A technician is diagnosing a vehicle with a complaint of intermittent no-start. The technician connects an oscilloscope to the fuel pump control circuit at the fuel pump module. When the key is turned to the 'ON' position, the oscilloscope shows a brief 12V pulse, then drops to 0V. When the engine is cranked, the oscilloscope shows no further voltage pulse. Which of the following is the MOST likely cause?
- AAn open circuit in the fuel pump's ground wire.
- BA faulty fuel pump relay.
- CA short to power in the fuel pump circuit.
- DA problem with the engine control module (ECM) not receiving a crankshaft signal.
Show answer & explanationAnswer & explanation
Correct answer: D. A problem with the engine control module (ECM) not receiving a crankshaft signal.
Modern vehicles typically provide a brief fuel pump prime (12V pulse) when the key is first turned ON, then shut off if the engine isn't cranked. The fuel pump is reactivated by the ECM once it detects a crankshaft signal, indicating the engine is cranking or running. If the pump doesn't receive power during cranking, and the prime pulse was normal, it strongly suggests the ECM isn't getting the crank signal, preventing it from commanding the fuel pump ON.
Why the other options are wrong
- A. An open ground would prevent the pump from running at all, even during the prime pulse, or cause no voltage to be present at all if the oscilloscope is measuring across the pump.
- B. The brief 12V prime pulse indicates the fuel pump relay is functioning at least partially for the prime cycle.
- C. A short to power would likely keep the pump running constantly or blow a fuse, not exhibit this specific behavior.
Fuel Pump Control Logic
The Engine Control Module (ECM) controls the fuel pump, typically providing a brief prime cycle upon key ON, and then continuous operation only when a crankshaft signal is detected during cranking or running.
- Initial prime pulse (2-3 seconds) upon key ON.
- Pump turns off if no crank signal is detected after prime.
- ECM reactivates pump when crankshaft signal is present.
Memory trick: Prime's Good, Crank's Silent: ECM Needs Crank's Voice.