General Motors 6T70

The larger of GM's two transverse six-speed automatics, in production since 2006 and fitted to the mid-size and full-size front-drive cars and crossovers — the Malibu and Impala, the Equinox and Terrain, the Acadia and Outlook, the LaCrosse, the SRX and the Saturn Aura.

It is not the 6T40, and the numbering makes that easy to get wrong. The 6T40 family is the smaller unit for compact cars; this is the physically larger one for bigger engines, and they share no parts. The names decode the same way — six speeds, T for transverse, and a strength rating on the scale that runs to 90 — so 6T70, 6T75 and 6T80 are one design at 380, 407 and 500 Nm.

It was designed with Ford, which is unusual enough to be worth knowing. The two companies began joint work in 2002 and put something like $720 million into the plants that build it; GM makes its version at Warren, Michigan. Ford's equivalents carry 6F50 and 6F55 codes and sit in the Edge, Taurus, Explorer and Lincoln range. A Chevrolet Traverse and a Ford Explorer of the same era are running the same architecture, which is not a sentence one often gets to write.

And it has the same signature failure as the 6T40, which makes it a family trait rather than a coincidence: the 3-5-R wave plate. The name again records which gears depend on it — third, fifth and reverse — and on units built before 2009 the plate was not stress-relieved and could simply break. When it does it comes apart inside the case and sends metal through the fluid to everything downstream. GM released a revised plate in 2009, so the year is what separates a car that is a candidate for this from one that is much less likely to be.

What else the trade meets:

• The solenoid block, a documented weakness on units built before about 2012, producing electrical faults rather than mechanical ones.
• The torque converter, which degrades and contaminates the fluid in the ordinary way.
• Clutch wear, generally downstream of one of the above rather than a starting point.
• Overheating, which is what accelerates all of it in a heavy vehicle worked hard.

On fluid: Dexron VI, around nine litres of it, on a service interval of roughly 60,000 km. As with its smaller relative, that will not save a pre-2009 wave plate — that part breaks because of how it was made — but it is what keeps the debris from a failure that has already happened from turning a repair into a replacement.