How an Automatic Transmission Works (FWD)

Here's how a front wheel drive automatic transmission works in your passenger automobile.

The automatic transmission is responsible for providing a selection of gear ratios between the engine and drive wheels of a car. The transmission uses the principle of planetary gears, which are always meshed together to vary gear ratios. This results in speed reductions and torque multiplication.

Most gasoline engines are tuned to generate more power as the RPM increases. Thus a low gear ratio, which multiplies torque, helps a vehicle to take off from a stop. Once inertia is achieved and the vehicle is coasting, the transmission will engage the next gear, resulting in a speed increase.

Increasing the speed output also decreases the amount of fuel an engine will consume, since it will be rotating at a slower RPM for a given output. Newer transmissions may have 6, 7, 8, 9 and upto 10 speed ratios to choose from! The downside to having too many gears is that the tranny will tend to hunt or get confused as to which is the optimal gear to be in at a given throttle input.

In this video an automatic transmission from a Toyota Corolla is torn down to the basics to show it works. The main components include the input shaft, planetary gearset, one way clutch, clutch discs and bands, valve body, case, countershaft, differential, oil pump and torque converter. This transmission is the 4 speed automatic transmission (A245E) used in Corollas from 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008.

A full writeup and discussion of this transmission breakdown can be found here:

More information on how the torque converter can be found in my separate video here:

The removal of the engine and transmission:

Get your supplies and tools to work on your transmission here:
Transmission Fluid:
Impact gun:

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[ronin-dpls]

Finally a person who actually explains how the clutchplates are triggered

[mancard]

Much respect to all the transmission mechanics out there!

[treyblacksmith]

I’m an engine guy and I was honestly confused on the ins and outs of automatic transmissions but you cleared it up. Your a very smart individual and a very good instructor

[CrazyCandyCrush]

This is so complicated, I think I better switch back to using a horse.

[Zetapeep]

Ya gotta wonder how the hell someone came up with this design.
It's so vast.

[bartwaggoner]

Not only is the design complex, but imagine the metallurgy and manufacturing involved!

[johnkubik]

Its just amazing considering that most of this design as been achieved by mechanical engineers in the 50's/60's without any help from CAD/CAM programs, not even a pocket calculator.

[bartwaggoner]

SO COMPLEX - the people that developed this were unsung geniuses

[protoslashwyl]

what an amazing piece of engineering. It's so complicated, yet it needs to operate reliably over the life of the car. It amazes me just thinking the amount of design that goes into this. Thanks for the video

[louisedwards]

I am an Automatic Transmission professional and You are a very good fast pace teacher, , , , when people realize how complicated these things are Thay don't complain about the cost !!!

[trishanuray]

Precise, to-the-point and practical. No one explains like you in all of YouTube.
Your brother must be very lucky.

[PhunkBustA]

i love how this makes more sense that all those (probably) expensive animations and narrations

[User_fr]

I've watched over 5 videos on how automatics work and still confused

[AliShaarawi]

I will never look at my car the same way again. That was amazing bro, good job!

[MoosesValley]

Very well explained. Automatic transmissions can almost be considered to be a fluid filled mechanical computer - but instead of processing input data and generating output data, they are processing inputs (acceleration requirements, etc) and producing outputs (increased speed via smoothly changing gears). Great video. Thanks !

[gbudning]

Wow. After years of wrenching I finally understand how an automatic transmission actually decides on and selects gears. I'll be showing this to my 8th grade class! Your explanations are so clear and concise while managing to account for every key detail. Thank you!

[petar]

Your channel is like college of automotive engineering...One more year of watching your videos and i'll be ready to open my own auto service :D

[mrflamewars]

The best way to describe the valve body is that its an analog, hydraulically operated computer of sorts.

[billgateskilledmyuncle]

I have pretty much understood all of this for a few years, but the one thing that had me scratching my head is how in the world the clutch packs physically get pushed together. Finally your video showed the round ring that puts pressure on them so they will engage. Finally!

[claudionieto]

Much respect to all the transmission designer engineers. They must be a genious to create something like this. And every new model has more devices in a very small piece, I remember the first FWD vehicles from GM in 1982 (Citation, Celebrity) their automatic transmission used to be very large and heavy and now, if you compare those with a 2020 automatic FWD car is less than the half part of those automatic transmissions and besides that the new ones have more speeds than the first ones (6 or 8 speeds versus 3 or four in the 80´2). It is amazing, the engineers are really a God mental health persons.