Document ID# 706177
2001 Chevrolet Cavalier (CNG)
________________________________________
Fuel Lock-Off Relay Diagnosis

Circuit Description
The lock-off relay is energized whenever the gasoline control module (VCM/PCM or ECM) supplies power to the gasoline fuel pump circuit. The gasoline control module (VCM/PCM or ECM) supplies power to the gasoline fuel pump circuit for about 2 seconds at every ignition ON and whenever 7X reference pulses indicates the engine is cranking or running. When energized the lock-off relay supplies power to the high and low pressure lock-off solenoids.
At initial ignition ON the gasoline control module energizes the GASOLINE fuel pump relay supplying power to the Fuel Pump Power circuit for a two second prime pulse. When the prime pulse is supplied to the Fuel Pump Power circuit the lock-off relay is energized.
When the engine is started the GASOLINE control module energizes the GASOLINE fuel pump relay supplying power to the Fuel Pump Power circuit. 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 monitored (the engine is running).
The high and low pressure lock-off solenoids are energized by the AF ECU when the vehicle is operating in the CNG mode. At initial ignition ON the AF ECU will close the high pressure lock-off solenoid control circuit driver (completing the circuit path to ground) for a one second CNG prime pulse. The prime pulse occurs at the same time as the gasoline fuel pump prime pulse occurs. This ensures that the lock-off relay will be energized and supplying power to the lock-off solenoids at the same time that the AF ECU closes the driver and supplies the ground path.
When the engine is cranking or running the AF ECU closes BOTH lock-off solenoid control circuit drivers allowing the engine to operate on CNG.