Just doing a quick double check here.

[BrassDrummer]

The alternator warning bulb is the source of exciter power for the alternator. If the bulb is bad there will not be exciter power to the alternator and thus it won't charge properly. This configuration is a lot more common than you might think, and not just on E30's. Though very late E30 clusters, and many other implementations, also include a bypass resistor to make sure that exciter power is available if the bulb burns out.

I know exactly what it is, and how it works, it's just one of those things you don't think about. Especially when you work with newer BMW's all day, and they do not have that wiring configuration anymore. Just something easily over looked and forgotten.

[TurboJake]

^ I'm sure Jlevie was just trying to inform stamar.

[cabriodster87]

Cars have been like this for as long as i can remember. When you turn ignition on, if the alt lamp doesn't light...its blown/open circuit.