The MB owners manual advises all unnecessary electric loads be kept off if you are driving a lot of stop and go, and idling a lot, so I wouldn't expect much more from stock alt. Check the output at secondary road RPM, 2000 or so, and see what you get. The alternator is rated at 14 volts (anyone with better/ more exact figures is welcome to correct me any time, please) so at speed anything less than 13.8 is probably not good. Keep in mind that voltage and amperage tests require a fully charged battery to be accurate.

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1984 300 D runs, interior
1984 300 D light hit in front, RUST, mint/perfect dash, decent interior (see above), parts car
(hate to kill one)
1998 Dodge diesel