When should you check the voltage output of an alternator?

Checking the voltage output of an alternator is most effectively done when the engine is running. This is because the alternator produces electrical current when the engine is operating, allowing you to assess its performance under normal operating conditions. When the engine runs, you can observe if the alternator is producing the correct voltage, usually between 13.8 to 14.5 volts, indicating it is functioning correctly and supplying adequate power to the electrical system and battery.

Performing this check while the engine is off would not provide an accurate representation of the alternator's performance, as it would only show the voltage level at rest, which is not indicative of the load and operational capacity needed when the vehicle is running. Checking after battery replacement or with the battery disconnected also does not simulate the true operating conditions necessary for assessing the alternator's output.