Sounds like you were running the square back alt on a pre-’70 application. Likely one field terminal is intentionally grounded by the removal of a brush assembly insulating washer, substituting a metal washer. As mentioned, on a later isolated field system, there can be no continuity from either field brush/terminal to the case. Check the washer under the mounting screw head that the green wire currently connected to, make sure it’s an insolating washer.