How Does An Automatic Transmission Know When to Change Gears?

Posted by on Thursday, January 15th, 2015

An automatic transmission is an integral part of your car. Image source: Wikimedia Commons

Ever wonder how an automatic transmission “knows” when to change gears? Scroll to learn more.
Image source: Wikimedia Commons

By this point in your automotive career, you’ve probably figured out how the engine sends power to the transmission. But you still may be wondering:  how does an automatic gear know when to change gears? That’s a good question, and one that we’ll be discussing below.

The Answer: Planetary Gear Sets

While every component of a transmission plays a role in helping it figure out when to change gears, perhaps none of them are more important than the planetary gear sets. The planetary gear sets, or as they are also known, “epi-cyclic gear sets”, are composed of the following parts:

  • A sun gear
  • Planet gears
  • Planet carrier
  • Ring gear

Basically, the sun gear sits in the center, while the planet gears rotate around it (similar to how the “planets” rotate around the sun in space). The planet carrier is what’s responsible for connecting the planet gears, while the ring gear sites on the outside and associates with the planet gears.

The basic premise is simple: when accelerating or breaking, you essentially prevent various components of the planetary gear sets from moving. When this happens, you change the output and input of the system, which changes the vehicle’s overall gear ratio. It is in this way, partly, that the automatic gearbox “knows” when to change gears. The RE5R05A Nissan 5-Speed Transmission is a perfect example of an automatic transmission that not only “knows” when to change gears, but do so in a manner that is efficient for the vehicle.

A good way to picture it would be like this: a planetary gear set will allow you to experience different gear ratios without having to engage each individual one. That’s because all of them are already engaged. All that you need to do is use your gas pedal or brakes to determine which ones remain stationary and which ones rotate.  The final gear ratio will be directly correlated with which one of the components is “fixed”.

For example, if a ring gear is fixed then the gear ratio is going to be much shorter than if the sun gear were fixed. Quite obviously, with today’s technology there’s no doubt that computer’s play a role in how automatic transmissions change gears. But if you’re looking for the bare bone, fundamental theory for how it all works then the previous paragraphs explain it quite clearly.

Need To Have Your Planetary Gear Sets Repaired?

Planetary gear sets can be repaired for as little as a few hundred dollars and about 10 hours in labor (assuming that you do it yourself). If you hire an auto shop, you should expect to pay much more. Either way, now you have a clear and thorough understanding for how automatic transmissions “know” when to shift from gear to gear while moving.

If you’re looking for a quality remanufactured transmission, then you won’t do better than ETE REMAN. That’s why we offer a three-year/unlimited mileage warranty on all our products. Browse our online catalog and contact us today.

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