2003 Tahoe Battery light and not charging notification

[FineLinePimpin 10+ year member]

So i get my 320a alt on my 2003 Tahoe and after getting it in I have a battery light and a notification on the screen for system not charging. I have already done the Big 3, all quality 0ga wire to the amp and good grounds. I also started getting a few other codes from the ecu...I'm guessing its because of the excessive volts. When the battery light comes on it thinks its not charging but its fully charging at 14.8. The company I got the alt from tells me that oh yeah that's normal with a high output unit.
Well I can NOT stand driving around like that. Someone linked me to a harness that plugs in between the alternator and the factory plug, but someone also mentioned I can get a " S" to the terminal and run that to the batteries positive and that fixes it.....so I'm not sure what that is. Can someone shed some light on it.

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