Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairEasy
A technician is diagnosing a vehicle with a 'no-crank' condition. When the ignition is turned to START, the starter motor spins freely and rapidly, but the engine does not turn over. The battery is fully charged. What is the most likely cause?
- AExcessive resistance in the main positive battery cable
- BA shorted starter motor armature
- CA faulty overrunning clutch (bendix drive)
- DA seized engine
Show answer & explanationAnswer & explanation
Correct answer: C. A faulty overrunning clutch (bendix drive)
If the starter motor spins freely but the engine does not turn over, it indicates that the starter's pinion gear is not engaging with the flywheel, or if it engages, it is not transferring power. This is the classic symptom of a faulty overrunning clutch (bendix drive) which allows the pinion to spin independently of the armature.
Why the other options are wrong
- A. Excessive resistance in the main positive battery cable would lead to slow cranking or a 'no-crank' condition, not a freely spinning starter.
- B. A shorted starter motor armature would typically cause the starter to draw excessive current, crank slowly, or not at all, not spin freely.
- D. A seized engine would prevent the starter motor from spinning at all, or cause it to draw very high current and possibly hum or click loudly.
Overrunning Clutch Failure
The starter's overrunning clutch (bendix drive) fails to engage the flywheel or transmit torque, causing the starter motor to spin without turning the engine.
- Starter motor spins freely and often rapidly.
- Engine does not crank or turn over.
- Requires replacement of the starter motor or bendix drive.
Memory trick: The 'Spin' is in, but the 'Engine' won't begin.