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The regulator that sets the alternator's output has failed or is misbehaving. On a modern vehicle it is rarely a simple voltage regulator any more — the engine module commands a target over a communication line, and the regulator obeys, raising output when the battery needs charge, backing off under hard acceleration, and pushing harder on the overrun to recover braking energy.

That command link creates a failure mode worth knowing about, because it looks like a dead alternator and is not. If communication fails, the regulator falls back to a fixed default and charges at a fixed voltage regardless of what the module wants. The module sees an alternator ignoring instructions; the alternator is doing exactly what it should with no instructions to follow. Check the command line before condemning the unit.

Failures go in both directions and undercharging is the less damaging one. A regulator stuck high overcharges, which boils electrolyte, shortens battery life dramatically, and can damage electronics across the vehicle. A battery that has repeatedly needed replacing is worth reading as a regulator symptom.

On most alternators the regulator is replaceable separately and is a fraction of the cost of the assembly — worth asking about before agreeing to a complete unit.