The pivot housings of the front axle pivot assemblies are equipped with threaded plugs (1)
1. To check the hinge housing of the right drive shaft, the wheels should be turned all the way to the left and vice versa.
2. Unscrew the threaded plug (1) from the casing being tested.
3. The lubricant level in the casing should reach the lower edge of the inspection hole. If necessary, make the appropriate adjustment by adding the appropriate amount of lubricant of the required grade to the casing using a grease gun with a flexible nozzle, the end of which should be inserted into the inspection hole.
Original source of information [MercedesMan]
4. Do not forget to screw in and tighten the plug (1) to the required torque.