General Motors 3T40 (THM125)

3-speed automatic · transverse · 1984–2001

Also known as: 3t40, thm125, th-125, turbo-hydramatic 125, 125c, gm 3-speed transverse, th-125c

General Motors' three-speed transverse automatic — behind the four-cylinder Cavaliers, Corsicas, Berettas, Skylarks, Grand Ams and the Lumina and Lumina APV.

It is the gearbox the 4T40-E replaced, and its three ratios are the whole of what people remember about it: an engine that is loud at motorway speed because there is no overdrive to drop into.

That is not a fault, and it is the commonest reason one of these is looked at. A 3T40 revving at 3,000 rpm at 110 km/h is doing exactly what it was built to do; there is no fourth gear to be missing, and a scan for a stuck solenoid will find nothing.

The genuine failures are the forward clutch and the direct clutch, presenting as a slip from rest or a flare on the 2–3 change, and the governor — a purely mechanical part on this transmission — which produces changes that arrive at the wrong road speed rather than not at all. A gearbox that changes up far too early or far too late, consistently, is the governor, and that is a distinctly different symptom from slipping.

These are old enough that fluid history is unknowable and usually bad. A pan drop tells more about one of these in ten minutes than any amount of scanning.

Cars fitted with this gearbox

7 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.

Buick

Skylark Sixth generation

Chevrolet

Pontiac

Grand Am Fourth generation

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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