Set off in my 900i (1990) the other day and noticed the battery light was
glowing dimly when idling, after a while the light became brighter but again
only while idling or at low revs, after a while driving the problem went
away only to return about half an hour later. When I stopped I found the
battery to be completely discharged.

I've had the AA out to take a look, he found the alternator to be charging
but intermitantly so he cleaned all the contacts in particular the earth and
that seemed to do the job, it was even charging better than ever, but 10
minutes down the road with the AA guy gone and the light came back on... I
managed to get back home but now the light is brighter than ever - it will
fade and even dissappear but it needs about 3000rpm to do so.

Does this mean the alternator is completely knackered or should I try and
dismantle it? I saw there is a section on checking the brushes in the Haynes
manual.

If I do need a new one would a scrap yard job do or am I just setting myself
up for more probs? The cars done 130k so I guess its time for a new one...

Any help would be much appreciated.
Cheers, G.