Fuel Lock-Off Relay Diagnosis: Description

Lock-off relay is energized whenever Powertrain Control Module (PCM) supplies power to gasoline fuel pump circuit. PCM supplies power to gasoline fuel pump circuit for approximately 2 seconds at every ignition on cycle and whenever 7X reference pulses indicate engine is cranking or running. When energized, lock-off relay supplies power to high and low pressure lock-off solenoids. At initial ignition on cycle, PCM energizes gasoline fuel pump relay, supplying power to fuel pump power circuit for a two second prime pulse. When prime pulse is supplied to fuel pump power circuit, lock-off relay is energized.

When engine is started, PCM energizes gasoline fuel pump relay, supplying power to fuel pump power circuit. PCM will supply power to fuel pump power circuit, regardless of fuel system that in operation, as long as crankshaft pulses are monitored (engine is running).

High and Low Pressure Lock-Off (HPL and LPL) solenoids are energized by Alternate Fuel Engine Control Module (AF ECM) when vehicle is operating in CNG mode. At initial ignition on request, AF ECM will close the HPL solenoid control circuit driver (completing circuit path to ground) for a one second CNG prime pulse. Prime pulse occurs at same time as gasoline fuel pump prime pulse occurs. This ensures lock-off relay will be energized and supplying power to lock-off solenoids at the same time AF ECM closes driver and supplies ground path. When engine is cranking or running, AF ECM closes BOTH lock-off solenoid control circuit drivers, allowing engine to operate on CNG.