Checking tranny fluid is tricky on Mercedes. Your owner's manual will tell you how and when to do it. Per my TD's owner's manual, if you have been running on the highway, let the tranny cool for 30 minutes. ATF is temp. sensitive

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member