Mercedes-Benz 722.6 (5G-Tronic)

Mercedes' five-speed automatic for longitudinal engines, and one of the most widely sold gearboxes ever built by a carmaker for its own cars — three planetary gearsets, three brakes, three clutches, and a production run that outlasted two generations of the models it launched in.

It is also in a great many cars that are not Mercedes, which is why an owner can arrive here from an unexpected direction. Chrysler bought it and called it NAG1, putting it behind the Jeep Grand Cherokee and Wrangler, the Dodge Charger and Challenger, the Chrysler 300 and the Crossfire. Porsche used it in the 911, Jaguar in the XK, and SsangYong across its range. A repair described for a Grand Cherokee is a repair for an E-Class, which is not obvious to either owner and is most of this page's value.

The variant codes decode cleanly and are worth knowing before buying parts. W5A is the family; the number is the torque rating in newton-metres, so a W5A330 is the light unit behind a six-cylinder and a W5A580 the heavy one behind a V8. The AMG applications go to a thousand. They are the same gearbox scaled, not different designs.

What fails is electrical before it is mechanical, and the sequence is specific enough to diagnose from. The speed sensors live on a conductor plate inside the gearbox, in the fluid, and they fail from a combination of heat and contaminated ATF. That is the first fault most cars show. Left alone it becomes a second and worse one: the connector that carries those signals through the case leaks, fluid wicks up the loom, and it reaches the control unit at the other end. A gearbox fault and a soaked control module are frequently the same fault six months apart, which is the argument for not driving on a known sensor fault.

After that the list is hydraulic and then mechanical:

• The valve body, working with degraded fluid and pressure it can no longer regulate cleanly.
• General oil starvation once pressure has fallen, which is what actually wears the rest of the unit.
• Bushings and the pump cover's friction surfaces, worn by vibration from a tired torque converter rather than by age.
• Front and rear overrunning clutch separators, and the K1 freewheel, which the trade replaces as a matter of routine at overhaul.
• The turbine shaft and the planetary bearings, which is where a repair stops being economic.

Mercedes declared the fluid a lifetime fill and it is not one. Both the sources behind this page reach the same conclusion from different evidence — one from the fluid specification and a roughly 60,000 km interval, the other from what contaminated ATF does to the electrics above. The gearbox is not fragile; the maintenance advice it shipped with was.

One characterful detail rather than a fault: the Winter setting starts the car in second gear to cut wheelspin on ice. A driver who has knocked the switch and cannot work out why the car feels flat from rest has found a feature, not a problem.