The ZF eight speed automatic (BMW Steptronic 8HP family) has two faults, and you can feel both.
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The first is the mechatronics unit: the valve body and the control electronics in one. If the solenoids or the pressure regulation weaken, the gearbox jerks, the D/R engagement is delayed, it logs a gear ratio fault code, and finally drops into limp mode (often staying in 2nd or 3rd).
The second, and often misdiagnosed, is the connector sleeve of the mechatronics unit: the O-ring of the plastic sleeve ages, and ATF leaks from the side of the housing, exactly where the wiring harness enters. This is easy to mistake for a rear crankshaft seal. The trail starts from the driver's side of the housing, not from the clutch side.
On cars built in 2023 there was also a separate safety recall for a welding fault in the mechatronics unit. That is not the same as the wear related sleeve and the valves.
Check the fluid level, the area around the sleeve and the fault codes first; do not accept a "full gearbox replacement" quote straight away.
What to check
- On the test drive watch for delayed D/R engagement, a jerk when shifting, and limp mode (a stuck gear)
- Look at the driver's side of the gearbox housing where the wiring harness enters: an ATF trail from the sleeve is suspect
- Do not confuse it with a rear crankshaft seal leak: the direction of the trail decides
