BMW SMG / SMG-II / SMG III

BMW's Sequential Manual Gearbox — in the E46 3 Series, E60 5 Series, E63 6 Series and E85 Z4.

It is a manual gearbox with a hydraulic actuator on it, which is what people find strange about it and what the name says plainly. There is no torque converter, so it does not creep; the clutch is a real clutch and wears like one; and the lurch on a slow change is the robot doing what a driver's left foot would do more smoothly.

The failure that defines it is the hydraulic pump and its accumulator. The pump runs on demand, the accumulator holds pressure between operations, and when either fails the car refuses to select a gear and shows a warning — usually intermittently first, and often after standing overnight. It is a very well-known fault with a well-known remedy, and it is not the clutch.

A pump that runs constantly rather than in short bursts is the accumulator, and that is audible from outside the car with the door open — one of the few faults in this catalogue with a diagnosis you can hear.

A clutch replacement must be followed by a bleed and an adaptation reset. Without it the car judders from rest and changes badly, which is exactly the complaint the work was meant to fix.

It is not the DCT that replaced it. BMW's later dual-clutch has two clutches, no single-clutch pause, and entirely different faults.

SMG and SMG-II are six-speeds; SMG III in the E60 M5 and E63 M6 is a seven. That last one is a different gearbox with a different pump and a different failure pattern, and it is the one most owners arrive asking about.