BMW: charging issues, belt tension, 320i

Question
Hi,perhaps you can help? My battery light is no longer going out after I start.('85 320i) As I reved the engine it dimmed and eventually went out when reaching 4500rpm. I repeated this with a multimeter on the battery and saw that obviously it wasnt getting a charge and when it cleared, was reading 13.5 v. I've checked the belt tension, all the various bolts, cleaned the connections on the alt. aswell as trying a different (new) battery but it hasn't helped. Do you think the alt. is on the verge of death or do I have a damaged lead somewhere? Any help would be great!

Answer
Gavin,
 It does sound like you have an alternator issue.  Make sure I understand this correctly.  The battery light stays on unless you rev the engine to 4500rpm?  At 4500rpm the alternator is supplying 13.5v, and as soon as you let the engine idle the voltage drops back down?

 If that is the case, it is almost certain that you have a bad alternator.  You "can" replace the voltage regulator on the back of the alternator to see if that helps, and if not then replace the whole alternator.
 
 Hope this helps,
 Josh