Under Pressure

Welcome back to our blog! This week we are going to jump right into the rather complex and winding part of the automatic transmission– the Hydraulic System. But before that, like always, time for a quick recap & review some definitions.


Previously on Anthony’s Automotive Blog….


Last week, we learned all about the One Way Clutch (also known as the Sprag Clutch) and how it will allow movement in one direction but not in the opposite direction. You can do a re-read of it here.

For ease of understanding, the One Way Clutch works very similar to how a bicycle works in that when you move the pedal in a forward motion, it moves the bike wheels. But, on most bicycles, if you try to move it backwards, the wheels won’t move at all. In fact, engaging the One Way Clutch puts you in a “coasting” mode – the engine is still running but no gears are activated and the wheels are kind of moving from the energy already generated from the engine.


There are a couple of new words added to our vocabulary this week. You can find them at the bottom of the list. As always, you can find our entire and current automotive dictionary here. Feel free to save its link so that you can find it again in case we use a word that happens to not be on this week’s list.


Our Growing Automotive Vocabulary


Here are the related words for this week’s topic. Enjoy expanding your vocabulary.

Transmission – The mechanism by which power is transmitted from an engine to the wheels of a motor vehicle.

Torque Converter – A device that transmits or multiplies torque generated by an engine.

Gear – One of a set of toothed wheels that work together to alter the relation between the speed of a driving mechanism (such as the engine of a vehicle or the crank of a bicycle) and the speed of the driven parts (the wheels).

Automatic – A device or process that works by itself with little or no direct human control.

Automatic Transmission – An automotive transmission that can automatically change gears as the vehicle moves, freeing the driver from having to shift gears manually.

Clutch – A mechanism for connecting and disconnecting a vehicle engine from its transmission system.

Band – Something that binds, ties together, restrains, etc.; a strip or ring of wood, metal, rubber, etc. fastened around something to bind or tie it together.

Pump – Any of the various machines that force a liquid or gas into or through, or draw it out of, something, as by suction or pressure.

Sprag – A device for preventing a vehicle from rolling backward on a grade.

Radiator – A device of tubes and fins, as in an automobile, through which circulating water passes to radiate superfluous heat and thus cool the engine.

Tube – A hollow cylinder or pipe of metal, glass, rubber, tec., usually long in proportion to its diameter, used for conveying fluids, etc..

Fin – (this one is a little tricky) Anything like a fin in shape or use; specifically, any narrow edge or ridge formed in manufacturing, as on a casting by metal forced through the halves of the mold; and/or any fixed or movable airfoil whose chief function is to give stability in flight.

Hydraulic – Operated by the movement and force of liquid; specifically, operated by the pressure created when a liquid is forced through an aperture, tube, etc..


Can you feel the pressure….


The Hydraulic System in an automatic transmission is a complex labyrinth of passages and tubes. The whole purpose of this system is to send transmission fluid under pressure to all parts of the transmission and torque converter.  I have included three different examples of what a Hydraulic System can look like. You can try this yourself by searching for “automatic transmission hydraulic system” and see all the different diagrams that you can find. Long story short – there is a lot to choose from and they are all beyond complex.



Transmission fluid serves many purposes in this system. It is involved in the shift control process. It is used for general lubrication of the system. And part of it connects up to the radiator so that means it assists with keeping your transmission cool.  All of these aspects of a transmission’s functions are dependent on a constant supply of the transmission fluid under pressure.  You can think of the transmission fluid as being the blood of your car. In fact, it’s even red in color.


And just like your body, it is vastly important that this fluid is always under pressure. Any lack of pressure at any time can be harmful or even fatal to the life of your transmission. In fact, most of the problems that come from your transmission has to do with a lack of fluid or a lack of pressure. It simply does not work right without either.


And exactly how much fluid are we talking about here? Your typical transmission has an average of 10 quarts of transmission fluid that runs between the transmission, the torque converter and the cooler tank. That is 2.5 gallons of fluid! In fact, most of the components of the transmission are constantly submerged in this fluid. And this includes the clutch packs and bands. In fact, the friction surfaces of these parts are designed to operate properly only when they are submerged.

A Gallon Jug

As I mentioned earlier about keeping your transmission cool, a portion of the transmission fluid is sent through one of two steel tubes to a special chamber that is submerged in the combination of antifreeze and water that is in your radiator. The fluid that passes through this section is cooled and then returned to the transmission through the other steel tube.


For Next Week

Let’s continue examining the Hydraulic System of the Automatic Transmission and find out how the Oil Pump factors into this system.



Until Next Time….


Anthony’s Automotive


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