The brief
The owner had been seeing the battery warning light on the GLC250's dash, the displays would dim noticeably with the headlights and aircon both on, and the morning crank had grown slower than usual. He came in before a flat battery left him stranded.
The alternator charges the battery and runs the car's electrics while the engine's turning. Inside it a voltage regulator holds the output in a narrow band. When the regulator fails, the alternator stops charging properly, so the battery slowly runs down, the warning light comes on, there isn't enough spare power to hold the displays steady when something big switches on, and the starter struggles in the morning. That's exactly the pattern the owner described.
The diagnosis
The scan turned up a stored fault code logged against the alternator. A multimeter on the output confirmed it: well below charge voltage at idle, barely climbing under load, when it should be holding a steady charge across the range. The alternator's internal regulator was the failed part.
We checked the rest: the battery itself tested fine, the drive belt was tight, the charge harness clean. So it was the alternator. The serpentine belt and tensioner had taken some wear over the years, so those went on the list to do at the same time.
The work
The drive belt was released, the old alternator unbolted and lifted out, and a new Mercedes-spec alternator went in. A new Mercedes-spec belt and tensioner went on with it, and the belt was re-tensioned to spec.
Then the new alternator was load-tested with the engine running and the big consumers, headlights, aircon, blower, all switched on, to confirm the output held in the healthy band across the rev range.
A cold-start check afterwards confirmed normal cranking.
The outcome
Charging output is back to spec across the rev range, the dash stays steady with everything switched on, the engine cranks normally, and the battery warning is gone.
The GLC250 went home with the electrical system supplying what it should. A failing alternator left long enough flattens the battery and strands the car, so acting on the warning light and the slow crank meant a planned fix rather than a tow.