How does an automatic know when to change gears?

many ways. the older ones used shift valves, basicly comparing engine speed and load to throttle. the newer ones do somewhat the same but with more inputs. the computer can control most of the transmission activity if not all, but it looks at the same information that the older ones did, speed and load. if you ever get into a transmission nook you will hear a few different terms in regards to pressure. line pressure, what the pump is developing. governer pressure, this is what the transmission uses to sense speed. even a two speed transmission can get complex quickly. you should go a visit how stuff works and spend some time in the transmission area. you might be surprised.How does an automatic know when to change gears?
uses a cheat sheetHow does an automatic know when to change gears?
Some use vacuum sensor

Some use RPM

SOme are electronic

Take your pick
by computer on newer cars and on older cars it is controlled by vaccum from the engine.

how to deal with guys

pc automatically shuts down