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what's in a transmission?
• A modern automatic transmission is an assembly of electronic, hydraulic, and mechanical components that come together to change the output speed of the drive train. Its primary components include:
• Pump
Torque converter
Planetary gears
• Clutches
• Bands
• Servos
• Valve body
• Solenoids

what is Torque converter?
A Torque converter or "stall converter" is a fluid coupler that allows the engine to turn while the transmission is stopped. If you picture two fans placed face to face and you turn one on, it can spin while you hold the free fan but if you let go of the free fan it will start to spin faster and faster because of the air moving across it. A torque converter operates in a similar way but with fluid instead of air. Modern torque converters also contain a clutch to lock the two "fans" together, this locking of the torque converter results in better fuel efficiency during highway driving. When a torque converter fails it begins to shed friction material from this clutch. This material clogs the filter and can result in overheating other components in the transmission.

What are planetary gears?
A planetary gear set is the component that changes the output speed of the drive train, transmissions can contain as many as 4.
They contain 3 gears, an outer large ring gear, 3 to 6 small planet gears and a central sun gear. They work by holding one of the three and turning one of the three the remaining gear will rotate at a different rate than the drive gear.
The difference in circumference of each gear results in a different ratio of rotation between any two gears in the set. As illustrated in the lower image.
When these gear sets fail they break teeth off of one or more gears resulting in damage to the entire set and sometimes other components in the unit.

What are Clutches and Bands?
Clutches and bands are used to change which gear in a planetary gear set is being held to the case or driven by the engine.

Clutches are arranged in stacks of alternating steel plates and plates coated with a friction material similar to brake pads. When theses plates are stacked in a drum together they are known as a "clutch pack" these plates have teeth on there inside or outside edge that engage other components or the case. When they are applied hydraulic pressure pushes on a piston that squeezes the plates together locking the the inner teeth to the outer teeth and holding the two components together.

Bands are used in a similar manner to hold drums to the case, they are made of a flexible steel coated in the same kind of friction material. when they are applied, hydraulic pressure forces a servo piston pin against one end of the band while the other is anchored to the case this force squeezes the band around the drum locking it to the case.

What is a valve body and what are solenoids?
Valve bodies are the brain of the transmission, they are made from aluminum castings that contain valves and form the pathways for pressurized transmission fluid to travel to different components. The channels in a valve body are like "U" shaped pipes with the tops sliced off. When you bolt it to the case the plate in between becomes the other half of the pipe. If the casting is warped or damaged these "pipes" can leak pressure into places it should not be or jam valves damaging components. We check all valve bodies for flatness and surface them if necessary. the valves in a valve body are known as spool valves and can be driven by hydraulic force, spring force or with the use of a solenoid. Solenoids are valves that are moved electronically by the computer using an electromagnet.

For more information visit the Automatic Transmission Wikkipedia page