Charging System Troubleshooting links...
http://www.largiader.com/articles/charging/
http://bmwmotorcycletech.info/trbleshootALT.htm
Troubleshooting a dead GEN lamp:
Because a common problem is a GEN lamp that does not illuminate at all, this troubleshooting section is first.
The problem may not be a bad bulb. The GEN lamp SELDOM burns out. More often it is the rotor that has opened, or connection where the lamp fits, occasionally the VR, and sometimes worn brushes. You can easily test the rotor without any instruments.
As a double check: Turn on Ignition....GEN lamp should still be out.
Disconnect battery (removing all the wires to the negative post will do).
Remove front cover.
Reconnect battery.
Turn on ignition. GEN lamp should still be out.
Pull off the wire/connector going to the brush holder male spade that is marked as Df.
Df is the place the lamp and VR both apply the initializing current (and, after the alternator produces current, where the alternator self-energizes).
Stick a bent paper clip into the wire/connector, and ground the clip to the case. The lamp should light up. If you do not have a paper clip, UNplug the associated wire, which is the D- female spade connection, and plug that Df wire into the D- female. D- is ground. If the lamp lights up when grounded, and not when to the Df terminal, then the rotor is open, or the brush(s) not making contact. If the lamp does not light up when grounded, it is the lamp, or the wiring or pod connector or the VR.
To further test for the lamp circuit condition, and eliminate the VR as the problem:
Turn off the ignition.
Pull fuel tank.
UNplug the VR.
Turn on the ignition.
Ground the solid BLUE female connection wire in the voltage regulator's harness socket.
If the lamp does not light up, the wiring from the VR to the pod or its large rear plug, or internal wiring or lamp or lamp connection in the pod is at fault.
Do NOT forget to disconnect the battery before replacing the front cover.