Brushes are shorted to ground.

Make sure two things (or three or four)

1...That you do not have an early (69/ earlier) which DOES have one grounded brush

2...That the alternator is not some "rebuilt" which has had a grounded brush installed, or the insulating washers lift out or broken

3...With both brushes removed, check from slip rings to the alternator case, it should be OPEN (infinity)