General Motors 6T40

General Motors' six-speed automatic for transverse engines, in production since 2008 and fitted to most front-wheel-drive Chevrolets, Opels, Vauxhalls and Buicks of the last fifteen years. It shifts clutch-to-clutch, doing away with the bands older automatics used, and drives on-axis rather than through the chain that GM's earlier transaxles needed.

Its name decodes exactly like the 4L60E's, and reading one teaches you the other. The 6 is the number of forward gears, the T means transverse — where the 4L60E's L means longitudinal — and the 40 is the strength rating on the same scale that runs to 90. So a 6T45 is the same gearbox built stronger, and the family runs 6T30, 6T35, 6T40, 6T45 and 6T50 in ascending order of what they will take. The 6T30 is the small one behind a 1.8; the 6T50 went behind a V6.

Do not confuse this family with the 6T70 and 6T75. Those are physically larger units and a different design — a joint GM and Ford project — so parts, capacities and faults do not carry across, however similar the numbering looks.

One component defines this gearbox, and it is worth knowing the name before anything goes wrong: the 3-5-R wave plate. The name records exactly which gears depend on it — third, fifth and reverse — and on early units it is simply too weak. When it breaks it does not fail quietly. It comes apart inside the transmission and sends metal through the fluid into everything downstream, so what begins as one inexpensive stamped part ends as a contaminated valve body, contaminated solenoids and a rebuild. GM revised the part, and the aftermarket sells a strengthened replacement precisely because the original earned its reputation.

So the symptom to take seriously is a specific one. Erratic shifting, slipping between ratios, hard or delayed changes, and above all the loss of forward or reverse drive are how this unit announces itself. A car that has lost third, fifth and reverse together is describing the wave plate almost by name.

The rest of the list is ordinary by comparison:

• The transmission control module and its solenoid connection plate, a documented failure on the 6T40 and 6T50, which produces electrical faults rather than mechanical ones.
• Gaskets and seals, which are the commonest routine repair and are cheap until a leak has run the unit low.

On fluid: it takes Dexron VI, close to eight litres of it, and the trade changes it and the filter at roughly 40,000 km. That interval will not save a wave plate — that part breaks because of what it is, not because of what it sits in — but it is what keeps the debris from a failure that has already happened from turning a repair into a replacement.