I think I've joined the club of BMW owners with alternator related issues.
It's a 1990 535im with 314,000 miles. I just completed a trip of 1050 miles with no problems. Yesterday, after the car sat in 10 degree F weather over night, I went to run an errand. As I left the driveway, I felt a major brief stumble. It recovered quickly. Today it's 13 degrees F. I went to run an errand at night, radio started fluttering, dash lights dimmed, headlights seemed pretty dim, wipers ran very slow and didn't speed up with acceleration but did when I turned off the headlights. Before I left on my trip, I tightened the loose alternator belt. Tomorrow I plan on putting it on the charger and hope to get it to an auto parts store for the free load test. Any suggestions on what to do in what order? I will check the terminals when I pull the back seat. Voltage regulator perhaps? I don't know how many miles are on the alternator. The PO kept real accurate records so I will review them to see. Any ideas will be appreciated including location of components other than the battery, alternator and attached regulator. Are there suspect relays?