What is an automatic transmission and how does it work?

Until the 1980s, most passenger cars had manual transmissions which required you to perform gear changes yourself using the clutch and gearstick. Automatic transmissions, originally invented in 1921, took over that job, sensing the most appropriate gear for the driving conditions and performing the gear change with no input from the driver.

Automatic transmissions are made up of three main sections.

The torque convertor performs the same function as the clutch in a manual transmission but does it differently. It uses fluid to channel power to the transmission, prevent the engine from stalling, and allow the transmission to shift gears. The fluid dynamics not only make the car go forward, but they also absorb torque when the car is stationary. That’s why you can keep an automatic car in drive while you’re stopped at a red light, and why the car will start to creep forward when you release the brake.

Planetary gears enable the car to operate comfortably at different speeds, taking the power from the torque convertor and turning it into the right amount of torque and speed for the driving situation. These gears are so named because they look like planets orbiting a star - the central gear is called the sun gear. An automatic transmission generally has two planetary gearsets that work together as a single component, creating all the gear ratios that the car needs.

Brake bands and clutches create the different gear ratios in the planetary gearset. The bands tighten to hold a gear still or loosen to allow it to spin, making the specific combination of stationary and spinning gears that produces the desired gear ratio. The clutches perform a similar function, working together with the bands as the car shifts gears. These components are controlled by hydraulics and electronics.

To put all of this together, when you put the car into drive, the system activates and power flows from the engine into the torque convertor. If you press the accelerator, the power increases, propelling you forward. The hydraulics and electronics in your transmission sense the car’s speed in relation to the engine’s speed and adjust the gears to the most appropriate ratio. You can start, stop, speed up, slow down, climb hills and turn corners without having to worry about stalling the engine or not having enough power – the automatic transmission does it all for you.

Of course, not all automatic transmissions are the same. Commonly autos will fall into one of four main categories: Automated Manual Transmission (AMT), Continuously Variable Transmission (CVT), Dual Clutch Transmission (DCT), and Fully Automatic Transmission. They all have different components and operate slightly differently to each other, but all perform the same function of allowing you to drive without having to change gears manually.

Regardless of which kind of automatic transmission you have in your car, we can help you keep it in top shape. We’ve got over 40 years of experience fixing and servicing automatic transmissions and have industry-leading diagnostic equipment and highly experienced staff. Get in touch if you need help with your automatic transmission.