within the fuel gauge select switch near the headlight switch assembly. The FIL is also turned ON for a two-second bulb check whenever the ignition switch is turned to RUN position. The fuel gauge select switch allows the vehicle operator the ability to request fuel gauge display of either fuel tank.

The PCM also determines the CNG fuel level. The PCM monitors the Fuel Tank Pressure (FTP) and Fuel Tank Temperature (FTT) sensor information. The FTP and FTT sensors are located on the CNG tank HPL solenoid. The PCM performs a calculation of the CNG pressure and CNG temperature to determine an accurate CNG fuel level. The gasoline fuel level is determined by the PCM in a conventional manner using an in-tank variable resistor sensor.