AUTOMATIC TRANSMISSION - MODE 2 or S(SECOND)

This mode limits the 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. 

It is usually recommended to use second gear for starting on snow and ice, and use of this position enables this with an automatic transmission. 

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.

AUTOMATIC TRANSMISSION - MODE 3(THIRD)

This mode limits the transmission to the first three gear ratios, or sometimes locks the 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 gear is also recommended while towing a caravan.

AUTOMATIC TRANSMISSION MODE - Overdrive (D, OD, or a boxed [D])

This mode is used in some 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 is engaged under steady speeds or low acceleration at approximately 35–45 mph (56–72 km/h).


Under hard acceleration or below 35–45 mph (56–72 km/h), the transmission will automatically downshift.


Vehicles with this option should be driven in this mode unless circumstances require a lower gear.

AUTOMATIC TRANSMISSION - DRIVE(D)

This position allows the transmission to engage the full range of available forward gear trains, 


allows the vehicle to move forward and accelerate through its range of gears. 


The number of gear ratios a 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 and Honda/Acura)


Six-speed automatic transmissions are now probably the most common offering Toyota Camry V6 models, the Chevrolet 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).


seven-speed autos are becoming available (found in Mercedes 7G gearbox, Infiniti, VW Group DSG, as are eight-speed autos in the newer models of Lexus, BMW ZF 8HP and VW, cars (VW Touareg).

AUTOMATIC TRANSMISSION MODE- N(NEUTRAL)

-This disengages all gear trains within the transmission

-effectively disconnecting the transmission from the driven roadwheels, so the vehicle is able to move freely under its own weight and gain momentum without the motive force from the engine (engine braking).

-This is the only other selection in which the vehicle's engine can be started.

AUTOMATIC TRANSMISSION - MODE R(REVERSE)

In order for the driver to select reverse in modern transmissions, they must come to a complete stop,

push the 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 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 in reverse 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 shifters with a shift button allow the driver to freely move the shifter from R to N or D, or simply moving the 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 transmission.

AUTOMATIC TRANSMISSION - MODE P(PARK)

Usually, Park (P) is one of only two selections in which the car's 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 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.

Advice is given in some owner's manuals that if the vehicle is parked on a steep slope using the park lock only, it may not be possible to release the park lock (move the selector lever out of "P"). Another vehicle may be required to push the stuck vehicle uphill slightly to remove the loading on the park lock pawl.

Most automobiles 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)

Park (P) selection mechanically locks the output shaft of transmission, restricting the vehicle from moving in any direction.

A parking pawl prevents the transmission from rotating, and therefore the vehicle from moving, although the vehicle's non-driven roadwheels may still rotate freely

 it is recommended to use the hand brake (or parking brake) because this actually locks (in most cases) the rear wheels and prevents them from moving.

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.

 It should be noted that locking the transmission output shaft does not positively lock the driving wheels.

It is typical of front-wheel-drive vehicles for the parking brake to be on the rear (non-driving) wheels

use of both the parking brake and the transmission park lock provides the greatest security against unintended movement on slopes.

A car should be allowed to come to a complete stop before setting the transmission into park to prevent damage