Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairHard
A technician is replacing a starter motor. After installation, the engine cranks and starts normally, but the technician notices a strong burning smell from the starter motor after a few starts. What is the most likely cause?
- AThe starter motor is improperly grounded.
- BThe starter motor was incorrectly shimmed, causing excessive pinion-to-ring gear mesh.
- CThe starter motor's overrunning clutch is sticking.
- DThe battery cables are undersized for the application.
Show answer & explanationAnswer & explanation
Correct answer: B. The starter motor was incorrectly shimmed, causing excessive pinion-to-ring gear mesh.
Incorrect shimming, leading to excessive pinion-to-ring gear mesh (too tight), causes the starter motor to bind and draw excessive current, leading to overheating and a burning smell. While the engine might still crank, the increased load on the starter motor rapidly generates heat.
Why the other options are wrong
- A. Improper grounding would cause high resistance in the ground circuit, leading to slow cranking or no crank, not necessarily a burning smell from the starter itself.
- C. A sticking overrunning clutch would cause the starter to remain engaged with the engine after it starts, leading to a whining noise and eventual starter failure, but the immediate burning smell after a few starts points more to binding.
- D. Undersized battery cables would cause excessive voltage drop and slow cranking, but the burning smell implies an issue with the starter motor itself or its engagement.
Starter Shimming Importance
The process of adding or removing shims (spacers) between the starter motor and the bell housing to achieve the correct pinion-to-ring gear clearance, preventing binding or improper engagement.
- Incorrect shimming (too tight) causes binding, high current draw, and overheating.
- Incorrect shimming (too loose) causes grinding and premature wear.
- Critical for proper starter motor operation and longevity.
Memory trick: Burning smell means too much heat, check the shims, make it neat.