How can you test for a bad alternator?

Testing the battery voltage output while the engine is running is an effective way to determine the condition of the alternator. When the engine is started, the alternator is responsible for generating electricity to power the vehicle's electrical systems and charge the battery. If the alternator is functioning properly, the voltage across the battery terminals should typically read between 13.7 to 14.7 volts when the engine is running.

If the voltage reading is below this range, it indicates that the alternator may not be charging the battery adequately. Conversely, if the voltage is significantly higher, it could suggest that the alternator is overcharging, which can also cause potential damage to the battery and electrical components. Therefore, measuring the battery output effectively assesses the alternator's performance in the charging system.

The other options, while relevant to vehicle maintenance, do not directly relate to testing the function of the alternator. Checking the oil level and examining the brake fluid or inspecting the fuel filter focus on different systems within the vehicle and are not indicators of alternator performance.