General Motors TH400 / 3L80

General Motors' three-speed Turbo-Hydramatic — in the Hummer H1, and in the Jaguar XJS.

A Jaguar with a GM gearbox is the thing worth knowing here, and it is not an oddity: Jaguar bought the TH400 because it was the strongest automatic available and their own was not. So an XJS owner's transmission literature is American, enormous, and filed under a name their car never mentions.

It is probably the most over-built automatic in this catalogue. It has three ratios, no overdrive and no lock-up on the early ones, which is why it feels the way it does at motorway speed — that is the design, not a missing gear, and no amount of scanning will find a fourth.

Its faults are almost all age rather than wear. The governor and the modulator are mechanical parts that drift, producing changes that arrive at the wrong road speed; the front pump seal leaks; and the fluid in an unknown example is unknowable. A gearbox that changes far too early or too late, consistently, is the governor — which is a distinctly different symptom from slipping and a far cheaper repair.

The 3L80 is the same transmission renamed when GM moved to its modern naming; a part listed under either fits.