Charging system issues after changing over to 7603 tune.

Changed my friend's 06 silverado 4x4 L33 over to a 04 pedal and tac module so we could use a 7603 tune on it. Used a spare p59 ecm I had on the shelf to put a light transmission tune on it for now. After putting in the new computer and associated parts I am getting an alternator not charging message on the cluster. I checked the battery with a multimeter and I am getting a solid 13.2 charge at the posts while running. Does anyone have any idea if the 7603 tune could be causing problems with the bcm to cause an issue?

Hello. I am running into the same issue with an 05 Avalanche. Apparently the 05-07 alternators use a different charging strategy than the 7603 OS uses. Looking at the alternator part numbers and pin outs shows 2 pins vs 4 pins. I am trying to find a work around and have not had luck yet. I have been trying to find an xdf for the 618 os from 2005 but it is incomplete. Hoping someone here has found a solution they can share with us.