General Motors TH400 / 3L80

3-speed automatic · longitudinal · 1964–2003

Also known as: th400, th-400, 3l80, turbo 400, turbo-hydramatic 400, thm400, thm-400

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.

Cars fitted with this gearbox

3 generations. Only where a source names the model and the generation together — a gearbox is documented as a product far more often than as a fitment, so this list is deliberately shorter than the truth.

Hummer

H1 First generation

Jaguar

Information, not instruction

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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.

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