Rebuilt? The ecu is very finicky with aftermarket regulators found in many rebuilt alternators. I would only replace with an oem unit or at a minimum, oem regulator. I would check the belt to ensure it is routed correctly. Flat side on smooth pulley, ribbed side on ribbed pulley. It can be routed differently.

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E53 X5 4.8is-Dinan S2-Street Brawler
E70 X5M-The stoplight shocker
AMG E55K-539 RWHP, 629RWTQ-my toy
GX470-the unbreakable truck.
F30 328i-JB4 STG 2. Daily MPG Work Beater
02-WS6, 6sp my other toy
...other stuff parked around here somewhere