What does the alternator do?
The alternator is the part that keeps your battery charged. It works with the battery to keep the electronic components of your car working. Here is a great article about everything you need to know about alternators :)
What are the signs of a failing alternator?
If your alternator is failing you will initially start seeing your electronics go out. For example, as we were driving the radio started cutting out and then went out completely. After that, the cruise control went out, then the lights on the climate control panel, the ABS (antilock break) light came on signaling that the ABS was not functional, then the entire instrument panel with the spedometer and odometer went out. Finally the car will just shut off and you will be left with a powerless car. No interior lights, no headlights, nothing.
What to do if you are driving when the alternator starts failing?
The best thing you can do is pull over immediately and call either AAA or another agency to come tow you home. If that isn't an option (for us we don't have any towing covered by insurance) you can call a friend or relative to come jump start your car, let the battery charge for a good while and try to make it home on that charge. It's really bad to jump the car, drive it, jump it again, drive it, etc. That's what we did since we were about 30 minutes away from home and it is really really really bad for the batteries of both cars, but like I said, if you have no other choice then go for it.
How to replace the alternator:
In my 1996 328is BMW it was fairly simple. Generally alternators aren't in tough spots to get to so as long as you can find it this DIY should be somewhat helpful.
There were two large bolts holding the alternator in, one was easy to reach the other not so much.
If you have the ability to get a replacement alternator from a mechanic or a BMW dealer I would suggest doing that but if it's the weekend and you have to go to Autozone, they should carry one that works in your car, even though it might not be the best quality. They do have a lifetime warranty on the alternator at Autozone so if they do sell you a crap part, it won't cost you anything to replace. We took the old alternator in and got $90 off (the copper cores are valuable).
Once we got everything back in place we had to jump the battery to get the car started and drove around for about an hour to let the alternator do it's job and juice up the battery a bit. After that drive there haven't been any problems with the battery or alternator. The blower on the other hand...I'll save that for a later post :)