For starters alternator output is measured in amps. Maybe to check how that light is wired in for a diesel because it might not be "turned on" by the DME and instead be hooked right to the alternator. When did the light first come on ?? Did you buy the vehicle this way?

__________________
1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)