The brief
Mr Tang's S300 lit up a transmission warning and then wouldn't engage gear at all, and when he asked around he was told the gearbox needed overhauling, a big bill. He came to us for a second opinion before committing, which was the right move, and we had it towed in rather than risk driving it. Not every transmission warning means the gearbox is worn out. The automatic on this car is run by an electronic transmission control unit, and when that control plays up the box can't shift or engage properly and throws fault codes, codes that to an inexperienced eye look like a worn valve body and lead to a full overhaul being quoted. The right move is to read it properly on the Mercedes diagnostic system and find out which it actually is, the control or the box itself.
The diagnosis
On arrival we ran the Mercedes diagnostic protocols, no guesswork, and pinned the fault to the electronic transmission control, which was faulty, not the valve body and not a worn gearbox. The similar-looking fault codes had led other workshops to the overhaul quote, but the box itself was sound. That's an electronic transmission control replacement, a fraction of a gearbox overhaul, rather than rebuilding a box that didn't need it.
The work
The faulty electronic transmission control was removed and a new genuine Mercedes-spec unit fitted, coded and adapted to the car so the gearbox and the control talk to each other properly. The transmission fluid was checked, the fault codes cleared, and the box's adaptations relearned. A road test confirmed the transmission engaging properly, shifting up and down silky smooth, no warning light, and the box behaving exactly as it should.
The outcome
The transmission engaging first time and shifting silky smooth, no warning light, and a gearbox that's exactly as it should be, on a bill nowhere near an overhaul. Mr Tang got the S300 back with the gear issue solved and a lot of money saved. Reading it properly on the Mercedes system, rather than going by fault codes that mislead, is the difference between replacing one control unit and rebuilding a whole gearbox.