شادّ السير أو بكرة الفصل متآكلة

The automatic belt tensioner or the alternator's decoupler pulley worn. The tensioner keeps the belt at constant tension and the decoupler lets the alternator's rotor freewheel through the engine's torsional pulses — both absorb vibration the belt would otherwise transmit.

A tensioner whose damping has failed lets the belt flutter under load, so it slips exactly when the alternator is working hardest — which reads as a charging fault with a healthy alternator. A seized decoupler transmits every pulse into the belt, causing squeal and rapid belt wear. Watch the tensioner arm with the engine running: an arm oscillating visibly is the fault, and it is invisible with the engine off. Replace the tensioner with the belt, since a new belt on a failed tensioner slips too.