Alternator for that era with Bosch voltage regulator (Bosch sets them conservatively; no 14.4 v. for that era) should be around 13.7 -14.1 volts at ~2000 rpms. 12.13 v. would indicate a no charge situation (bad regulator/brushes/alternator), since a good battery standing alone puts out ~12.6 v.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)