How does an automatic gearbox work? The automatic transmission explained

An automatic gearbox chooses the right gear for you, so you only work the accelerator and the brake. Sensors measure road speed, throttle position and engine load. A control unit decides which gear fits, and oil pressure closes the clutch packs that select it inside a set of planetary gears. There are four main types, and inside they work very differently.

From behind the wheel, every automatic feels much the same: you select D and drive. That changes the moment the gearbox starts to behave oddly, because a jolt in a torque converter automatic means something quite different from a jolt in a DSG. At ATR Transmissies in Montfoort, in the province of Utrecht, we diagnose and repair all four types, and the way they fail differs a lot from one type to the next. Below you can read what actually happens inside an automatic gearbox, what the letters on the selector do, and which parts wear first.

How does an automatic gearbox work, step by step?

An automatic gearbox works in a loop of measuring, deciding and switching, many times a second. In a conventional torque converter automatic, a gear change goes like this:

1. The engine drives the torque converter, which passes the power on to the gearbox through a fluid coupling instead of a friction clutch.
2. Sensors report road speed, engine speed, accelerator position and engine load to the transmission control unit (TCU).
3. The TCU compares that data with its shift programme and decides that a different gear fits.
4. It energises solenoids in the valve body, which route pressurised automatic transmission fluid (ATF) to the right clutch packs and brake bands.
5. One clutch pack releases while another applies. Holding or driving a different part of the planetary gear sets changes the ratio between the engine and the wheels.

The whole sequence takes a fraction of a second. How smoothly the old clutch lets go while the new one bites decides whether you feel a soft shift or a thump.

What types of automatic gearbox are there?

There are four main types of automatic gearbox, plus the special case of the hybrid e-CVT:

• Torque converter automatic: a torque converter plus planetary gear sets. The oldest and best-known design, and the one most people mean by “an automatic”.
• Dual-clutch gearbox (DSG, DCT, PDK): two half-gearboxes in one casing, each with its own clutch and input shaft.
• CVT: continuously variable, with adjustable pulleys and a steel push belt or chain. No fixed gears at all.
• Automated manual (semi-automatic): an ordinary manual gearbox where actuators work the clutch and select the gears.

Hybrids such as Toyota’s add a fifth case, the e-CVT. It is often lumped in with the normal CVT, but technically it is something else entirely, as you can read further down. For a side-by-side comparison of every gearbox type, including manuals and electric cars, see our overview of the car gearbox. Not sure which one your car has? Our guide on which gearbox is in my car shows how to find out.

How does a torque converter automatic work?

A torque converter automatic replaces the clutch pedal with a fluid coupling and replaces the separate gear pairs of a manual with planetary gear sets. Hydraulic pressure, directed by a valve body and controlled by electronics, does the shifting. General Motors put the first mass-produced automatic of this family, the Hydra-Matic, on sale in 1939, and the basic layout is still recognisable in today’s eight- and nine-speed gearboxes.

The torque converter: fluid instead of a clutch pedal

The torque converter is an oil-filled housing between the engine and the gearbox. It is not a clutch with plates that clamp together but a hydrodynamic connection with three main parts:

• The impeller (or pump) turns with the engine and flings ATF outwards.
• The turbine catches that flow of oil and starts to turn with it. It is connected to the input shaft of the gearbox.
• The stator sits between the two and redirects the oil flowing back from the turbine so that it helps the impeller instead of braking it.

Because of the stator, the converter delivers more torque than the engine puts in when there is a big speed difference between impeller and turbine, which is exactly the situation when you pull away from standstill. That torque multiplication is why an automatic pulls away so smoothly.

The lock-up clutch

Transferring power through oil always costs some slip, and slip costs fuel and creates heat. That is why almost every modern torque converter has a lock-up clutch. As soon as conditions allow, usually at a steady speed, it locks the engine and gearbox together mechanically: no slip, lower fuel use and lower revs on the motorway. A lock-up clutch that starts to shudder or slip is a well-known cause of a vibration at constant speed, which drivers often mistake for an engine or wheel problem.

Planetary gear sets: several gears from one set

Behind the torque converter there are no separate gear pairs like in a manual gearbox, but planetary gear sets. Each set has a sun gear in the middle, planet gears around it in a carrier, and a ring gear with internal teeth around the outside. By holding one of those three parts still and driving another, you get a different ratio out of the same set, including reverse. Combining two or more sets gives the six, eight or nine forward gears of a modern automatic.

Clutch packs and brake bands: how a gear change happens

The holding and driving is done by wet multi-plate clutches, which are packs of friction plates and steel plates running in oil, and in many gearboxes by brake bands that clamp around a drum. Each gear is therefore a combination of which packs are closed and which are open.

When the gearbox shifts, one pack opens while the next one closes. The overlap between the two is the whole art. Too much overlap and the gearbox briefly fights itself, which you feel as a thump. Too little and the engine revs flare for a moment before the new gear bites. Worn friction plates, low oil pressure or old fluid all change that timing, which is why a harsh or slipping shift is often the first sign of wear.

Valve body, solenoids and the mechatronic unit

The valve body decides which packs close and when. It is a block full of channels, valves and spools through which the ATF flows under pressure. In older gearboxes this was done purely hydraulically and mechanically. In modern gearboxes electronics sit on top: solenoids and pressure regulators controlled by a transmission control unit, often built together into one assembly called a mechatronic unit.

That unit decides not only when the gearbox shifts but also with how much pressure. Its software adapts to your driving style and to the wear of the clutch packs. If a solenoid, a pressure sensor or the wiring goes wrong, the gearbox usually starts to behave wrongly rather than stopping altogether. The control unit then often stores a transmission fault code that shows which circuit is out of range, and in more serious cases switches to limp mode, where the gearbox stays in one gear to protect itself.

Why automatic transmission fluid does more than lubricate

In a manual gearbox the oil mainly lubricates the gears and bearings. In an automatic gearbox, ATF does three jobs at once:

1. It lubricates the bearings, gears and planetary sets.
2. It cools. The torque converter produces heat, especially in heavy town traffic and when you tow a trailer or caravan.
3. It transmits power and delivers pressure. The fluid drives the turbine in the converter and, through the valve body, supplies the pressure that closes the clutch packs.

That third job makes ATF a working part of the gearbox, not just a lubricant. If the fluid is old, contaminated with friction material or the wrong type, its friction behaviour and the pressure build-up change. You feel that as slow engagement, slipping or jolting. The level matters as well: too little oil lets air in and pressure drops, while too much makes it foam, with the same effect. Every manufacturer specifies its own fluid, and those fluids are not simply interchangeable. More on intervals and what a fluid change involves in our article on changing gearbox oil.

How do the other automatic gearboxes work?

The other automatic gearboxes reach the same goal, no clutch pedal, with very different hardware. That is why they wear differently too.

Dual-clutch gearbox (DSG, DCT, PDK)

A dual-clutch gearbox has no torque converter. It has two clutches on two input shafts: one shaft carries the odd gears (1, 3, 5, 7) and the other the even gears. Where reverse sits depends on the design. While you drive in third, fourth is already preselected on the other shaft. Changing gear is then nothing more than opening one clutch and closing the other, which happens extremely quickly.

There are wet versions, where the clutches run in oil and can handle more torque and heat, and dry versions, where the clutch plates run in air and the gears have their own gearbox oil. Dry versions are mostly found in lighter cars and are more sensitive to heat from a lot of pulling away and stopping. Volkswagen calls its dual-clutch gearbox a DSG, and Porsche uses the same principle under the name PDK. The weak points are usually the clutches themselves and the mechatronic unit, so the gearbox tends to judder at low speed and when pulling away, as with a juddering 7-speed DSG.

CVT

A CVT gearbox has no fixed gears at all. Two pulleys, each made of a pair of cone-shaped discs, change their width, with a steel push belt or a chain running between them. When one pulley narrows and the other widens, the effective diameter changes and so does the ratio, without steps. The engine speed can therefore stay the same while the car speeds up, which gives the familiar rubber-band feeling. The Dutch carmaker DAF put this idea on the road in 1958 with its Variomatic, and Audi later sold its own chain-driven version under the name Multitronic.

Automated manual (semi-automatic)

A semi-automatic or automated manual gearbox is exactly what the name says: a manual gearbox with actuators that work the clutch and engage the gears. You notice it in the pause at every gear change, because there is only one clutch that has to open and close each time. Its wear therefore resembles that of a manual: when the clutch is slipping, the car behaves just like a manual car with a worn clutch plate.

The e-CVT in hybrids

The e-CVT in, for example, a Toyota hybrid is not a belt CVT. There are no pulleys and no push belt inside. It is a planetary gear set, which Toyota calls the power split device, that divides the power between the petrol engine and two electric motor-generators. The ratio between engine speed and wheel speed is set by controlling those electric motors, not by moving or shifting anything. So there are no clutch packs that engage and release and no shift points. That is why this design does not suffer from the shift complaints of the other types: there is simply nothing to shift.

What do P, R, N, D and the other letters on an automatic gearbox mean?

The letters on the selector tell the gearbox which mode to work in. Most automatics use the same basic layout, sometimes with a few extra positions:

• P (Park): a parking pawl drops into a toothed ring in the gearbox and locks the drive mechanically. It is a latch, not a brake. On a slope, apply the handbrake first and only then select P, so the weight of the car does not rest on that pawl.
• R (Reverse): reverse gear.
• N (Neutral): no drive to the wheels, but nothing locked either. The car can roll.
• D (Drive): the gearbox shifts through the forward gears by itself.
• S (Sport): shifts up later and down sooner, for more response.
• M or + and –: manual mode, where you choose the gear, usually with protection against over-revving.
• L, 1 or 2: limits the gearbox to the lowest gears, useful for engine braking on a long descent or when towing. In hybrids, B usually stands for extra engine braking.

Driving an automatic: what is normal and which habits cause wear

A few behaviours are typical of an automatic and are not a fault. A few habits, on the other hand, wear it out faster than necessary.

Creep. Put a torque converter automatic in D, release the brake and the car creeps forward on its own. That is because the converter already passes on some torque at idle. Dual-clutch gearboxes imitate this electronically by letting the clutch slip slightly, which is one reason they dislike long stretches of crawling in traffic.

Kickdown. Press the accelerator past the pressure point all the way down and the gearbox shifts down for maximum acceleration. In old gearboxes a cable or a pressure valve did this. Today it is a switch or a pedal sensor value that the control unit takes into account.

Stop before you go from D to R. Switching between forward and reverse means the gearbox has to close a different set of clutches and bands, against the direction the drivetrain is turning. Do that while the car is still rolling and those packs have to grind away the speed difference. That costs friction material and creates heat. So wait until the car is completely still, with your foot on the brake.

Handbrake before P on a slope. If you select P first and then release the footbrake, the car settles onto the parking pawl. You often hear and feel that as a clunk when you move off again. Applying the handbrake first keeps that load off the pawl.

Neutral at traffic lights. For a normal stop there is no need: an automatic is designed to stand still in D with your foot on the brake. For a long wait, N or P with the handbrake on is fine. What you should not do is coast downhill in N, because you lose engine braking and control.

What wears inside an automatic gearbox, and how do you notice it?

Every type has its own weak points, but in a torque converter automatic the same patterns come back. The table links each part to its job and to what you feel when it starts to fail.

Heat speeds all of this up. Towing, a lot of town traffic or a blocked oil cooler make the ATF age faster, and aged fluid makes every other part on this list wear sooner.

The table also shows why we never judge an automatic on one symptom. The same jolt from first to second can come from a sticking solenoid, a worn clutch pack or simply old fluid, and each needs a very different repair. At ATR Transmissies we therefore read the fault memory, look at the fluid and take the car for a test drive before we say anything about the cause. Only then do we decide whether a targeted repair is enough or whether the gearbox needs a rebuild. If you recognise jolting, slipping or jerking when pulling away, find out what your symptom could mean, or read about a car that jerks or stutters, where engine-related causes are covered too. A complete list of warning signs is on gearbox failure symptoms, and what is known per make is on automatic gearbox problems by make.

For garages: a second opinion without moving the car

Are you a garage with an automatic on the lift that behaves oddly, and no transmission specialist in house? With ATR Remote we can read and assess the gearbox remotely, so you know where to look before you take anything apart.

Frequently asked questions about how an automatic gearbox works

How does an automatic gearbox work in short?

An automatic gearbox decides which gear fits your speed, throttle position and engine load, and engages it without a clutch pedal. In a torque converter automatic it does that with planetary gear sets and clutch packs that are closed by oil pressure, controlled by a transmission control unit and a valve body.

Does an automatic car have a gearbox?

Yes. Every automatic car with a petrol or diesel engine has a gearbox, it just changes gear by itself. Electric cars are different: most use a reduction gearbox with one fixed ratio, and only a few models, such as the Porsche Taycan, have two gears.

What is the difference between a CVT and a DSG gearbox?

A CVT has no fixed gears: two pulleys and a steel belt or chain change the ratio continuously. A DSG is a dual-clutch gearbox with real gears on two shafts and two clutches that hand over from one gear to the next. A CVT feels seamless, a DSG feels like very fast gear changes.

What is the difference between a torque converter automatic and a DSG?

A torque converter automatic passes the power to the gearbox through oil and uses planetary gear sets. A DSG has two mechanical clutches and two input shafts, and preselects the next gear. You drive them the same way, but inside they are different constructions with different wear patterns.

Should I put my automatic in neutral at traffic lights?

For a short stop, no. An automatic is built to stand in D with your foot on the brake, and shifting back and forth all the time adds wear. For a long wait, N or P with the handbrake applied is fine. Never coast in N to save fuel.

Can I switch from D to N while driving?

Yes, selecting N while moving does not damage the gearbox, for example if the car needs to be pushed or the engine stalls. Do not use it to coast, though, because you lose engine braking. Never select R or P while the car is still rolling.

What kills automatic transmissions?

Heat and neglected fluid do the most damage. Old, low or wrong fluid, regular towing, a lot of stop-start driving, shifting between D and R while still rolling and driving on with an early symptom all shorten the life of an automatic. Most failures we see started as a small complaint that was left too long.

Do you need to change the fluid in an automatic gearbox?

ATF is not just a lubricant but also the medium that transmits power and delivers the shift pressure, so it ages with use. Whether your car has a set interval and which fluid it needs is in its service schedule and differs per gearbox. With heavy use, such as towing or lots of town driving, fresh fluid matters even more.

How much does it cost to repair an automatic gearbox?

That depends on the type of gearbox, the cause and what is damaged inside. A solenoid or valve body repair is a very different job from a complete rebuild with a new torque converter. That is why ATR only gives a price after diagnosis. Contact us for a price indication based on your registration number and complaint.

Questions about your automatic?

An automatic gearbox is a hydraulic machine controlled by electronics: a torque converter or clutch passes on the power, planetary gears or shafts provide the ratios, and oil pressure does the shifting. Once you know that, most odd behaviour starts to make sense, and so does the value of clean fluid and calm habits.

Not sure which type of automatic your car has, or does the gearbox react differently from what you are used to? As a gearbox specialist we work on all four types every day. What a repair or rebuild costs depends on the type of gearbox, the cause and what is damaged inside. How such a price is built up is explained on gearbox rebuild costs. We only give a guide price for your car once we know what we are looking at.

Book your car in at ATR Transmissies in Montfoort, even if you only want to know whether that little jolt is serious.