A technician is replacing a starter motor on a 12 V heavy-duty vehicle. After installation, the starter engages with a loud clunk but spins very slowly and fails to crank the engine. The battery is fully charged. What is the most likely issue?

Question: A technician is replacing a starter motor on a 12 V heavy-duty vehicle. After installation, the starter engages with a loud clunk but spins very slowly and fails to crank the engine. The battery is fully charged. What is the most likely issue?

Answer options:

• Damaged starter drive (Bendix) assembly not engaging flywheel properly.
• Incorrectly shimming the starter motor, causing misalignment. ✅ A high resistance connection in the main battery cable to the starter.
• A faulty starter solenoid failing to make full contact.

Correct answer: A high resistance connection in the main battery cable to the starter.

Explanation: A loud clunk and slow cranking with a fully charged battery strongly suggests insufficient current reaching the starter motor. This is most commonly caused by high resistance in the main high-current circuit, such as a loose or corroded battery cable connection to the starter. A damaged Bendix or incorrect shimming would likely result in grinding or failure to engage, not slow cranking after engagement. A faulty solenoid that fails to make full contact would prevent the starter from spinning at all or result in just a click.