Figure 10. Cooling system expansion tank (the arrow shows the filler cap of the engine cooling system)
Since the expansion tank of the cooling system is made of opaque material, to check the fluid level in the system, you need to unscrew the threaded plug of the tank at the top of the radiator (figure 10). The engine must be stopped and its temperature must be above 50°C. Coolant must be visible in the expansion tank. Before the first night frosts, it is necessary to check the concentration of antifreeze (freezing temperature no higher than -30°C) through the filler cap of the radiator expansion tank. More detailed information on this issue can be found in section. "Cooling system".
Attention! The expansion tank cap may be opened when the coolant temperature is below 90°C. First, turn the cap to the first lock, pause briefly to release steam, and only then open it completely. To replenish small losses of coolant, ordinary water should be used. Do not use fully descaled, rainwater, distilled or demineralised water as they promote corrosion. If a large amount of coolant has leaked from the cooling system during the journey, never add cold water to a hot engine to avoid deformation or cracking of the cylinder head due to thermal stress. A small amount of cold water can also be added to a hot engine.
(Publication taken from an online resource: Mercedesman.ru)