page_title: Automatic transmission
section_title: Automatic transmission modes
hierarchical_section_title: Automatic transmission / Automatic transmission modes
caption_attribution_description: English: Paddle shifter (upshift paddle) being actuated by the driver in a 2013 Honda Accord EX coupe with a continuously variable transmission.
context_page_description: An automatic transmission, also called auto, self-shifting transmission, n-speed automatic, or AT, is a type of motor vehicle transmission that automatically changes the gear ratio as the vehicle moves, meaning that the driver does not have to shift the gears manually. Like other transmission systems on vehicles, it allows an internal combustion engine, best suited to run at a relatively high rotational speed, to provide a range of speed and torque outputs necessary for vehicular travel. The number of forward gear ratios is often expressed for manual transmissions as well.
  The most popular form found in automobiles is the hydraulic planetary automatic transmission. Similar but larger devices are also used for heavy-duty commercial and industrial vehicles and equipment. This system uses a fluid coupling in place of friction clutch, and accomplishes gear changes by hydraulically locking and unlocking a system of planetary gears. These systems have a defined set of gear ranges, often with a parking pawl that locks the output shaft of the transmission to keep the vehicle from rolling either forward or backward.
context_section_description: Conventionally, in order to select the transmission operating mode, the driver moves a selection lever located either on the steering column or on the floor (as with a manual on the floor, except that automatic selectors on the floor do not move in the same type of pattern as manual levers do). In order to select modes, or to manually select specific gear ratios, the driver must push a button in (called the shift-lock button) or pull the handle (only on column-mounted shifters) out. Some vehicles position selector buttons for each mode on the cockpit instead, freeing up space on the central console. It follows the classic PRND gate
  Vehicles conforming to US Government standards must have the modes ordered P-R-N-D-L (left to right, top to bottom, or clockwise). Previously, quadrant-selected automatic transmissions often used a P-N-D-L-R layout, or similar. Such a pattern led to a number of deaths and injuries owing to driver error causing unintentional gear selection, as well as the danger of having a selector (when worn) jump into reverse from low gear during engine braking maneuvers.
  Depending on the model and make of the transmission, these controls can take several forms. However, most include the following:
  Park (P)
  This selection mechanically locks the output shaft of the transmission, restricting the vehicle from moving in any direction. A parking pawl prevents the transmission from rotating, and therefore the vehicle from moving. However, the vehicle's non-driven wheels are still free to rotate, and the driven wheels may still rotate individually (because of the differential). For this reason, it is recommended to use the hand brake (parking brake) because this actually locks (in most cases) the wheels and prevents them from moving. It is typical of front-wheel-drive vehicles for the parking brake to lock the rear (non-driving) wheels, so the use of both the parking brake and the transmission park lock provides the greatest security against unintended movement on slopes. This also increases the life of the transmission and the park pin mechanism, because parking on an incline with the transmission in park without the parking brake engaged will cause undue stress on the parking pin, and may even prevent the pin from releasing. A hand brake should also prevent the car from moving if a worn selector accidentally drops into reverse gear while idling.
  A car should be allowed to come to a complete stop before setting the transmission into park to prevent damage. Usually, Park (P) is one of only two selections in which the car's engine can be started, the other being Neutral (N). This is typically achieved via a normally open inhibitor switch (sometimes called a "neutral safety switch") wired in series with the starter motor engagement circuit, which is closed when P or N is selected, completing the circuit (when the key is turned to the start position). In many modern cars and trucks, the driver must have the foot brake applied before the transmission can be taken out of park. The Park position is omitted on buses/coaches (and some road tractors) with automatic transmission (on which a parking pawl is not practical), which must instead be placed in neutral with the air-operated parking brakes set.
  Reverse (R)
  This engages reverse gear within the transmission, permitting the vehicle to be driven backward, and operates a switch to turn on the white backup lights for improved visibility (the switch may also activate a beeper on delivery trucks or other large vehicles to audibly warn other drivers and nearby pedestrians of the driver's reverse movement). To select reverse in most transmissions, the driver must come to a complete stop, depress the shift-lock button (or move the shift lever toward the driver in a column shifter, or move the shifter sideways along a notched channel in a console shifter) and select reverse. The driver should avoid engaging reverse while the vehicle is moving forwards, and likewise avoid engaging any forward gear while traveling backward. On transmissions with a torque converter, doing so at ver
caption: One type of manumatic shifting system available on automatic transmissions are paddle shifters.  The paddle depicted here is the upshift paddle in a 2013 Honda Accord, with the driver's hand on it.  Manumatic and paddle shifters may control any type of automatic transmission, including the continuously variable transmission in the Accord.
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