After replacing the alternator, the vehicle's battery continues to show a low voltage reading. What might be the underlying issue?

The underlying issue in this scenario is likely related to the malfunctioning of the voltage regulator. The voltage regulator’s primary function is to maintain the voltage output from the alternator within a specified range, ensuring that the battery is charged correctly and does not receive too much or too little voltage. If the voltage regulator is not functioning properly, it may not effectively regulate output, which can lead to insufficient charging of the battery, even if the alternator itself has been replaced.

A defective battery could potentially also present low voltage readings, but in this case, the issue appears to stem from the charging system's inability to deliver adequate voltage to the battery due to the voltage regulator's malfunction. The alternator belt being loose might also affect charging, but this would usually present itself in a different way, commonly associated with unusual noises or physical signs of slippage. A failing starter motor typically would not directly affect the battery's voltage reading when the vehicle is off, as it only draws power when the vehicle is starting. Thus, focusing on the importance of the voltage regulator is crucial in diagnosing the cause of the low voltage reading following an alternator replacement.