Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
System Description
Alternative Fuel (AF) fuel pump relay coil is supplied power when ignition is in RUN and START position. Alternative Fuels Engine Control Module (AF ECM) supplies ground path to AF fuel pump relay coil when engine starts and runs on CNG. The purpose of AF fuel pump relay circuitry is to allow AF ECM the ability to turn OFF gasoline fuel pump and to supply a voltage source for lock-off relay. At initial ignition ON, AF fuel pump relay is not energized by AF ECM. The gasoline Powertrain Control Module (PCM) energizes gasoline fuel pump relay, supplying power to Fuel Pump Relay Control circuit for a 2 second prime pulse. When prime pulse is supplied to Fuel Pump Relay Control circuit, lock-off relay is energized and gasoline fuel pump relay receives the 2 second prime pulse power. The 2 second prime pulse is for gasoline fuel pump durability purposes. When engine is operating on CNG, AF ECM energizes AF fuel pump relay in order to turn OFF gasoline fuel pump. The lock-off relay continues to be energized by PCM gasoline Fuel Pump Relay Control circuit.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.