Mercedes-Benz 722.9 (7G-Tronic)

The seven-speed automatic that replaced the 722.6, and the first production gearbox with a magnesium alloy casing — a weight saving Mercedes took because seven ratios and four planetary gearsets would otherwise have cost more than they returned. It served almost the whole rear-drive range for fifteen years before the nine-speed took over, and Infiniti and SsangYong bought it too.

Its converter locks up in all seven gears rather than only the upper ones, which is where its efficiency comes from and also where its commonest failure comes from — see below. The revised 7G-Tronic Plus of 2010 is the same architecture with a new torque converter and control strategy rather than a new gearbox.

The AMG cars are a genuine exception and not a trim level. SpeedShift MCT deletes the torque converter altogether and starts the car on a wet clutch instead, shifting in about a tenth of a second. It shares the type number and much of the gearset with the ordinary 722.9 and does not share its start-off hardware at all, so converter advice simply does not apply to it. SpeedShift TCT, confusingly, is the version that does keep the converter.

The controlled slip that makes it smooth is what contaminates it. The lock-up clutch is deliberately allowed to slip under control, and the friction material it sheds goes into the fluid faster than most owners expect. That single mechanism drives the two repairs the trade sees most: a torque converter rebuild, and a valve body that has been working in dirty fluid for years. A 722.9 that shifts harshly is far more often describing its fluid than its gears.

What is replaced, and what each one looks like from the driver's seat:

• Valve body springs and solenoids — small springs break and solenoids clog, and the unit is unusually sensitive to fluid that is hot or dirty. Erratic or harsh shifting.
• The Teflon sealing rings, which are what hold shift pressure where it belongs. When they go, shift timing goes vague rather than sharp, and this is the failure most specific to the unit.
• The K2 friction pack, the commonest clutch burnout.
• The conductor plate and its Y3/8 speed sensor, which burns out and takes gear selection with it — the same electrical-before-mechanical pattern the 722.6 shows.
• The bellhousing, damaged by vibration from a converter that has already been left too long.

Limp-home is a feature here, and worth recognising rather than fearing. On a critical fault the gearbox locks itself into a single gear so the car can be driven to a workshop. A Mercedes that will suddenly only do about 40 mph and will not change gear has not destroyed itself; it has decided that shifting is the thing most likely to destroy it.