Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairHard
A client complains that their vehicle occasionally fails to start, with no sound from the starter motor. When the technician checks, the battery is fully charged, and all connections are clean and tight. A test light connected to the S-terminal (solenoid trigger wire) on the starter solenoid illuminates brightly when the key is turned to START. What is the most likely cause?
- AA parasitic draw on the battery
- BA faulty ignition switch
- CA faulty starter solenoid
- DAn open circuit in the starter motor field windings
Show answer & explanationAnswer & explanation
Correct answer: C. A faulty starter solenoid
If the test light at the S-terminal illuminates brightly, it confirms that the ignition switch, neutral safety switch, and associated wiring are correctly sending power to trigger the solenoid. Since there is no sound from the starter and the battery/connections are good, the solenoid itself is failing to engage and pass current to the motor.
Why the other options are wrong
- A. A parasitic draw would discharge the battery, leading to a weak or no-crank condition, but the question states the battery is fully charged when checked.
- B. A faulty ignition switch would prevent power from reaching the S-terminal, so the test light would not illuminate.
- D. An open circuit in the starter motor field windings would cause the motor not to turn, but the solenoid would still engage, typically producing a 'click' sound.
Starter Solenoid Trigger Diagnosis
Testing the S-terminal (trigger wire) of the starter solenoid to confirm power delivery from the ignition switch and related safety switches.
- A test light on the S-terminal should illuminate brightly with the key in START.
- If power is present at the S-terminal but no starter action, the solenoid is faulty.
- If no power at S-terminal, the fault is upstream (ignition switch, neutral safety switch, wiring).
Memory trick: No click, but power to 'S', means the solenoid's stuck in its 'no-go' phase.