Hi

That car is just like mine. The reservoir you see is for the Automatic locking differential (ASD) : that system is hydraulically controlled and effective up to 19 mph. That traction system was offered on W124 only for the diesel. The gaz model had a more fancy system : automatic slip control (ASR) which electronically modulate brake pressure on the spinning wheel and power to the drive train ; that system operate up to highway speeds.

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DR
1992 300D 2.5T 351 000 km