How Does An Automatic Gearbox Work?


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Taylor Edgar answered
Until electronic wizardry came along, an automatic gearbox, also known as automatic transmission, was controlled mechanically, gear selection being worked out from road speed, gear lever position and the throttle setting.

Hydraulic servos would then engage the required gear and lock the torque converter when a given road speed was reached. And this was all done mechanically, with nothing more sophisticated than an electrical switch to prevent the engine starting unless it was in P or N and to work the reversing lights.

Today, though, mechanical governors and kick down cables are no more, it is electronics that control the show. Thanks to the computing power of these new components, automatic transmissions are far more efficient and less prone to wear. Electronic sensors now compute the road speed down to several decimal places and allow far more variables to be taken into account when setting the gears. Among the measurements taken now by these sensors are such things as engine load, temperature and even the altitude to allow adjustments for power losses due to the thinner air at higher altitudes.

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