Alternator or ? fails

[donohue@azstarnet.com (Terry Donohue)]

    We just moved to Tucson, and I drove the family on the very scenic
mountain road to the top of Mt. Lemmon (9000 ft).  Unfortunately, the
charging system in my '90 200 TQ quit right at the top.  The indications
are:  Alternator warning light on, as well as the "Antilock Off" and brake
warning lights on.  The voltmeter initially read about 12V.  I decided to
see how far I could get running off the battery (fortunately new as of last
fall); I made it within a few miles of home (45 minutes of driving!; the AC
was off, and the outside temp. was 105), and AAA got me the rest of the
way.  Other useful info:  The car will run until the voltage drops to
slightly below 10V.
     The Bentley Manuals are not very helpful, in fact, downright obscure
when it comes to charging problems.  Anybody have an idea on the most
likely culprit?  Bad wire; bad voltage regulator; bad alternator; or else?
Also could use recommendations on sources for used alternators, if that is
the way to go.  TIA.

Terry Donohue
donohue@azstarnet.com
'71 BMW 2002Ti
'90 200 TQ
'95 BMW M3