The turn lights are working also as an emergency stop flashing but when I turn them off, the relay starts buzzing. The windshield wipers and heating of the rear window seems in working condition but do not stop buzzing when they are turn on.
I see no RPM, oil pressure, coolant temperature on the display, only speed and economy gauge working.

Why the relay is buzzing? It should be direct current (DC) in a car – constant +12V or 0??

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1987 MB260E 187K