Just had my alternator replaced last week. Car never really seemed right after i got it back.. had a slight whinning noise. Was going down the highway and heard a pop. The battery light came on and it was making a horrible noise. When I got it pulled over black smoke was pouring out of the alternator area... couldn't see exactly where it was originating from. Smelled like burnt rubber ... but the belt did not break. Car will start but a horrible kind of high pithed noise comes from the front of the engine compartment and my battery light is on. Any ideas as to what this could be? Did I get a faulty alt. or maybe a belt tensioner is to blame? Any suggestions on to how to narrow it down would be greatly appreciated

My guess is either faulty belt tensioner or the mechanic routed the belt incorrectly.

wiring harness
 

I don't know what the wiring connectors are like on the BMWs, but I've had ford alternators catch on fire internally because of dry wiring connectors (originally packed with dielectric grease) where they plug into the alternator (bad connection). Just replaced the wiring harness and alternator (there was also a pigtail harness repair available for the fords.