Your low fluid sensor is probably defective. You can drain the fluid from the reservoir and take it out and clean the connections with dielectric grease to see if that will help. Looks like you can buy a new one from Fastlane for $30.

http://catalog.peachparts.com/ShopByVehicle.epc?q=1984-Mercedes--Benz-190d-Body--Electrical&yearid=1984%40%401984&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6119%3AMBC%7C1481%3AED%7C10000092%40%40190D&catid=241959%40%40Body+Electrical&subcatid=242438@@Washer+Level+Indicator&mode=PA