black cylinder with twist knob on top
 

This item sits on top of the aircon compressor under the hood. It has a twist knob on top of it with a 2way arrow. To be exact, it is next to the oil filter cylinder. My car is the 1994 C180 Euro. What is this item? What will happen if I start to twist the top knob either clockwise or anticlockwise? I thought this was only in the diesels, maybe there is another lookalike in the diesels.

Hi Hocky1,
I think it is the power steering fluid reservoir. Should have to hoses attached to it. In and out going to steering. If you do a search on it, there will be pictures of it.
If you loosen it, hold on to the lid as there is a spring underneith.

Reinhard Kreutzer