Valve Body

 The valve body is the control center of the automatic transmission.  It contains a maze of channels and passages that direct hydraulic fluid to the numerous valves which then activate the appropriate clutch pack or band servo to smoothly shift to the appropriate gear for each driving situation.  Each of the many valves in the valve body has a specific purpose and is named for that function. For example the 2-3 shift valve activates the 2nd gear to 3rd gear up-shift or the 3-2 shift timing valve which determines when a downshift should occur.

The most important valve, and the one that you have direct control over is the manual valve.  The manual valve is directly connected to the gear shift handle and covers and uncovers various passages depending on what position the gear shift is placed in.  When you place the gear shift in Drive, for instance, the manual valve directs fluid to the clutch pack(s) that activates 1st gear. it also sets up to monitor vehicle speed and throttle position so that it can determine the optimal time and the force for the 1 - 2 shift.  On computer controlled transmissions, you will also have electrical solenoids that are mounted in the valve body  to direct fluid to the appropriate clutch packs or bands under computer control to more precisely control shift points.

Computer Controls

The computer uses sensors on the engine and transmission to detect such things as throttle position, vehicle speed, engine speed, engine load, stop light switch position, etc. to control exact shift points as well as how soft or firm the shift should be.  Some computerized transmissions even learn your driving style and constantly adapt to it so that every shift is timed precisely when you would need it.

Because of computer controls, sports models are coming out with the ability to take manual control of the transmission as though it were a stick shift, allowing the driver to select gears manually.  This is accomplished on some cars by passing the shift lever through a special gate, then tapping it in one direction or the other in order to up-shift or down-shift at will.  The computer monitors this activity to make sure that the driver does not select a gear that could over speed the engine and damage it.

Another advantage to these "smart" transmissions is that they have a self diagnostic mode which can detect a problem early on and warn you with an indicator light on the dash.  A technician can then plug test equipment in and retrieve a list of trouble codes that will help pinpoint where the problem is.

Governor,  Vacuum Modulator,  Throttle Cable

These three components are important in the non-computerized transmissions. They provide the inputs that tell the transmission when to shift.  The Governor is connected to the output shaft and regulates hydraulic pressure based on vehicle speed. It accomplishes this using centrifugal force to spin a pair of hinged weights against pull-back springs.  As the weights pull further out against the springs, more oil pressure is allowed past the governor to act on the shift valves that are in the valve body which then signal the appropriate shifts.

Of course, vehicle speed is not the only thing that controls when a transmission should shift, the load that the engine is under is also important.  The more load you place on the engine, the longer the transmission will hold a gear before shifting to the next one.

There are two types of devices that serve the purpose of monitoring the engine load: the Throttle Cable and the Vacuum Modulator.  A transmission will use one or the other but generally not both of these devices.  Each works in a different way to monitor engine load. 
The Throttle Cable simply monitors the position of the gas pedal through a cable that runs from the gas pedal to the throttle valve in the valve body.  
The Vacuum Modulator monitors engine vacuum by a rubber vacuum hose which is connected to the engine.  Engine vacuum reacts very accurately to engine load with high vacuum produced when the engine is under light load and diminishing down to zero vacuum when the engine is under a heavy load.  The modulator is attached to the outside of the transmission case and has a shaft which passes through the case and attaches to the throttle valve in the valve body.  When an engine is under a light load or no load, high vacuum acts on the modulator which moves the throttle valve in one direction to allow the transmission to shift early and soft.  As the engine load increases, vacuum is diminished which moves the valve in the other direction causing the transmission to shift later and more firmly.

Seals and Gaskets

An automatic transmission has many seals and gaskets to control the flow of hydraulic fluid and to keep it from leaking out.  There are two main external seals: the front seal and the rear seal. The front seal seals the point where the torque converter mounts to the transmission case. This seal allows fluid to freely move from the converter to the transmission but keeps the fluid from leaking out.  The rear seal keeps fluid from leaking past the output shaft.

A seal is usually made of rubber (similar to the rubber in a windshield wiper blade) and is used to keep oil from leaking past a moving part such as a spinning shaft. In some cases, the rubber is assisted by a spring that holds the rubber in close contact with the spinning shaft. 

A gasket is a type of seal used to seal two stationary parts that are fastened together. Some common gasket materials are: paper, cork, rubber, silicone and soft metal.

Aside from the main seals, there are also a number of other seals and gaskets that vary from transmission to transmission. A common example is the rubber O-ring that seals the shaft for the shift control lever.  This is the shaft that you move when you manipulate the gear shifter.  Another example that is common to most transmissions is the oil pan gasket.  In fact, seals are required anywhere that a device needs to pass through the transmission case with each one being a potential source for leaks.