Check where the negative battery cable attaches to the body. If it is loose or rusted out, you can have a complete power loss situation. Another area worth checking is under the vehicle. There is a woven ground strap that runs from the transmission to the body.

A working alternator will produce 13.5-14V at idle at the battery.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Excellent advice. Thank you