On my '87, it came with "clear" washer fluid. I replaced it with standard "blue" washer fluid. The low fluid light comes on. After some testing, I found out Mercedes sells concentrated washer solution you mix yourself (yup the "clear" fluid). My guess is the alcohol content in the blue fluid is less dense, so the level sensor in my car sinks, light goes on. Put the MB water based solution back in and light goes out. May be an old sensor, but I am living with the clear solution at least until winter sets in. Let us know what you find out.