Of late, the charge light comes on and stays on after starting the engine. The intensity varies, but in the end, it's on. Until I twist the engine up to about 2100 - 2500. The light goes out and stays out. I checked the battery this afternoon and found 12.9V with the key off; with the engine idling, 12.1V(!). This is Not Good. Checking the wiring diagram, there really isn't very much to get worked up about. There's blue wire from the alternator to the charge light and a red wire to the battery (and also off to I forget which black box) - "Nothing to see here, folks, move along."

I'm beginning to suspect the alternator is going but maybe it's something else? Before I BMW (Break My Wallet) with the dealer, to troubleshoot and R&R expensive bits and pieces, does anyone have any suggestions about what's going on here?