Quick guide to the modern Automatic Transmission.
A manual gearbox is a simple and straight forward affair; each gear has its own cog in the box, a lever shifts them, power ensues. Automatic transmissions are a little more complicated. Rather than specific cogs for each ratio, they use planetary gears much like you find in a winch, with one feeding into the next. By using a series of brakes and clutches, the transmission is able to control what gears are engaged and what the final ratio will be.
The clutch has a complicated alternative too. Up front, a torque converter acts like two fans pointing at each other; although, they pump fluid not air. It lets the engine-side fan spin harder and harder until the transmission-side fan starts turning, which is what provides torque multiplication and what stops the 4WD from stalling when you come to a stop.
At certain speeds the transmission can completely lock up the torque converter, giving you direct drive through the transmission.
What’s all this got to do with programming?
In the not so long ago “olden days” the automatic transmission controlled all these different operations by pumping transmission fluid through complicated valve bodies that would engage the different clutch packs, giving different gear ratios.
This complicated system has now been replaced with simpler solenoids, all controlled by the “Transmission Computer”.
The Transmission Computer relies on a whole bunch of inputs, with everything from the throttle position and road speed, temperature and gear selection etc, all being considered before it will kick up or down a gear. The result is a more intelligent transmission that does what you want, when you want it, and one that can be easily modified without even dropping the fluid.