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.
Fuel Pump Relay Diagnosis: Description
Alternate Fuel Engine Control Unit (AF ECU) supplies power to AF fuel pump relay coil when engine starts and runs on CNG. AF fuel pump relay allows AF ECU to turn off gasoline fuel pump and supply a voltage source for lock-off relay. At initial ignition on, AF fuel pump relay is not energized by AF ECU. Powertrain Control Module (PCM) energizes gasoline fuel pump relay, supplying power to fuel pump power circuit for 2 second prime pulse. When prime pulse is supplied to fuel pump power circuit, lock-off relay is energized and gasoline fuel pump receives 2 second prime pulse power. When the engine is started, PCM energizes gasoline fuel pump relay, supplying power to fuel pump power circuit. When engine is operating on CNG, AF ECU energizes AF fuel pump relay, turning off the gasoline fuel pump. Lock-off relay continues to be energized by gasoline fuel pump relay.
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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.