Mala conexión a masa del alternador o del motor

A high-resistance earth between the alternator, the engine and the body, so charging current cannot return cleanly. An alternator's output is only as good as its earth path, and that path runs through the engine block and a strap to the body — a route full of joints that corrode.

The result is charging voltage that measures correctly at the alternator and low at the battery. The difference is the voltage lost in the earth path, so the battery is undercharged while the alternator appears to be working — and the module, seeing low system voltage, reports charging rather than earthing.

Measure voltage drop on the earth side under load, which is the test that finds it: probe between the alternator case and the battery negative with the engine running and loads switched on, and anything beyond a couple of tenths of a volt is the fault. The earth strap between engine and body is the usual culprit, corroded internally under intact insulation or loose at a painted mounting point. Temporarily adding a jump lead as a supplementary earth confirms it in seconds.