1992 500sl What is the black electrical box with a knob on the top that is located R/S rear of engine compartment, I turned the knob thinking that it would tighten down the unit. ( it seemed loose) now most of my warning lights are out.

That is your module box for all the control unit if you remove the nut there is 38 pins connetor under the nut(38 pins is for HHT or SDS tester)

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MB STARS Master Guild Technician,17 years MB tech
14 years independent shop owner