Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisEasy
A technician is diagnosing a vehicle with an intermittent no-start condition. The technician determines that the fuel pump is not receiving power. Using a multimeter, the technician measures 12 volts at the fuel pump relay's coil input, but 0 volts at the relay's switched output when the engine is cranked. Which of the following is the MOST likely cause?
- AA faulty fuel pump motor.
- BA short to ground in the fuel pump circuit.
- CA corroded ground connection for the fuel pump.
- DAn open in the fuel pump relay's switched power supply circuit.
Show answer & explanationAnswer & explanation
Correct answer: D. An open in the fuel pump relay's switched power supply circuit.
If the relay coil has power but the switched output has no voltage, it indicates that the power supply to the relay's switched contacts is missing or the relay itself is faulty. Since the question specifies 0 volts at the switched output, an open in the power supply to the switched side of the relay is the most direct cause.
Why the other options are wrong
- A. A faulty fuel pump motor would still allow voltage to reach the pump, but the pump wouldn't operate.
- B. A short to ground would likely blow a fuse or cause continuous current flow, not 0 volts at the output.
- C. A corroded ground would prevent the pump from operating but would not cause 0 volts at the relay's switched output.
Relay Power Supply
A relay requires both control (coil) power and load (switched) power to operate correctly and deliver voltage to a component.
- Coil energization closes internal contacts.
- Switched power must be present for output voltage.
- Absence of switched power results in no output voltage.
Memory trick: Relays need two types of power to make things flow, coil to click, switched to glow.