Alternator started sparking

One thing that used to happen to these is that either the housing comes loose, or the "step" in the housing where the stator sits becomes worn from loose/ vibration, and the stator either vibrates and moves or in bad cases the rotor can rub. In any case the vibration can loosen the winding which begin vibrating and rubbing against each other. Then the insulation wears off and the turns short together. I've seen this in darkness, looks like a mini lightning storm inside.