very likely the reg itself. way less likely would be alt/brushes & unplug the green field wire at the alt (later type 2 field wire system) & if it stops charging then it ain't the alt/brushes. If it's an early "box" reg it could have lost it's ground continuity but fairly rare. unplug the reg triangle connector till you get it taken care of as that full fielding (17+ volts) will cause multiple problems quickly