Fuel Lock-Off Relay Diagnosis

CIRCUIT DESCRIPTION
The lock-off relay coil is supplied power whenever the gasoline control module (VCM/PCM or ECM) energizes the gasoline fuel pump relay. The Alternative Fuels ECM controls the ground circuit of the lock-off relay. When energized the lock-off relay supplies power to the high pressure lock-off solenoids.

At the initial ignition ON, the gasoline control module energizes the gasoline fuel pump relay which supplies power to the Fuel Pump Power circuit for a 2 second prime pulse. At the initial ignition ON, the Alternative Fuels ECM will close the Lock-Off Relay Control circuit driver, completing the circuit path to ground, to allow for a 2 second CNG prime pulse. The prime pulse occurs at the same time as the gasoline fuel pump prime pulse occurs.

The gasoline control module will supply power to the Fuel Pump Power circuit regardless of the fuel system that is in operation as long as crankshaft pulses are received.

When the engine is running on CNG, the Alternative Fuels ECM closes the Lock-Off Relay Control circuit driver which opens the high pressure lock-off solenoids.

TEST DESCRIPTION

Steps 1-5:

Steps 6-13:

Steps 14-23:

Steps 24-27:

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.
4. The Fuel Indicator Lamp (FIL), located within the fuel gauge select switch button, will illuminate when the engine is operating on gasoline.

In addition, the AFO Signal Command scan tool parameter will indicate the Alternative Fuel ECM's commanded state of the AFO Signal circuit, while the AFO Signal parameter will indicate the gasoline control modules monitored state of the AFO Signal circuit.

When operating on CNG, the displays should indicate that the AF ECM is in command. When operating on gasoline the displays should indicate that the PCM is in command.

Regardless of the fuel system in operation, these two data parameters will always indicate the same data unless an AFO Signal circuit fault is present.