doubt it is serious; it just requires attention. Black smoke is carbon, not a problem; blue or white, that's a problem. Drive the hell out of the car after you do the following and it will be great...

You don't need a "new" alternator (there's no such thing). You need yours serviced. It can be done at a pro shop reasonably. Boxed alts from brightly lit retailers are a rip off. They take yours, which is fine except it needs brushes and a bench test, and sell you one that they put ten bucks into for $$$$$$.

I get brushes and testing while I wait for $20 at my local starter/alternator place.

Confirm the battery is no good by checking voltage with accessories on. If not 14 volts, charging isn't working, so take the alt off and hi thee to a real shop.

Also, tighten and clean wiring connections and grounds.

The only other thing that can be problematic (not a showstopper) is the PCV system. The engine metal is close to forever on 240s.
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Stef (81 245 B21A SU M46 3.91 330000km, 81 244 BW55 3.31 220000km)