Final answer:
The voltmeter should read around 13.5 to 14.5 volts when the engine is running, indicating a properly functioning charging system.
Step-by-step explanation:
When the engine is running, the voltmeter should read around 13.5 to 14.5 volts for a properly functioning charging system. This indicates that the alternator is generating sufficient electrical power to recharge the battery and power the vehicle's electrical components.
If the voltmeter reading is significantly below or above this range, it may indicate a problem with the charging system. For example, a reading below 12 volts could indicate a weak or faulty alternator, while a reading above 15 volts could indicate a defective voltage regulator. In either case, it's recommended to have the vehicle inspected by a qualified mechanic.