Fuel System Unwanted Switch to Gasoline - BI-Fuel/Alternative Fuels

CIRCUIT DESCRIPTION
The Alternative Fuels Engine Control Module (AF ECM) and the gasoline control module (VCM/PCM or ECM) are connected by the Alternative Fuels Operation (AFO) signal circuit. In order for the engine to switch-over to CNG operation the gasoline control module must be commanded to disable the gasoline fuel injectors, switch to a different ignition spark calibration and disable some of the gasoline control module DTCs. The Alternative Fuels ECM performs the switch-over command by grounding the AFO Signal circuit when CNG operation is desired.

This vehicles primary fuel source is Compressed Natural Gas (CNG). The vehicle will only switch-over to gasoline operation if one of the following conditions is present:
^ The Fuel Tank Pressure (FTP) sensor indicates the CNG tanks are empty.
^ A system fault is detected by the Alternative Fuels ECM.
^ The engine crank time exceeds 8 seconds.
^ Every 100th start the vehicle will start and run on gasoline. A switch-over to CNG will occur during the first decel condition.

DIAGNOSTIC AIDS
This diagnostic information should be utilized only if the concern of the vehicle operator can be duplicated. An unwanted switch may occur only during specific vehicle operation. An example follows:
1. The vehicle has an intermittently leaking Low Pressure Lock-off (LPL) solenoid.
2. After turning the engine OFF and waiting several minutes all of the CNG purges from the lines.
3. During the next ignition ON key cycle the High Pressure Lock-off (HPL) solenoids are opened for about 1 second.
At the same moment that the HPLs are opened the Alternative Fuels ECM monitors the fuel pressure sensor.
If the LPL is still leaking the CNG pressure cannot build within the system and low fuel pressure is observed by the Alternative Fuels ECM.
Detecting the low fuel pressure in the system, the Alternative Fuels ECM performs a switch-over to gasoline.
4. The vehicle may start and run on CNG during any ignition cycle in which the LPL does not leak.
The customer concern would be an intermittent unwanted switch to gasoline with no DTCs stored.

Fault Not Present
Fault Not Present indicates the condition that caused the concern is intermittent and not currently present. Refer to Intermittent Conditions symptom diagnosis for additional information. If any additional concerns were observed return to the Diagnostic System Check - Alternative Fuels (Computers and Control Systems). Initial Inspection and Diagnostic Overview Intermittent Conditions

TEST DESCRIPTION

Steps 1-8:

Steps 9-13:

Steps 14-19:

The numbers below refer to the step numbers on the diagnostic table.
1. The Diagnostic System Check MUST be performed before proceeding with this diagnostic table. Failure to perform the System Check will result in misdiagnosis.
2. This step tests if the Alternative Fuels ECM has commanded the gasoline control module, using the AFO Signal circuit, to start on CNG.
The AFO Signal Command data parameter is the commanded state of the AFO Signal circuit from the Alternative Fuels ECM. The AFO Signal data parameter is the monitored state of the AFO Signal circuit from the GASOLINE control module (VCM/PCM or ECM).
3. This step inspects if the FIL is illuminated during CNG operation. The FIL should illuminate only during gasoline operation.
5. This step inspects for an AFO Signal circuit failure or for a PCM utilizing standard gasoline software and calibration.
6. This step inspects if the CNG Fuel Tank Pressure (FTP) sensor is monitoring adequate fuel pressure for the engine to start on CNG. Once fuel pressure drops below about 200 psi (70°F) the Alternative Fuels ECM will not attempt to start on CNG until fuel pressure is greater than 1,000 psi (70°F).
9. This step tests the ignition system of the vehicle for proper operation. A vehicle that operates correctly on gasoline but not on CNG, may have a faulty ignition system.
12. This step tests for proper fuel tank pressure sensor operation.