The brief
Royton brought his 740i in with a transmission fault code showing on the iDrive, and the gearbox shifting poorly. The mechatronic unit is the brain-and-hydraulics of the automatic box, the plate that holds the electronics and the valve body that does the hydraulic shifting, and when it fails the box can't shift cleanly and the car flags a transmission fault. Over the years the mechatronic wears, the solenoids and the hydraulics in particular, and degraded fluid speeds it along. When it fails the box hesitates, jolts, or won't shift right, and the iDrive shows a transmission fault. A failed mechatronic doesn't recover, so it gets replaced with a genuine unit, coded to the car, and the fluid refreshed with it.
The diagnosis
The transmission fault code and the way the box was shifting pointed at the mechatronic unit, which had failed. The rest of the gearbox was sound, this was the mechatronic, not worn internals. That's a mechatronic replacement, coded to the car, with fresh fluid, rather than chasing a gearbox problem that's really the mechatronic.
The work
The transmission pan came off, the failed mechatronic removed and a new genuine BMW-spec mechatronic fitted, coded and adapted to the car so the box and the unit talk to each other properly. The box was refilled with the correct BMW fluid to the right level at the right temperature, the way it wants it, the fault code cleared and the adaptations relearned. A road test confirmed the gearbox shifting cleanly up and down, no hesitation or jolt, no transmission fault on the iDrive, and the box behaving as it should.
The outcome
The gearbox shifting cleanly, no hesitation, no jolt, no transmission fault on the iDrive, fresh fluid, and a box that behaves the way it should. Royton got the 740i back with the gearbox sorted. Reading the fault properly meant we replaced the mechatronic that had actually failed rather than guessing at the box, and doing the fluid with it kept it to one visit.
