Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairEasy
During a fuel system inspection, a technician notices that the fuel pump relay clicks rapidly and erratically when the ignition is turned to the 'ON' position, but the fuel pump does not prime. All fuses are good. Which of the following is the MOST likely cause?
- AA short circuit in the fuel pump wiring.
- BAn open circuit in the fuel pump motor.
- CA low battery voltage.
- DA faulty fuel pump relay.
Show answer & explanationAnswer & explanation
Correct answer: C. A low battery voltage.
Rapid and erratic clicking of a relay is a classic symptom of insufficient voltage. If the battery voltage is too low, the relay coil may not be able to hold closed consistently, causing it to chatter or click erratically. This prevents the fuel pump from receiving consistent power to prime.
Why the other options are wrong
- A. A short circuit in the fuel pump wiring would likely blow a fuse or cause the relay to click once and then stop, or not click at all, depending on the location of the short.
- B. An open circuit in the fuel pump motor would prevent the pump from running, but the relay itself would still actuate normally (a single click) if it's receiving proper voltage and command.
- D. A faulty fuel pump relay would typically either not click at all, or click once and stay on/off, not click rapidly and erratically.
Relay Chatter Diagnosis
Rapid and erratic clicking (chattering) of a relay is a strong indicator of low control voltage or insufficient power supply to the relay coil.
- Commonly caused by low battery voltage.
- Can also be caused by poor ground connections.
- Prevents consistent power delivery to the controlled component.
- Check voltage at relay terminals.
Memory trick: Relay chatter, battery's sad patter.