MB's limited slip used clutches coated with brass & they alternate with steel clutches..the fluid uses a special additive that allows the brass to expand & grab the steel plates upto about 40% lock-up..

IF regular fluid has been used then the brass plates are worn & most likely there is too much clearance for them to expand & lock-up.
IF the car still has the limited slip you would need to disassemble & replace the brass plates...

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MERCEDES Benz Master Guild Technician (6 TIMES)
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190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
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