Carrier x4 7500 not charging

Hope someone can help me with this issue..
I have a 2014 carrier x4 7500 that won’t charge. I turn the unit on it dose the whole start up cycle and It starts up . but roughly in about five minutes it throws code a00051 alternator not charging. And I’ve checked all the fuses. Also change the alternator twice , cleans all the connection including the ones to the starter and battery and still nothing .. On the display getting 12.2 V and showing negative 8.3 on the amps the only way I can get it to charge is by exciting the alternator with the wire to the screw that’s at the bottom of the alternator and in the middle of positive and negative .. Anybody has any ideas why is not self activating?