Final answer:
To test an alternator, observe for dimming lights upon starting the engine, which can indicate an issue, and measure the battery voltage with a multimeter to ensure it's between 13.8 and 14.2 volts while the engine is running. Dimming lights with connected accessories could also imply alternator problems.
Step-by-step explanation:
To determine if an alternator is good or bad, you can observe a few signs that indicate its condition. One common symptom of a bad alternator is the dimming of lights. This can be compared to the situation where a refrigerator light dims when the compressor motor starts due to the initial surge in power required. If you observe that the lights in your car, such as the passenger compartment light, dim when you start the engine, it may indicate an issue. However, this dimming could also be caused by resistance inside the battery itself.
Another way to check the health of an alternator is by using a multimeter to measure the voltage output at the battery terminals while the engine is running. It should typically read between 13.8 volts and 14.2 volts. If the voltage is lower than 12.6 volts, which is the full charge level of a car battery, it might suggest that the alternator is not charging the battery properly.
The scenario where connecting excessive 12-V lights in parallel causes them to dim shows a similar principle. In a healthy electrical system, the lights should remain bright because the battery should maintain its voltage output. If the lights are dim, this might imply that the electrical system is overwhelmed, and in the context of a car, this could be related to the alternator's inability to provide sufficient power.