I think the height adjustment is for additional ground clearance for bad roads. I don't think it is for normal driving. I think the light comes on when the height adjustment is used. I could be wrong. If the car has a rough ride you might need new nitrogen spheres. There is one for every wheel. Acts like a shock absorber for the hyd. fluid. You should only use the correct Mercedes fluid. I would check for leaks first. Change the filter and fluid second and then attack the rest. Very similar to the system used on the rear of mid '80s 300D station wagons.