Mercedes-Benz 724.0 (7G-DCT)

Mercedes-Benz's transverse seven-speed dual-clutch — in the A-Class, B-Class, CLA, GLA and GLB.

It is a WET dual-clutch, and that is the first thing to establish, because most of what circulates about dual-clutch gearboxes is about dry ones. The clutches run in oil, so the low-speed judder that defines the dry units — Ford's PowerShift, Hyundai's 7DCT, Fiat's DDCT — is not a characteristic of this gearbox. A shudder from rest here is a fault rather than the design meeting a driving habit.

Its dominant failure is the mechatronic unit, and it presents electrically: a gear refused, a limp mode, a car that will not leave neutral, a code naming a solenoid or a clutch position sensor. Nothing slips first, which is what distinguishes it from a clutch problem.

Fluid is a control question rather than a maintenance one. A wet dual-clutch shares its oil between the clutches and the hydraulics that operate them, so worn fluid makes the gearbox change badly long before it makes anything fail — and the interval is far shorter than the word lifetime suggests.

It is not the 725.0 nine-speed, which is the longitudinal torque-converter automatic in the rear-drive cars; the two share a maker and nothing else.