Transmission leaks from above the right (passenger side) rear corner of the pan. Thought it was a pan gasket, but then noticed the leak was from some component mounted just above the pan on the right side of the transmission.
Can you tell me what component that is?
Is there a seal/gasket/o-ring I can buy?

did you find out what the leak was? I think I have a similar problem on an 89 560 sel. The fluid drips onto the exhaust at highway speeds only and smokes off.

If you have the same tranmission as a 1989 420 SEL, then it is probably the O-ring on the sender guage. In fact, I think there maybe 2 rings. I dropped the lower oil pan to replace them. You'll also need a new oil pan gasket. Dropping the pan is a little tricky. There are 2 nuts on the pan screws around the AC compressor. Also, be sure to get the torque on the pan screws correct. There are other posts here, that describe the job.

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1989 420 SEL

My mechanic found the leak. I'm not exactly sure what component it was, but it's mounted on the side of the transmission above the right rear corner of the pan. Apparently, it was just a seal and nothing major. It's just very hard to see the source but easy to fix when the car's up on a lift. Good luck.