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How an Automatic Transmission Works

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Old 10-24-17, 05:37 AM   #1
speedkar9
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Default How an Automatic Transmission Works

Hereís the basics of how an automatic transmission works in your car.


Hereís a few photos of the tear down I did on my Toyota Corollaís 4 speed automatic tranny:

For more details on how I removed the engine and tranny on the Corolla, see my engine removal video here:
The major components of an automatic transmission are the case, planetary gear set, clutch packs, valve body, differential, oil pump and externally, the torque converter.

Pan removed:


With the valve body removed, we can see the guts, including the 2nd gear clutch bands and some clutch packs. Along the side are accumulator pistons.

With the bell housing opened in half, weíve got full sight of the input shaft, which carries two planetary gearsets, the counter shaft which has one planetary gearset, and the differential that the axles plug into.

Whatís interesting is this is the parking pawl that holds your car from rolling when you park up. Even more reason to use the parking brake.


A closer look at the input shaft. The torque converter powers this. Its hollow and has oil feeds for the torque converter and oil pump.

For more info on how the torque converter works, see my video here:

The guts of the tranny is its planetary gear sets. It achieves different gear ratios by holding one of the three components (planet carrier, ring gear or sun gear) steady and using the other two as inputs and outputs. The selection of these are controlled by the clutches.

The final drive of the tranny is the differential. This splits power left and right between the axles on a FWD car. The fluid is shared within the transmission.

The valve body is the brain of the transmission. It is responsible for directing fluid through the accumulator pistons and the clutches to change gears.

The tranny oil pump sits at the front of the transmission and is powered by the torque converter. It provides fluid flow throughout the tranny so valves and passages can create pressure to make things work.

And thatís pretty much all the major components that go into a transmission. Itís quite complex and thereís a lot of thought and engineering put into making these things work and last a long time. I appreciate the work of these powertrain engineers!

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Old 10-24-17, 10:12 AM   #2
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Thank you for sharing those sir. Really interesting and helpful!
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Old 10-24-17, 07:20 PM   #3
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As usual, good illustrations, speedkar.

I'll just add one small detail. On modern FWD vehicles, that's where the term Transaxle comes from......the automatic transmission and the differential/final-drive unit being contained in one common housing (usually aluminum) and sharing the same lubricating-fluid. A few rear-engined vehicles, of course, such as Porsches, have rear-mounted transaxles, but most of them are up front.

Last edited by mmarshall; 10-24-17 at 07:27 PM.
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Old 10-24-17, 08:23 PM   #4
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Originally Posted by mk416 View Post
Thank you for sharing those sir. Really interesting and helpful!
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As usual, good illustrations, speedkar.

I'll just add one small detail. On modern FWD vehicles, that's where the term Transaxle comes from......the automatic transmission and the differential/final-drive unit being contained in one common housing (usually aluminum) and sharing the same lubricating-fluid. A few rear-engined vehicles, of course, such as Porsches, have rear-mounted transaxles, but most of them are up front.

Thank you guys!
And yes I agree, transaxle is when they're integrated together, I forgot to mention that.
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Old 10-24-17, 11:40 PM   #5
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Now I can see why the Uhaul truck I rented years ago would roll away if you put it in park(and didn't set the parking brake). It was a pretty damn big GMC or Chevy medium duty truck, like the biggest one you could get from them. 90's model, gas truck, probably a 454 Chevy with the 4L80e if I had to guess, hydraulic brakes(air brakes require a CDL).

That parking pawl in the transmission I'm sure was toast, broke a long time ago from careless people parking 30,000lbs worth of truck and cargo on a hill without using the parking brake.
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Old 10-25-17, 07:47 AM   #6
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Now I can see why the Uhaul truck I rented years ago would roll away if you put it in park(and didn't set the parking brake). It was a pretty damn big GMC or Chevy medium duty truck, like the biggest one you could get from them. 90's model, gas truck, probably a 454 Chevy with the 4L80e if I had to guess, hydraulic brakes(air brakes require a CDL).

That parking pawl in the transmission I'm sure was toast, broke a long time ago from careless people parking 30,000lbs worth of truck and cargo on a hill without using the parking brake.
Whenever parking on any significant grade, I always suggest that people first apply the parking brake, make sure the brake is securely attached (you can do that by gently easing your foot off the regular brake pedal and check for rolling), and then put it in Park. The Park feature is intended to be used only as an emergency back up....not to regularly substitute for the parking brake, or to hold the entire weight of the vehicle on hills. On GM's new 9-speed, it automatically goes into Park when you shut the engine off, whether you push the Park button or not.....so it's doubly-important to set the parking brake first. On level or nearly level surfaces, of course, it is much less important to set the brake first (or to set it at all), as the parking prowl will not receive much if any stress from gravity.


BTW.....if that was allowed to happen on a UHaul truck and not get detected, the company is probably not inspecting those trucks enough. As you note, there are enough bozos out there driving and parking them.

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Old 10-25-17, 11:55 AM   #7
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Great write-up, and to think as complex and well engineered as this is, this specific transmission is rather simple compared to other offerings on the market.
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Old 10-25-17, 10:36 PM   #8
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Whenever parking on any significant grade, I always suggest that people first apply the parking brake, make sure the brake is securely attached (you can do that by gently easing your foot off the regular brake pedal and check for rolling), and then put it in Park. The Park feature is intended to be used only as an emergency back up....not to regularly substitute for the parking brake, or to hold the entire weight of the vehicle on hills. On GM's new 9-speed, it automatically goes into Park when you shut the engine off, whether you push the Park button or not.....so it's doubly-important to set the parking brake first. On level or nearly level surfaces, of course, it is much less important to set the brake first (or to set it at all), as the parking prowl will not receive much if any stress from gravity.


BTW.....if that was allowed to happen on a UHaul truck and not get detected, the company is probably not inspecting those trucks enough. As you note, there are enough bozos out there driving and parking them.
U-haul rents junk equipment, old worn out trucks, or at least the ones in Nashville used to. After a couple bad experiences with U-haul in that job I had, company started renting from Penske. Rates were a little cheaper but the big plus was trucks were newer, nicer and always well maintained. You can't believe how infuriating it is having to unload a whole truck worth of sound equipment on the side of the interstate because the truck broke down.
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Old 10-26-17, 07:55 PM   #9
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Quote:
Originally Posted by mmarshall View Post
Whenever parking on any significant grade, I always suggest that people first apply the parking brake, make sure the brake is securely attached (you can do that by gently easing your foot off the regular brake pedal and check for rolling), and then put it in Park. The Park feature is intended to be used only as an emergency back up....not to regularly substitute for the parking brake, or to hold the entire weight of the vehicle on hills.
I feel bad for saying this but I'm guilty of not using the parking brake myself.

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Great write-up, and to think as complex and well engineered as this is, this specific transmission is rather simple compared to other offerings on the market.
Thank you, glad you enjoyed it. Your right, this is pretty much as basic as it gets, now a days transmissions are much more complex and computer controlled.
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Old 10-26-17, 08:11 PM   #10
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I feel bad for saying this but I'm guilty of not using the parking brake myself.
No problem, as long as you are on level or near-level ground. Just don't try it in a neighborhood like this LOL

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