More than likely the LB3 clutches are worn out, actually that is a safe bet.
LB3 clutches are for reverse only and are located behind the front pump housing surrounding the K1 drum.
Transmission has to be R & R to access. remove the filter first, open and inspect for clutch and metal material. If present trans has to come apart.

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Mercedes-Benz Master Guild - 2 years
Shop Foreman - 20 + years - dealer.
Part owner - MBZ Transmission remanufacture
2000 SL500
1999 ML430
1989 300SE