Conventionally, in order to select the automatic transmission operating 'mode', the driver moves a selection lever (gear shifter) located either on the steering column or on the floor. In order to select transmission modes, or to manually select specific transmission gear ratios, the driver must push a button in called the shift lock button or pull the handle only on column mounted transmission shifters out. Some vehicles position transmission selector buttons for each mode on the cockpit instead, freeing up space on the central console. Automobiles conforming to United States Government standards must have the transmission modes ordered P-R-N-D-L (left to right, top to bottom, or clockwise). Prior to this standard, quadrant-selected automatic transmissions often utilized a P-N-D-L-R layout, or similar. Such a pattern led to a number of deaths and injuries owing to unintentional transmission gear selection, as well as the danger of having a transmission selector jump into Reverse gear from Low gear during engine braking maneuvers.
Automatic transmissions have various modes depending on the model and make of the transmission. Some of the common transmission gears and modes include:
Park (P): This gear selection mechanically locks the output shaft of the transmission, restricting the car or truck from moving in any direction. A parking pawl prevents the automatic transmission from rotating, and therefore the car or truck from moving, although the vehicle's non-driven road wheels may still rotate freely. For this reason, it is recommended to use the hand brake or parking brake because this actually locks the rear wheels and prevents them from moving in most cases. This also increases the life of the automatic transmission and the park pin mechanism, because parking on an incline with the automatic transmission in park without the parking brake engaged will cause undue stress on the parking pin. An efficiently-adjusted hand brake should also prevent the car or truck from moving if a worn transmission gear selector accidentally drops into reverse gear during early morning fast idle engine warm ups. It should be noted that locking the automatic transmission output shaft does not positively lock the driving wheels. If one driving wheel slips while the automatic transmission is in "park," the other will roll freely as the slipping wheel rotates in the opposite direction. Only a properly adjusted parking brake can be relied upon to positively lock both of the parking-braked wheels.
It is typical of front-wheel-drive cars for the parking brake to be on the rear non-driving wheels, so use of both the parking brake and the transmission park lock provides the greatest security against unintended movement on slopes. Unfortunately, the rear of most front wheel drive automobiles has only about half the weight on the rear wheel as is on the front wheels, greatly reducing the security provided by the parking brake as compared to either rear wheel drive vehicles with parking brake on the rear wheels which generally have near half of the total vehicle weight on the rear wheels, except for empty pickup and open bed trucks or to front wheel drive vehicles with the parking brake on the front wheels, which generally have about two thirds of the vehicle's weight unloaded on the front wheels.
A car or truck should be allowed to come to a complete stop before setting the automatic transmission into park to prevent auto trans damage. Usually, Park (P) is one of only two transmission gear selections in which the car or truck engine can be started, the other being Neutral (N). In many modern cars and trucks, the driver must have the foot brake applied before the automatic transmission can be taken out of park. The Park position is omitted on buses / coaches with automatic transmission on which a parking pawl is not practical, which must be placed in neutral with the parking brakes set.
Most car and trucks require P or N to be set on the selector lever before the internal combustion engine can be started. This is typically achieved via a normally open 'inhibitor' switch, which is wired in series with the starter motor engagement circuit, and is only closed when P or N is selected, thus completing the circuit when the key is turned to the start position.
Reverse (R): This engages reverse gear within the transmission, giving the ability for the vehicle to drive backwards. In order for the driver to select reverse gear in modern day transmissions, they must come to a complete stop, push the transmission shift lock button in or pull the shift lever forward in the case of a column shifter and select reverse. Not coming to a complete stop can cause severe damage to the automatic transmission. Many modern automatic transmissions have a safety mechanism in place, which does to some extent prevent but does not completely avoid inadvertently putting the car or truck in reverse gear when the vehicle is moving forwards. This mechanism usually consists of a solenoid-controlled physical barrier on either side of the Reverse position, which is electronically engaged by a switch on the brake pedal. Therefore, the brake pedal needs to be depressed in order to allow the selection of reverse. Some electronic transmissions prevent or delay engagement of reverse gear altogether while the car is moving.
Some transmission shifters with a transmission shift button allow the driver to freely move the transmission shifter from R to N or D, or simply moving the transmission shifter to N or D without actually depressing the button. However, the driver cannot put back the shifter to R without depressing the shift button to prevent accidental shifting, especially at high speeds, which could damage the automatic transmission.
Neutral / No Gear (N): This disengages all gear trains within the transmission, effectively disconnecting the automatic transmission from the driven road wheels, so the vehicle is able to move freely under its own weight and gain momentum without the motive force from the engine braking). This is the only other transmission gear selection in which the vehicle's engine can always be started.
Drive (D): This position allows the automatic transmission to engage the full range of available forward gear trains, and therefore allows the vehicle to move forward and accelerate through its range of transmission gears. The number of gear 'ratios' an automatic transmission has depends on the model, but they initially ranged from three predominant before the 1990s, to four and five speeds losing popularity to six-speed autos, though still favored by Chrysler, Honda and Acura. Six speed automatic transmissions are now probably the most common offering Toyota Camry V6 models, the Chevrolet
Overdrive (D, OD, or a boxed D): This mode is used in some automatic transmissions to allow early computer controlled transmissions to engage the automatic overdrive. In these transmissions, Drive (D) locks the automatic overdrive off, but is identical otherwise. OD (Overdrive) in these cars and trucks is engaged under steady speeds or low acceleration at approximately 35–45 mph. Under hard acceleration or below 35–45 mph, the transmission will automatically downshift. Vehicles with this option should be driven in this mode unless circumstances require a lower gear.
Malibu LTZ, Corvette, GM trucks, Pontiac G8, Ford Falcon BF 2005-2007 and Falcon FG 2008 current in Australia with 6 speed ZF, and most newer model Ford, Lincoln, Mercury vehicles. However, seven-speed automobiles are becoming available found in Mercedes 7G gearbox, as are eight speed automobile in the newer models of Lexus and BMW cars and trucks.
Third(3): This mode limits the automatic transmission to the first three gear ratios, or sometimes locks the automatic transmission in third gear. This can be used to climb or going down hill. Some vehicles will automatically shift up out of third gear in this mode if a certain RPM range is reached in order to prevent engine damage. This transmission gear is also recommended while towing.
Second (2 or S): This mode limits the automatic transmission to the first two gear ratios, or locks the transmission in second gear on Ford, Kia, and Honda models. This can be used to drive in adverse conditions such as snow and ice, as well as climbing or going down hills in the winter time. Some vehicles will automatically shift up out of second gear in this mode if a certain RPM range is reached in order to prevent engine damage. Although traditionally considered second gear, there are other names used. Chrysler models with a three-speed automatic since the late 1980s have called this gear “3” while using the traditional names for Drive and Low.
First (1 or L Low): This mode locks the automatic transmission in first gear only. It will not change to any other transmission gear range. This, like second gear, can be used during the winter season, or for towing. As well as the above transmission modes there are also other modes, dependent on the manufacturer and model.
Some Other Transmission Modes Include:
D5: In Honda and Acura vehicles equipped with five speed automatic transmissions, this mode is used commonly for highway use and uses all five forward transmission gears.
D4: This transmission mode is also found in Honda and Acura four speed automatic transmissions or five speed automatic transmissions, and only uses the first four transmission gear ratios. According to the manufacturer manual, it is used for "stop and go traffic", such as city driving.
D3 or 3: This transmission mode is found in Honda, Acura, Volkswagen and Pontiac four speed automatic transmissions and only uses the first three transmission gear ratios. According to the manufacturer manual, it is used for "stop & go traffic", such as city driving.
S or Sport: This is commonly described as 'Sport mode'. It operates in an identical manner as 'D' mode, except that the transmission up shifts change much higher up the engine's rev range. This has the effect on maximizing all the available engine output, and therefore enhances the performance of the car or truck, particularly during acceleration. This transmission mode will also down change much higher up the rev range compared to 'D' mode, maximizing the effects of engine braking. This transmission mode will have a detrimental effect on fuel economy. Hyundai has a Norm / Power transmission switch next to the gearshift for this purpose on the Tiburon.
Winter (W): In some Mercedes-Benz, BMW and General Motors Europe models, a transmission 'Winter mode' can be engaged so that second gear is selected instead of first when pulling away from stationary, to reduce the likelihood of loss of traction due to wheel spin on snow or ice. On GM cars, this was D2 in the 1950s, and is second gear start after 1990. On Ford, Kia, and Honda automatic transmission, this feature can be accessed by moving the gear selector to 2 to start, then taking your foot off the accelerator while selecting D once the car is moving.
Brake (B): A transmission mode selectable on some Toyota models. In non-hybrid cars, this mode lets the engine do compression braking, also known as engine braking, typically when encountering a steep downhill. Instead of engaging the brakes, the engine in a non-hybrid car switches to a lower transmission gear and slows down the spinning tires. The engine holds the car back, instead of the brakes slowing it down. For hybrid cars, this mode converts the electric motor into a generator for the battery. It is not the same as downshifting in a non-hybrid car, but it has the same effect in slowing the car without using the brakes. GM called this HR (hill retarder) and GR (grade retarder) in the 1950s.
+ -, and M: This is for the 'manual transmission mode' selection of gears in certain automatics, such as Porsche Tiptronic transmission.. The M feature can also be found in Chrysler and General Motors products such as the Dodge Magnum and Pontiac G6, as well as Toyota's Camry, Corolla, Fortuner, Previa and Innova. Mitsubishi and some Audi models (TT), meanwhile do not have the M, and instead have the + and -, which is separated from the rest of the shift modes; the same is true for some Peugeot products like Peugeot 206. Meanwhile, the driver can shift up and down at will by toggling the console mounted shift lever like a semi automatic transmission. This transmission mode may be engaged either through a selector / position or by actually changing the transmission gears (e.g., tipping the gear-down paddles mounted near the driver's fingers on the steering wheel).