2000 BMW 323 charging system / computer

Electrical problem
2000 BMW 323 4 cyl Two Wheel Drive Automatic

the charging system is not working on this car. I had the alternator bench tested at 3 locations in which case the alternator pass the bench test in all 3 cases. My research tell me that the computer controls the alternator charging process. With the car runing you only have battery volyage at the battery. The wire between the alternator and computerand the intrument cluster is good. When you turn the key forward engine off the battery light stop comming on. I added ground to the body of the alternator. My research indicates that the computer sends a signal out to turn the indicator light on or off and controls the charging process by duty cycle control. Can you help me.