Yeah, those old S-Class use a ton of current from the battery. Not having a 100% good battery can get you in trouble in a hurry. Not having a good alternator can ruin a good battery very quickly.

Double check your main ground connections and make sure the battery terminals are pristine before driving very far on your new battery. Last thing you want to do is ruin a brand new battery. Also check the connections at the alternator. You could just have a bad connection.

Curious, does that model have the removable brush assembly, or is that just the older diesels?

-tp