The only thing that comes to mind would be a grounded field coil winding. If you disconnect the field coil from ground and disconnect the other end from the eunit, there should be no continuity between the isolated field coil and the motor frame. If there is, a field coil turn has probably gone to ground effectively shunting the field. If this turned out to be the case, either the field coil would have to be rewound or the motor replaced.
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