Automatic Transmission Problems on Mercedes ML

Having reviewed your description, the symptoms you are experiencing suggest a number of potential faults. It would be prudent to ascertain whether the hydraulic fluid level is correct and that the fluid is free from particulate contamination. However, have you had the vehicle's electrical system assessed recently? Specifically, was the battery condition checked? In certain Mercedes-Benz models, undervoltage conditions can induce erratic behavior in the automatic transmission control module, leading to the symptoms you have described. It is a situation akin to a computer glitching due to insufficient power.

Undervoltage, huh? Interesting. I haven't checked the battery specifically, but the car starts fine. Could that still be the culprit even if it starts okay? What's the connection between the electrical system and the automatic transmission acting up?

Indeed, even with adequate starting power, a marginal battery can present issues. Modern vehicle systems, including your Mercedes-Benz's automatic transmission control unit, are highly sensitive to voltage fluctuations. Insufficient voltage can lead to corrupted data transmission and, consequently, the symptoms you observe. The connection is analogous to a chain reaction, wherein a weak link (undervoltage) affects the entire system (transmission control). While I cannot definitively diagnose the issue remotely, given the described symptoms, it would be judicious to present the vehicle to a qualified technician for diagnostic assessment. A diagnostic scan tool should be employed to interrogate the transmission control module for stored fault codes, and the battery's state of charge and health should be evaluated under load.

Thanks for the advice! Turns out you were spot on. Took it to a workshop and they found undervoltage was the issue. Apparently, it messed with the transmission control unit like you said. They sorted out the electrical problem and cleared the codes. Cost me 240€, but at least the slipping transmission is gone!