The automatic gearbox: torque converter, dual clutch and CVT
Today almost half of new cars are automatics, but inside they have nothing in common. Here are the three types, what each selector position really does and what tends to fail in each one.
Three ways to change gear by itself
The gearbox is bolted to the engine, in violet in illustration 1, but inside there are three different worlds.
The traditional automatic, the torque converter type, joins engine and gearbox through oil: one turbine drives another without touching, like a fan blowing on another fan. That is why it pulls away so smoothly. Inside, planetary gears and oil-bathed clutches combine the gears, six to ten of them today. Above a certain speed, a clutch locks the converter so no energy is lost.
The dual clutch is really two manual gearboxes in one: one clutch for the odd gears and another for the even ones. While you are in third, fourth is already engaged and waiting, and the change is almost instant. Some have dry clutches, which suffer in traffic jams, and some have oil-bathed ones, which are tougher.
The continuously variable transmission, the CVT, has no gears: two pulleys that open and close their groove with a steel belt or chain between them, so the ratio changes without steps. Very smooth and efficient, with that sound of an engine that revs up and stays there. Note that Toyota’s hybrid gearbox is called an e-CVT but is something else: a planetary gear with two electric motors.
The selector
P locks the gearbox output with a pawl; it is not a brake: on a slope, apply the handbrake first and then P, so the car does not end up hanging on the pawl. R is reverse, N leaves the box free and D changes by itself. Many also have a manual or sport position. On modern cars the selector no longer moves anything mechanical: it sends an electric signal, which is why today they are buttons, dials or small levers.
The most common faults
- Old oil
Harsh shifts, thumps when selecting D or R, jerks. The "lifetime" oil idea falls short: many manufacturers recommend changing it between 60,000 and 100,000 kilometres (roughly 40,000 to 60,000 miles), or sooner with hard use.
Typical codes: P0700, P0868 - Worn dual clutch
Judder or vibration when pulling away, especially with dry clutches and lots of town driving.
- Gearbox control unit
The mechatronic unit of dual clutch boxes, which combines electronics and hydraulics, is a well-known fault: the gearbox goes into limp mode and stays in one gear.
Typical codes: P0700, P0730 - Torque converter
Shudder at constant speed, between 40 and 55 mph, when the converter locks up.
Typical code: P0741 - Speed or temperature sensors
Odd shifts or limp mode because a sensor is reporting wrongly.
Typical codes: P0715, P0720, P0711, P0218
What you can see with Final OBD
With Final OBD you read the faults from the gearbox control unit, not just the engine, each one explained, which is the first thing to check when an automatic starts behaving oddly. And on cars that provide it, its data, such as the gear it is in.
Open the bonnetthe in-depth explanation
Inside the torque converter
It has three bladed wheels in an oil-filled housing: the pump, joined to the engine; the turbine, joined to the gearbox; and between them, the stator. At low revs the pump throws oil at the turbine and the stator redirects it so it comes back pushing: that way the converter multiplies the force when pulling away, up to almost double. When the two spin at nearly the same speed, it stops multiplying and a lock-up clutch joins them so no energy is lost in the oil. That lock-up, if its clutch wears or the oil is old, is what gives the typical shudder at constant speed.
Gears without a lever
A planetary gear is a central sun gear, planet gears around it and an outer ring gear. By braking or releasing each of those three parts with clutches and band brakes, the same gear set gives different ratios. Combining two or three planetary sets gives all the gears. The gearbox control unit decides which clutches to apply, at what pressure and when, and it learns: it corrects for wear so shifts stay smooth. That is why, after an oil change or a repair, it sometimes has to be left to relearn.
Inside the dual clutch
Two input shafts, one inside the other, each with its own clutch. The mechatronic unit, a block with the control unit and solenoids, preselects the next gear and crosses the clutches: one releases while the other engages. It is fast and efficient, but demanding: in traffic, pulling away from a stop is done by slipping the clutch, and with dry clutches that heats and wears them. On a slope, holding the car on the throttle instead of the brake is the worst thing you can do to it.
Inside the CVT
Each pulley is two cones that move together or apart with oil pressure. When the input one closes and the output one opens, the belt runs on a large diameter at the front and a small one at the back: a tall ratio. The other way round, a short ratio. The belt is a stack of steel pieces that push rather than pull, and it all depends on the oil: a CVT is very sensitive to having exactly the oil the manufacturer specifies, changed on time.
How to look after an automatic
Oil on time and the right one. P only with the car completely stopped. Do not go from D to R while the car is still moving. On dual clutches, the brake on hills and no holding the car on the throttle. Warm it up for a few minutes before asking much of it, because gearbox oil takes longer than engine oil to reach temperature. And if an automatic starts thumping, read its faults first: very often it is a sensor or the oil, not the whole gearbox.
All sections of the mechanics guide
See it in your own car
Almost everything in this guide can be seen with Final OBD and an adapter plugged in: faults explained, live data and the details of each system.
