Most automatic transmissions have a defined set of gear ranges, often with a parking pawl feature that locks the output shaft of the transmission. Continuously variable transmissions (CVTs), which are very different to conventional automatic transmissions, can change their ‘ratios’ over a wider ’stepless’ range, rather than between a set of fixed gear ratios. CVTs have been used for decades in two-wheeled scooters; but have only seen use in a few automobile models. Recently, however, CVT technology has gained greater acceptance among manufacturers and customers, especially in Audi and Nissan automobiles, and gas-electric hybrid vehicles.

Some machines with limited speed ranges or fixed engine speeds, such as some forklift trucks and lawn mowers, only use a torque converter to provide a variable gearing of the engine to the wheels.

Comparison with manual transmission

Most cars sold in the North America since the 1950s have been available with an automatic transmission. Conversely, automatic transmission is less popular in Europe, with 80% of drivers opting for manual transmission. In most Asian markets and in Australia, automatic transmissions have become very popular since the 1990s.

Vehicles equipped with automatic transmissions are, arguably, easier to drive. Consequently, in some jurisdictions, drivers who have passed theirdriving test in a vehicle with an automatic transmission will not be licensed to drive a manual transmission vehicle. Examples of driving license restrictions are Bulgaria,Croatia, Dominican Republic, Israel, United Kingdom, some states in Australia, France, Portugal, Latvia, Lithuania, Ireland, Estonia, Belgium, Germany, Pakistan, the Netherlands,Sweden, Austria, Norway, Poland, Hungary, South Africa, Trinidad and Tobago, China, Hong Kong, Macau, South Korea, Singapore, United Arab Emirates, India, Finland, Spain,Switzerland, Slovenia and New Zealand (Restricted licence only)

Automatic transmission modes

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 next to him/her. 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. Vehicles conforming to US Government standart must have the modes ordered P-R-N-D-L (left to right, top to bottom, or clockwise). Prior to this, 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 gear selection, as well as the danger of having a selector (when worn) jump into Reverse 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 modes are:

Park (P)

This 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. For this reason, 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. An efficiently-adjusted hand brake should also prevent the car from moving if a worn selector accidentally drops into reverse gear during early morning fast-idle engine warm-ups.

A car should be allowed to come to a complete stop before setting the transmission into park to prevent damage. Usually, park is one of only two selections in which the car’s engine can be started. 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.

Most automobiles require P 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 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, 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.

Neutral/No gear (N)

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 without the motive force from the engine. This is the only other selection in which the vehicle’s engine can be started.

Drive (D)

This position allows the 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 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)^{[citation needed]}. Six-speed automatic transmissions are now probably the most common offering Toyota Camry V6 models, the Chevrolet Malibu LTZ, Corvette, GM trucks, Pontiac G8, and most newer model Ford/Lincoln/Mercury vehicles). However, seven-speed autos are becoming available (found in Mercedes 7G gearbox), as are eight-speed autos in the newer models of Lexus and BMW cars.

Some cars, when put into D, will automatically lock the doors or turn on the daytime running lamps^{[citation needed]}.

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.

Second (2 or S)

This mode limits the transmission to the first two gear ratios, or sometimes locks the transmission in second gear. 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 transmission in first gear only. It will not change to any other gear range. This, like second, can be used during the winter season, or for towing.

As well as the above modes there are also other modes, dependent on the manufacturer and model. Some examples include;

D5

In Hondas and Acuras equipped with five-speed automatic transmissions, this mode is used commonly for highway use (as stated in the manual), and uses all five forward gears. Lincoln LS also used a “D5″ & “D4″ in shiftgate….

D4

This mode is also found in Honda and Acura four- or five-speed automatics, and only uses the first four gear ratios. According to the manual, it is used for “stop and go traffic”, such as city driving. Lincoln LS also used a “D5″ & “D4″ in shiftgate….

D3 or 3

This mode is found in Honda, Acura and Pontiac four-speed automatics and only uses the first three gear ratios. According to the 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 upshifts change much higher up the engine’s rev range. This has the effect on maximising all the available engine ouput, and therefore enhances the performance of the vehicle, particularly during acceleration. This mode will also downchange much higher up the rev range compared to ‘D’ mode, maximising the effects of engine braking. This mode will have a detrimental effect on fuel economy.

+ −, and M

This is for the ‘manual mode’ selection of gears in certain automatics, such as Porsche’s Tiptronic. 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, meanwhile does not have the M, and instead has 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 mode may be engaged either through a selector/position or by actually changing the gears (e.g., tipping the gear-down paddles mounted near the driver’s fingers on the steering wheel).

Winter (W)

In some Mercedes-Benz, BMW and General Motors Europe models, a ‘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 wheelspin on snow or ice.

Brake (B)

A 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 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.

The fluid in your car’s transmission should be replaced every 30,000 miles. Over time, the assembly’s friction creates an enormous amount of internal heat. That causes the fluid to oxidize and lose its ability to lubricate the moving parts. Eventually, the lack of lubrication will cause the operational temperature to rise further, leading to varnish, cracked seals, and a loss in pressure. If your transmission fails, you can expect a hefty repair bill.

Below, I’ll quickly take you through the steps involved with changing your car’s transmission fluid and filter (it’s recommended that you do both at the same time). You can pay a mechanic to do the job, but you’ll save time and money by doing it yourself.

Draining The Fluid

If your car has an automatic transmission, it will also have a service pan that contains both the fluid and the filter. On the pan’s underside, there will normally a plug. Remove it and drain the fluid into a catch container. If the pan does not have a plug, you can drain the fluid when you remove the entire pan.

Removing The Transmission Pan

After completely draining the fluid, you’ll need to remove the pan. Look for bolts around the pan’s perimeter that are securing it in position, and remove them. Before removing the last two bolts, place your hand on the pan’s underside to hold it. Then, remove the remaining bolts.

Cleaning The Pan

After you have removed the pan, place it on a flat surface. Clean out the remaining fluid and look for small residual metal particles. There will usually be some, but an excessive amount can eventually cause problems within your tranny. Clean the pan as thoroughly as possible. Then, place a new gasket (included inside transmission service kits available at auto supply stores) along the rim of the pan.

Replace The Transmission Filter

In most cases, the filter will be housed inside the pan. Occasionally, you’ll find it secured with bolts to the front of the tranny. Remove it and replace the corresponding seal. It’s critical that the seal is in good condition in order to prevent fluid leaks. Replace the filter and reinstall the pan. As with the filter seal, make sure the gasket along the rim of the pan will prevent leaks.

Replenish The Fluid

At this point, your work under your vehicle is complete. Lift the hood and replenish the fluid level in the appropriate reservoir. Then, turn your car on and allow the engine to idle for two or three minutes. Check the fluid level again and add more, if necessary.

That’s it. You’ve successfully serviced your vehicle’s transmission. It’s a good idea to check the fluid level after a few days. Also, check the ground directly underneath the assembly to make sure that a badly positioned seal or gasket isn’t causing leaks.