How does an automatic gearbox work?

A simple guide to your automatic gearbox

Every driver knows that there’s a range of gears to choose from depending on the incline of the road and the speed you’re travelling.

There are 2 ways your car’s transmission will be controlled; it will either have a manual or an automatic gearbox.

In the UK, the majority of car gearboxes have been manual for years. It’s easy to understand why they’re called manual; you choose and select the gears yourself, whereas, with an automatic, the car does it for you.

But how does an automatic transmission know when to change the gears? And is it even the same type of gearbox?

In this article, we’re going to cover the basics. We’re going to answer such questions as: ‘How does a gearbox work?’, ‘How much is a new gearbox?’ and ‘How to change gearbox oil’, leaving you better informed and equipped to make the decisions about your car maintenance and repair.

The differences between manual and automatic gearboxes

The first thing you’ll notice about an automatic car is that it has no clutch pedal. The second is that there’s no gear stick; this is replaced with a simpler, similar control with 3 standard options: drive, park and reverse.

The torque converter replaces a manual gearbox’s clutch

The torque converter’s primary role is to provide connection of your engine’s power to the wheels being driven. Unlike a manual clutch with a connecting flywheel, a torque converter is a fluid coupling that operates by hydraulic pressure.

The torque converter has 3 main components:

  1. The impeller—bolted to a flywheel
  2. The turbine—connected to the gearbox input shaft
  3. The central reactor—connecting these two components

As the engine speeds up centrifugal forces acting on hydraulic fluid transmits torque to the turbine—this makes the turning effort.

At this point, the torque converter acts as a fluid flywheel to connect the engine to the gearbox.

Ok, and now in plain English…

When you turn on your engine, and it’s idling, the torque converter isn’t creating enough torque to power the turbine, so your car sits stationary.

When you press the accelerator, the torque converter spins faster, delivering more fluid and torque to the gearset. This will help set your car in motion.

However, the torque converter is a more complicated beast than we’ve got room to delve into here. You also need to understand that the way it delivers the hydraulic fluid through its various components and back to the pump, creates additional torque and further power. The continuation of this process accelerates your car’s speed to the desired point, and once there, releases torque to prevent it from going any faster.

But what happens to that power once it’s released from the torque converter?

How automatic transmission gearset works

Instead of having different sets of cogs to create each different gear ratio for your car’s drivetrain, an automatic transmission utilises 3 main gearing components:

  1. The sun gear
  2. The planet gears and the planet gears’ carrier
  3. The ring gear

The planetary gearset is the central player in the automatic gearbox system. This system of gears connects the central sun gear to the outer ring gear. There may be several gearsets working together to achieve the varying gear ratios.

Without getting too technical, it’s by locking or connecting these gears to the larger outer ring gear (using clutches and brakes) that creates the different ratios to expand, reduce or reverse the drivetrain power.

Because all of the gears are connected all of the time, it’s simply by locking the different planet gears on and off that create a similar type of action to the manual movement of engaging the many different cogs in a manual gearbox.

Brake bands and clutches

Your automatic transmission contains a wealth of brake bands and clutches.

The brake bands can tighten to hold either the ring or sun gear, to allow them to spin or keep them stationary. These are controlled, once again, by a hydraulic system.

The clutches connect and release the planetary gears, activating the gearing required. The control of these clutches is delivered by a combination of mechanical, hydraulic and electrical functions.

It’s a complicated and ingenious system and not the simplest thing about your car to describe or understand.

Hopefully, this simple explanation will help you understand how the automatic gears work a little better. If you’re not much wiser—never fear—the main thing you need to understand about an automatic gearbox is how to look after yours, and what it could cost you if it’s broken.

How much does a gearbox repair cost?

Repairing, rebuilding or changing your gearbox can cost around £2k to £3k for the typical family car.

Luxury vehicles and sports cars will cost you relatively more, often as much as £6k and £7k.

That’s why when it comes to finding the best mechanic for a gearbox replacement—you’re in the right place. How much does a gearbox cost to fix in the UK? Well, if you choose Fixter, you could be saving up to 30% compared to franchises and dealerships.

How do I know if my gearbox has gone?

If your gearbox has broken your car probably won’t move. If it’s damaged and in need of repair, the following are a few tell-tale signs that could help you spot trouble in advance.

Because your automatic gearbox features many hydraulic components, you may spot leaking fluids leaving pools of liquid under your car.

Your car’s drive could well feel clunky, and you could hear all sorts of noises as your vehicle is changing its gearing. You’ll notice the once smooth transition between gears has disappeared and may be replaced by a whining, buzzing or humming sound.

Taking the best care of your automatic gearbox

The best ways to protect the life of your gearbox are regular servicing and to make periodical checks of your transmission fluid.

How to check gearbox oil

Your car will have a dipstick to check your transmission fluid, the same way as you would your engine oil. This is generally at the rear of an in-line engine for rear-wheel-drive cars.

If your car is a front-wheel drive, the dipstick will be located at the transaxle near the front of the vehicle.

You should check the oil/fluid level while your engine is idling. It’s a good idea to move the gear selector through each of its positions 3 or 4 times and then set it back into park before allowing the engine to idle for 2 or 3 minutes.

If you need to know how to change the oil in your gearbox, you’ll need to remove both the sump pan and the dipstick tube, or for smaller cars, there should be a drain plug.

It’s quite a messy and challenging job, so should only be handled by the most confident and proficient home mechanic.

However, most modern cars shouldn’t need draining so you won’t find a drain plug even if you go looking for one.

Topping up low levels is done through the dipstick tube. You’ll need a small funnel to deliver the transmission fluid, and you should add only a little at a time until you reach the ideal midway level.