That is for ASD, which is the fancy Mercedes version of a locking differential. It is tied into the ABS system to detect wheel spin at the rear axle. When either or both of the rear wheels start to loose traction under acceleration, the ASD computer sends hydraulic pressure to the differential, and internal clutches "lock up" so power goes to both wheels regardless of the wheel slip, which improves traction. When the system engages, a flashing yellow triangle with an exclamation point in the middle will show on the speedometer.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC