1 wire starter.

I have a jeep with the old style starters that have only one post on the rear of the starter for 12v and ground via the case. The solenoid is built into the starter for this reason. When I apply 12v to post and place negative cable from battery charger on the case the starter gets very hot and cranks slow. Am I bench testing this starter wrong? It's brand new. Had advanced auto test it they said it was good. I have power from ignition to relay and my power wire from battery to bat side of relay and other 12v wire to starter. The solenoid clicks and 12 v goes to starter but still no start. As I stated I can't even bench test the new starter