Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairHard
A technician is performing a bench test on a starter motor. The starter spins freely and quietly when applying battery voltage directly to the motor terminals, but the drive gear does not extend. What does this indicate?
- AAn internal short in the motor windings.
- BA faulty overrunning clutch (bendix drive).
- CThe battery used for testing is discharged.
- DAn issue with the starter solenoid's pull-in winding.
Show answer & explanationAnswer & explanation
Correct answer: D. An issue with the starter solenoid's pull-in winding.
If the starter motor spins but the drive gear does not extend, it means the solenoid's pull-in winding is not functioning or not receiving power. The pull-in winding is responsible for engaging the drive gear with the flywheel before the main contacts close to power the motor. Since the motor itself spins, the problem lies with the solenoid's engagement mechanism.
Why the other options are wrong
- A. An internal short in the motor windings would likely cause excessive current draw and slow or no motor rotation, not free spinning.
- B. A faulty overrunning clutch would allow the motor to spin but fail to transmit power to the flywheel once engaged; the drive gear would still extend.
- C. A discharged battery would prevent the motor from spinning freely, which contradicts the observed symptom.
Starter Solenoid Engagement Function
The starter solenoid has two primary functions: extending the drive gear via the pull-in winding and closing high-current contacts to power the motor.
- Pull-in winding extends the drive gear.
- Hold-in winding keeps the drive gear extended.
- Main contacts supply high current to the motor.
Memory trick: Motor spins, but gear sits, solenoid's pull-in it quits.