I think i heard, that they are hydraulic, but how does the gearbox “know” when to shift and how does it change gears?
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The oldest automatics used a simple design, a speed governor collecting wheel speed, and a throttle cable collecting gas pedal position.
They in turn move the valves in the gearbox valve body, finally the two variables work against each other moving the shift valve, higher speed push the valve towards higher gears, deeper gas pedal push it toward slower gears, same as you would normally decide on a manual gearbox.
The gear change is implemented as tightening/loosening various multi-plate clutches and break belts, connecting various parts (input/middle/output) of one or multiple planetary gearsets with various outcomes in terms of transmission ratio. Other mechanisms exist, like AMT/DCT that are almost exact like manuals actuated by hydraulic actuators, or CVT that is basically just two pulleys varying the clamping pressure pushing a transmission belt to desired positions, producing variable transmission ratio.
Now they are all electronic. Speed and gas pedal position sensors provide the input and the computer decides the best gear based on a schedule.
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