Torque converter automatic: how it works and what it is like to live with

• The torque converter transmits engine power through a flow of oil. It gives a smooth pull-away and multiplies torque as the car moves off. The individual gears are created by the planetary gearsets behind the converter.
• Once the car is moving, a lock-up clutch connects the engine to the gearbox mechanically. The converter then stops slipping, the oil does not heat up unnecessarily and fuel consumption does not rise.
• Today's automatics have 6 to 9 or even more gears. Their reputation for being thirsty belongs to the old three- and four-speed units.
• Their strengths are smooth low-speed manoeuvring, towing, off-road driving and the ability to handle high torque. Their weaknesses are extra weight, a more complex design and heat build-up during prolonged slip under load.
• You will find them mainly in mid-size and large cars, SUVs and pick-ups. In listings they hide behind names such as Steptronic (BMW), 9G-Tronic (Mercedes-Benz) or EAT8 (Peugeot, Citroën).

A torque converter automatic is what most drivers picture when they hear the word "automatic": it pulls away smoothly, creeps along in traffic on its own and has no clutch pedal. On the used market you will find it in family cars, premium saloons and SUVs, but the trade name in a listing often says nothing about how it works. Its reputation as a comfortable but lazy and thirsty gearbox dates from the era of three- and four-speed automatics and no longer reflects today's technology. The gearbox's behaviour makes sense once you separate three things: pulling away through oil, changing gear in the planetary gearsets and the mechanical connection made by the lock-up clutch. For an overview of all designs, see our article on types of automatic transmission; for fluid care, read automatic transmission fluid change.

A conventional automatic consists of three main assemblies. The torque converter transmits power when pulling away, the planetary gearsets change the gear ratio, and the electronics and hydraulics decide when and how the gearbox shifts.

The torque converter: power carried by a flow of oil

The converter sits between the engine and the gearbox itself. Inside, three main components do the work:

This is why the engine can keep running with D selected while the car stands still on the brake. The impeller turns, the turbine stays still and the oil absorbs the difference in their speeds. When the driver releases the brake, the oil flow starts to turn the turbine. The stator raises the output torque at the same time, so even a heavy car moves off easily. As soon as impeller and turbine speeds even out, torque multiplication ends. The converter therefore acts as an "extra gear" only when pulling away and when accelerating hard from low speed.

Planetary gearsets: where the individual gears come from

The converter itself has no first, second or any other gear. Those are created by the gearbox behind it, which in a conventional automatic uses several interconnected planetary gearsets.

A planetary gearset consists of a central sun gear, smaller planet gears around it, a planet carrier and an outer ring gear. Multi-plate clutches and brakes inside the gearbox connect or hold individual parts. Depending on which elements are engaged, a different gear ratio results. During a shift, the gearbox smoothly releases one set of friction elements and engages another, so the flow of power to the wheels is not interrupted. The control unit works together with the engine, and a well-calibrated automatic shifts quickly and without a noticeable jolt.

The lock-up clutch: why the engine does not have to keep churning oil

Transmitting power through fluid is pleasant when pulling away, but the speed difference between impeller and turbine means losses. Some of the energy turns into heat in the oil instead of driving the wheels.

That is why the converter has a lock-up clutch. Once it engages, there is a mechanical connection between engine and gearbox and the slip disappears. Modern automatics engage it soon after pulling away, often as early as second or third gear, and can also engage it partially with controlled slip. The car moves off gently through the oil and, once under way, transmits power as efficiently as a mechanical clutch.

Older automatics typically had three or four gears. Today you will commonly find 6 to 9 gears in used cars, and gearboxes with even more exist. The point is not to shift as often as possible, but to have a suitable ratio for every situation.

A short first gear helps a loaded car or a car with a trailer move off. A long top gear lowers engine speed on the motorway. Small steps between gears keep the engine in the range where it pulls well and runs economically. The eight-speed ZF 8HP, for example, is built around a wide ratio spread, which the manufacturer describes as a way of keeping the engine at a suitable operating point.

Do torque converter automatics use more fuel?

The reputation for thirsty automatics comes from gearboxes with big gaps between gears that engaged the lock-up clutch only in top gear, or had none at all. The converter then slipped for much of the journey and energy was lost as heat. Today's automatics engage the lock-up clutch early, run at low revs and have hydraulics with smaller losses. The difference in fuel consumption compared with a manual is therefore small, and for many models the manufacturer quotes the same or lower consumption for the automatic.

The greatest strength of a torque converter automatic is its smooth transmission of power when there is a big difference between engine speed and wheel speed. It shows wherever the car is moving slowly but needs a lot of tractive force.

The drawbacks are less visible, but they exist:

• Weight and complexity. The converter, several planetary gearsets, multi-plate clutches and hydraulics weigh more than a manual, and repairs often mean a rebuild or replacement of the whole gearbox.
• Losses during slip. When pulling away and driving slowly, some energy turns into heat. This is more noticeable in town than on the motorway.
• Slower response in older gearboxes. Older automatics with fewer gears "think about it" for a moment when you press the accelerator. Today's eight- and nine-speed units respond promptly.
• Sensitivity to temperature and fluid condition. Here the fluid does not just lubricate; it actually transmits power and controls the shifts.

Is a torque converter automatic durable?

This design copes with high torque and hard use, which is why it is found in large SUVs, pick-ups and powerful saloons. The converter also spares the friction elements: the car does not pull away through a slipping clutch, as with a manual or dual-clutch gearbox. Its lifespan, however, depends on how the whole gearbox is dimensioned, on its cooling and on the condition of the fluid, because multi-plate clutches and brakes, hydraulic valves, bearings and electronics are still at work inside.

The torque converter automatic is the standard in mid-size and large cars, bigger SUVs and pick-ups, where engines produce high torque and the cars often tow. It is also used in some compact models and SUVs. A listing usually gives the car brand's trade name; the name of the gearbox maker (ZF, Aisin) appears only rarely.

The trade name alone is not always enough. BMW also uses the Steptronic name for the dual-clutch gearbox in smaller front-wheel-drive models. In many model ranges the type of automatic varies by engine, and plug-in hybrids often have an electric motor with a clutch instead of a torque converter, even when the gearbox carries a similar name to the one in the ordinary petrol and diesel versions. For how a dual-clutch DSG differs, see our article on the DSG gearbox.

On a used car with a torque converter automatic, the warning signs are harsh jolts when shifting, judder when pulling away, a delay after moving the selector to D or R, and revs that suddenly flare while driving without matching acceleration. Only diagnostics and a test drive can determine the cause; for cars offered by Carvago, both are part of the CarAudit™ inspection.

The most common misconceptions concern the principle itself:

• "The converter smooths everything out." It damps engine torsional vibration and softens the pull-away, but it will not hide worn clutch plates or faulty hydraulics.
• "Slip is always a fault." When pulling away it is part of how the converter works. Once under way, the lock-up clutch should eliminate it. If revs fluctuate at a steady speed, that is a warning sign.
• "It can tow any load." Long periods of reversing and creeping with a heavy trailer heat the oil and can exceed the capacity of the cooling system. The approved towing limit of the specific car applies.
• "More gears means a better automatic." Smoothness and responsiveness are determined mainly by the software and how well the gearbox is matched to the engine. A nine-speed automatic that keeps hunting between two gears feels worse than a well-calibrated six-speed.

The fluid in an automatic transmits power, controls the shifts, lubricates and carries away heat, so the whole gearbox depends on its condition. The change interval is set by the manufacturer for each specific gearbox; there is no single rule for all torque converter automatics.

Carvago offers around a million used cars from dealers in several European countries. So when you are after a specific combination of model, engine and gearbox, such as a BMW 3 Series with Steptronic or a Peugeot 3008 with EAT8, you are choosing from a selection many times larger than a single local market offers.

Before sale, every car is inspected by a technician as part of the CarAudit™ check, covering up to 270 checkpoints. The inspection includes a check of the car's history and technical components, diagnostics and a test drive, which is exactly how the condition of an automatic gearbox is assessed. The customer knows the result before deciding to buy.

Carvago guarantees the condition stated in the CarAudit™ certificate and provides a free extended warranty with every car. Models that Carvago considers problematic are kept out of its offer altogether under the Carvago Blacklist.

Can I tell from the paddles behind the steering wheel that a car has a torque converter gearbox?

No. Paddles are simply a way of controlling the gearbox, and cars with a dual-clutch gearbox or a CVT with simulated gears have them too. They tell you nothing about how the gearbox is built inside.

How does a torque converter automatic reverse?

Reverse is created by a combination of the planetary gearsets that reverses the direction of rotation of the output shaft. Neither the engine nor the converter changes its direction of rotation.

Can you bump-start a car with a torque converter automatic?

No. The engine drives the gearbox's oil pump, so without the engine running there is no oil pressure in the gearbox and the gearsets cannot engage. A car with a flat battery has to be jump-started from another power source.

Does a torque converter automatic shift more slowly than a DSG?

Today's eight- and nine-speed automatics shift so quickly on the move that an ordinary driver will not notice the difference. A dual-clutch gearbox responds slightly faster and is lighter; the torque converter is smoother when creeping in traffic, reversing uphill and towing.

1. https://static.nhtsa.gov/odi/tsbs/2019/MC-10159424-0001.pdf
2. https://www.zf.com/products/en/cars/products_64238.html?pageLink=service_and_support_divider658792
3. https://carvago.com/car-delivery
4. https://carvago.com/caraudit