I ran the first two tests Rick (Motorrad Elektrik) provided me a
few years ago, and it is obvious the rotor on my R100GS is shot,
so it'll be replaced. I always first suspect the 'gen' lamp,
since if it is blown, the alternator doesn't receive the exciter
current necessary for it to operate. However, the lamp tested
'ok', (came on when I grounded the DF terminal against the
engine block with ignition on.)  Question - I cannot find info
on exactly how to wire a diode into the exciter current circuit,
so that if the 'gen' bulb fails, the alternator will still
receive exciter current. I recall reading about this fix several
years ago. (a) can someone point me to a link to that fix? (b)
is it necessary to install the fix in the dashboard housing
(undesirable), or can it be installed elsewhere on the bike that
is more accessible? (c) does the fix work in conjunction with
the 'gen' bulb, or must the bulb be removed for the diode to
work?