undo the 2 screws on the back of the alt that hold the bush pack on. check the 2 bushes have meat on and are about the same length, then clean up all the electrical mating surfaces. if you look in the hole you'll note there are 2 prongs, these contact a metal part of the bush pack. had a similar issue on a mates 16v, turned out this mating face was covered in rust! cleaned it all up and bit of grease, fixed!