General Motors 3T40 (THM125)

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.