When replacing an alternator, which symptom suggests the need for replacement?

The symptom indicating the need for alternator replacement is poor charging. The alternator is responsible for generating electrical power and charging the vehicle's battery while the engine is running. If the alternator is not functioning properly, it may fail to produce sufficient voltage or current to keep the battery charged, leading to symptoms like dimming headlights, a battery warning light illuminated on the dashboard, or difficulty starting the vehicle.

While overheating, noise, and corroded terminals can be associated with various electrical issues in a vehicle, they do not directly indicate a failing alternator. Overheating might occur due to other factors, noise could originate from bearings or loose components unrelated to the alternator’s charging capability, and corroded terminals typically indicate poor connections that might affect charging but don't necessarily mean the alternator itself is faulty. Therefore, the specific observation of poor charging is most clearly linked to alternator failure.