I think for once your mechanic mght be right. I would call MB and price a reman transmission. If I remember correctly, they're not terribly expensive. Also, I would recommend not having this guy service the car anymore.

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Rgds,
Aaron Greenberg
MB technician
'67 250SE Cabriolet
'77 450SL
'79 6.9
'79 6.9
'80 300SD
'81 240D
'85 380SE
'89 420SEL
'93 300E 2.8