The contact box is a box which you connect between the big connector for the cars computer, and the computer itself, it gives an outlet to which you can connect a multimeter and monitor everything while the car is running.

I did my testing without it, but it sure would make things a lot easier. It's an additional part used for diagnostics only, I would imagine your dealer can help you get one.

xp

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1985 190E 2.3L - a constant project.