Introduction:

Automatic transmissions are a cornerstone of modern automotive engineering, eliminating the need for manual gear changes and providing a convenient driving experience. Many rear wheel drive (RWD) and four wheel drive vehicles use automatic transmissions, while front wheel drive (FWD), all wheel drive, and some RWD vehicles use automatic transaxles units that combine the transmission and final drive. These systems select gear ratios based on engine speed, powertrain load, vehicle speed and other factors, requiring little driver effort. Upshifts and downshifts occur automatically no clutch pedal is needed and the vehicle can stop without shifting to neutral a major convenience in traffic. Drivers can manually select lower forward gears, reverse, neutral or park, and engine braking is available during deceleration depending on the range chosen. The number of forward gears varies from four to eight in current vehicles some have continuously variable designs with no fixed ratios.

Early automatics were three or four speed today, five and six‑speed units are most common, while seven and eightspeed units are mainly in luxury vehicles. Most new automatics feature a lockup torque converter. Some are fitted with a transfer case for four wheel or all wheel drive. Until recently all automatic transmissions were hydraulically controlled, but most now use computer control for the torque converter and shifting. Based on electronic sensors and switches, the computer sets converter operating mode, controls shift sequence and in some cases regulates oil pressure. Modern transmissions have at least one overdrive gear to reduce fuel consumption, emissions, and noise during cruising. The lockup clutch and shifting are computer controlled.

Torque Converter:

Automatic transmissions use a torque converter a fluid clutch to transfer engine torque to the transmission. It operates via hydraulic force from automatic transmission fluid (ATF). The converter changes or multiplies the crankshaft’s twisting motion and directs it through the transmission. It automatically engages and disengages power relative to engine rpm: at idle, fluid flow is insufficient for power transfer; as engine speed rises, increased flow creates enough force to transmit power. Nearly all torque converters are one piece, welded units repairable only by specialty shops. They are sealed, doughnut shaped units located between engine and transmission, always filled with ATF. A flexplate mounts the converter to the crankshaft, transferring rotation to the converter shell. The flexplate flexes as pressure builds, bolted to the crankshaft flange and to the converter shell. No heavy flywheel is needed because the converter and flexplate mass smooth power strokes. The flexplate or converter is surrounded by the starter ring gear.

Components:

A standard torque converter has three elements: the pump assembly (impeller), the stator, and the turbine. The impeller is the input member, receiving engine power. The turbine is the output member, splined to the transmission’s turbine shaft. The stator is the reaction member or torque multiplier, supported on a one‑way overrunning clutch that allows free rotation in one direction and locks in the opposite. The converter shell is shaped like two bowls facing each other. A short stubby shaft supports the converter in the crankshaft. At the rear, a pump drive hub with notches or flats drives the transmission pump. A pump bushing supports the hub. Some transaxles use a separate pump shaft. The impeller forms the internal shell section with curved blades that rotate with the shell at engine speed, circulating oil. The turbine faces the impeller with curved blades that are more curved than the impeller’s to reduce turbulence. The stator sits between them, redirecting oil from the turbine back into the impeller in the direction of rotation with minimal loss.

Operation:

ATF transfers energy in the converter. As the impeller rotates, centrifugal force throws oil outward and upward. Faster rotation increases force. When the turbine is mounted above, the oil strikes its vanes, causing rotation no direct mechanical link exists. An oil pump driven by the converter shell delivers pressurized oil through a hollow shaft into the converter; a seal prevents leakage. The turbine shaft, located inside this shaft, is splined to the turbine and transfers power to the transmission’s main drive shaft. Oil leaving the turbine goes to an external cooler and then to the sump.
At idle with brakes applied, little power is transferred. When throttle opens, speed and centrifugal force increase, directing oil against turbine blades to drive the transmission.

Types of Oil Flow:

Two oil flows occur: rotary flow i.e. around the circumference and vortex flow i.e. from impeller to turbine and back, at a 90‑degree angle. As turbine speed approaches impeller speed the coupling point they run at nearly the same speed due to slippage exact synchronisation requires a lockup clutch. The stator freewheels at coupling and redirects oil to improve efficiency. Torque multiplication only occurs when the impeller turns faster than the turbine. Stators can be rotating or fixed rotating types are more efficient at higher speeds.

Overrunning Clutch:

The overrunning clutch keeps the stator from rotating in one direction and permits free rotation in the other. Rotating stators typically use a roller type clutch with an inner race, rollers, accordion springs and outer race with cam‑shaped pockets. At vehicle start, the stator locks because of speed differences rollers wedge against the races. As speed increases, oil exiting the turbine strikes the stator’s back face, causing it to rotate freely. If the vehicle slows, oil direction changes, striking the front face, halting rotation, and rollers jam to lock the stator again, enabling torque multiplication.

Lockup Torque Converter:

A lockup torque converter eliminates the 10% slip at coupling, improving fuel economy, reducing heat and engine speed. The assembly is called a torque converter clutch (TCC). Various designs exist; the most common is the electronically controlled lockup piston clutch. Other types include fully mechanical, centrifugally controlled or viscous coupling.

Lockup Piston Clutch:

This clutch has a piston between the turbine front and the shell’s interior front face, with a piston plate, damper assembly (coil springs), and friction plate. The PCM controls hydraulic valves to engage lockup based on operating conditions. In a typical system, a three valve module includes a lockup valve, fail safe valve, and switch valve. The lockup valve controls the clutch the fail safe prevents lockup until third gear the switch valve directs fluid through the turbine shaft. When unlocked, fluid enters the converter and moves to the front of the piston, keeping it away from the shell, and exits around the stator support. In lockup mode, the switch valve reverses fluid flow, pushing the piston forward to apply the clutch. Fluid from the front side exits through the turbine shaft. During acceleration, higher pressure moves the fail safe valve, blocking lockup and spring tension moves the switch valve, returning to unlocked mode.

Planetary Gears:

Nearly all automatic transmissions use planetary gear sets to transfer power and multiply torque. Compound gear sets combine two simple sets to spread load over more teeth and obtain more ratios in a compact space. A simple set has three parts: a sun gear (center), a carrier with planetary pinions, and an internally toothed ring gear (annulus). The sun gear meshes with pinion teeth pinions are fitted into the carrier with shafts. They rotate on needle bearings. The ring gear surrounds them, providing strength.

How Planetary Gears Work?

Each member can spin or be held. Power transfer occurs only when one member is held or two are locked together. Any member can be input, another can be reaction (held); the third becomes output. Depending on which is which, the result is torque increase (underdrive), speed increase (overdrive) or reverse. When external gears mesh, rotation direction changes, when external meshes with internal, direction stays the same.

Maximum forward reduction:

Ring gear held, sun gear input (clockwise) drives pinions counterclockwise, carrier output high reduction, high torque multiplication.

Minimum forward reduction:

Sun gear held, ring gear input (clockwise) drives pinions and carrier forward less reduction.

Maximum overdrive:

Ring gear held, carrier input (clockwise) drives sun gear (output) faster speed increase.

Slow overdrive:

Sun gear held, carrier input drives ring gear faster overdrive.

Slow reverse:

Sun gear input, carrier held ring gear output rotates counterclockwise, slow reverse.

Fast reverse:

Carrier held, ring gear input drives sun gear output fast reverse.

Direct drive:

Both ring and sun gear input at same speed opposing forces lock the set, direct drive.

Neutral:

No member held or locked.

Compound Planetary Gear Sets:

Simpson Geartrain:

The Simpson arrangement uses two planetary sets sharing a common sun gear, with two ring gears and two carriers. It provides three forward gears. The front and rear planetary units may differ in size and tooth counts. Gear ratios depend on those counts. By applying torque to one member, holding another, and taking output from a third, neutral, first, second, direct and reverse are achieved. In neutral, no member is locked to the shaft. In first gear, the input shaft drives the front ring gear, which drives front planets, sun gear, and rear planets, producing reduction. In second, the front ring gear drives planets that walk around the held sun gear, turning the carrier and output with reduction. In third, both the front ring gear and sun gear are inputs, locking the carrier for direct drive. For reverse, the sun gear drives rear planets, with the rear carrier held, making the ring gear and output turn counterclockwise with reduction. In park, a pawl locks the output shaft to the case.

Ravigneaux Geartrain:

The Ravigneaux has two sun gears (small and large), two sets of planet gears (three long and three short), a common carrier, and a single ring gear. It is compact, handles high torque, and can have three different outputs, but is more complex. The small sun gear meshes with short pinions, which drive long pinions long pinions mesh with the large sun gear and ring gear. In neutral, the small sun gear freewheels. In first gear, the small sun gear drives short pinions, which drive long pinions and the ring gear output at reduced speed. In second, the long planets walk around the held large sun gear for further reduction. In third, both the small sun gear and carrier are inputs, giving direct drive. In overdrive (fourth), only the carrier is input, long planets walk around the held large sun gear, driving the ring gear faster. In reverse, the reverse sun gear is input, the carrier is held and the ring gear turns counterclockwise with reduction.

Planetary Gear Sets in Tandem:

Some transmissions use two simple planetary units in series without shared members. Instead, holding devices lock different members together. The front carrier locks to the rear ring gear, and the front ring gear to the rear carrier. A transaxle may have additional sets for the final drive.

Lepelletier System:

Late model six, seven and eight speed transmissions often use the Lepelletier system, connecting a simple planetary set to a Ravigneaux. This design was difficult to control but modern electronics have made it practical. It allows more forward speeds without increasing size and weight most six speeds are more compact and lighter than four or five speeds. The simple set’s ring gear serves as input, and can also connect to the Ravigneaux carrier. Different input combinations drive various gear members to produce forward ratios. The Ravigneaux ring gear is the output. In some seven speeds, the input shaft always connects to the simple ring gear and can also connect to the Ravigneaux carrier and large sun gear for extra combinations.

Continuously Variable Transmissions (CVT):

CVTs have no fixed forward speeds; the ratio varies with engine speed and temperature. They have a one speed reverse. Some CVTs lack a torque converter, using a flywheel with a start clutch. Instead of gear sets, CVTs use belts and pulleys. One pulley is driven, the other drives each has a movable and fixed face. Moving the face changes effective diameter and ratio. A steel belt links them. For a low ratio, high hydraulic pressure increases the driven pulley size while reducing drive pulley pressure, the opposite for high ratios. Ratios can change anytime the vehicle moves, allowing the engine to run at most efficient speed, improving fuel economy and reducing emissions. Many late model CVTs simulate manual shifts with predetermined pulley stops. Nissan’s Extroid CVT uses discs and rollers and can handle high torque.

Planetary Gear Based CVTs (Hybrid):

Planetary gear set as a power split device, with two motor/generators, a differential, and a simple planetary set. The engine and motors connect to the planetary unit. The device splits engine power to drive a motor/generator or the wheels or both. The other motor can drive wheels, assist, or be driven. Speed ratios change with torque applied to members. Two sources of torque (engine and electric motor) rotate in the same direction but at different speeds, allowing assistance or braking.

Two Mode Hybrid System:

Two mode full hybrid system develop using two planetary gear sets and two electric motors in a standard transmission housing. It operates in low speed/low load mode and highway cruising mode. It can run on electric, engine, or combined power. Motors can act as generators for regenerative braking.

Planetary Gear Controls:

To achieve different ratios, certain members must be held or driven. Controls include bands, servos, and clutches.

Transmission Bands:

A band is a braking assembly around a drum or carrier it wraps around and holds the drum. Hydraulically applied by a servo, it holds a planetary member stationary. Bands are space efficient and provide good holding. When closing they self energize. Bands are larger in diameter than the drum to promote self disengagement. Friction material lines the inside. Low speed drums use semimetallic linings high speed drums use paper based. Lugs connect the band to the servo and anchor. Bands can be rigid, flexible, single or double wrap. Single wrap steel bands are used for high output engines; thin flexible bands provide more self energizing. Double wrap bands have segmented design for cushioning heavy double wrap bands are for high output.

Transmission Servos:

Servos convert hydraulic pressure into mechanical force to apply a band.

Simple servo:

Piston in cylinder held by coil spring. Applying fluid pressure moves the piston against the spring, applying force through a lever to the band. When pressure is released, the spring returns the piston, releasing the band.

Compound servo:

Similar but has a release side. When releasing, pressure is applied to both sides, letting spring retract the piston. Some servos have larger release areas for positive release to ensure bands release before other members apply.

Transmission Clutches:

Clutches can both hold and drive members.

Overrunning clutches:

Allow rotation in one direction only. Roller type: rollers in cam pockets lock when one race turns one way, free when turned the other. Sprag type: figure eight sprags lock between hub and drum in one direction, free in the other.

Multiple disc clutches:

Use alternating friction discs and steel plates. Friction discs have internal teeth engaging a hub; steel plates have external teeth engaging a clutch drum. A pressure plate and snapring hold the pack. Hydraulic piston forces the pack together. Friction materials now include cellular paper, graphite, and ceramics (asbestos is phased out). The drum housing contains piston, springs, seals, and snaprings. The hub connects to the planetary member. Piston return springs disengage when pressure drops.

Control Terminology:

Manufacturers use different names for planetary controls some by speed gear, some by function, others by location. This can be confusing but is important to recognize.

Bearings, Bushings and Thrust Washers:

Bearing surfaces reduce friction between sliding or rotating parts. Sliding bearings are for low speed, large surfaces, or low use; rolling bearings for high speed, high load, small surfaces. Transmissions use bronze alloy sliding bearings, radial load types are bushings, axial load are thrust washers. Bushings are press fit and support rotating parts, guide valves and control fluid flow. Thrust washers control end play, available in various thicknesses; they are made of softer materials (copper, babbit faced steel, bronze, nylon, Teflon) to be the wear item. Selective washers set end play to specifications. Torrington bearings are roller thrust bearings that reduce friction and control end play. Needle bearings have rollers with length‑to‑diameter ratio ≥5:1, they may be loose or caged. Other roller bearings include radial and axial types. Tapered roller bearings handle both radial and axial loads but are rarely used in automatics (common in final drives). Ball bearings carry heavy radial loads and can handle light axial loads, they have grooved races.

Snaprings, Gaskets, and Seals Snaprings:

Retain assemblies and adjust clutch clearance; waved snaprings smooth clutch application.

Gaskets seal parts or direct fluid flow. Hard gaskets (paper) are for smooth surfaces (valve body, pump); soft gaskets (rubber/cork) for irregular surfaces (oil pan); some use RTV.

Seals prevent leakage. Static seals (pan, pump) between non‑moving parts; dynamic seals (piston seals) between moving parts. Positive seals prevent all leakage; nonpositive allow controlled leakage for lubrication.

Rubber seals: O‑rings (circular cross‑section, static or low axial movement dynamic, not for rotation), lip seals (flexible lip for shafts and pistons; piston seals have lip facing pressure; shaft seals have garter spring for uniform pressure, allowing a 0.0001 inch oil film), and square‑cut seals (rectangular cross‑section, withstand more axial movement).

Metal sealing rings (cast iron, nylon, Teflon) are used where some leakage is acceptable. Types: butt end, open end (gap), hook end (hooked ends for better sealing). Teflon seals (scarf‑cut or one piece) provide softer surfaces and longer life one piece Teflon seals give nearly positive sealing. General Motors uses Vespel (flexible, durable plastic like material) in some transmissions.

Final Drives and Differentials:

The final drive is the last gearset. In RWD cars, it is in the rear axle; in FWD, inside the transaxle; some longitudinally mounted FWD have separate cases; AWD/4WD have front and rear units. RWD uses hypoid gears turning power 90°. FWD transverse engines have parallel axis, so simple gear connections. Final drive configurations in FWD: helical, planetary, hypoid, chain. Helical uses two gears to a transfer shaft with a pinion driving the ring gear. Planetary final drive uses a sun gear driven by transmission output, pinions walking around a stationary ring gear, carrier driving the differential. Chain‑drive uses a chain between drive and driven sprockets, allowing remote differential positioning. Ratios depend on sprocket sizes.

Hydraulic System:

The hydraulic system uses ATF to perform work. The pump is the source of fluid flow, providing pressure to operate, lubricate, and cool. Pressure regulating valves control shift quality and points, flow‑directing valves direct fluid to apply devices. The reservoir is the oil pan. The valve body contains control valving; output devices are servos and clutches.

Functions of ATF:

ATF cools, lubricates and cleans; dirt is filtered. It moves valves and applies devices, pressurized fluid engages clutches and bands.

Reservoir and Venting:

The pan stores fluid, atmospheric pressure forces it to the pump. Dipstick or side plug checks/adds fluid. Vents allow air in and out, all reservoirs need venting to allow pump inlet suction and to exhaust built up air pressure from heat and moving parts.

Transmission Coolers:

Heat removal is critical. Overheated fluid oxidizes, forms varnish, damages seals, and leads to failure. Cooler lines direct hot fluid from converter to the radiator cooler, cooled fluid returns to the transmission either to bushings/bearings or to pan. Heavy duty vehicles may have an auxiliary cooler.

Valve Body:

The valve body (aluminum or iron) has precision bores and passages. It contains spool valves, check ball valves and poppet valves to regulate and direct fluid. The separator and transfer plates seal passages. The valve body senses load and driver needs.

Check ball valve:

Ball on seat; pressure unseats or seats it,, can have two seats.

Poppet valve:

Ball or disc, usually spring loaded, pops open/closed.

Spool valve:

It has lands and valleys, lands ride on fluid film, covers/uncover ports, reaction area sees spring, fluid or linkage forces.

Oil Pump:

Three types: gear, rotor, vane. Driven by the torque converter hub or shaft; it runs whenever the engine is running.

Pressure Regulator Valve:

A spool valve that opens/closes an exhaust port to regulate pressure. If pressure drops, it closes, if too high, it opens. Many use electronic pressure control (EPC) solenoids.

Governor Assembly:

Driven by output shaft, it senses road speed and sends pressure to the shift valve. Increasing speed raises governor pressure to upshift, decreasing causes downshift. Heavy load delays upshift because throttle pressure opposes governor pressure.

Pressure Boosts:

Under heavy load, line pressure is increased to prevent slipping. Electronic sensors (TP, MAP) send signals to the control unit; throttle pressure (via cable) also boosts pressure by acting on the regulator. Vacuum modulators on older transmissions increase pressure with low vacuum.

Kickdown Valve:

Quick wide open throttle increases throttle pressure, moves kickdown valve, sends mainline pressure to shift valve causing a downshift for passing or climbing.

Shift Quality and Timing:

Shift feel is controlled by apply/release pressures, rates, and timing. Bands may release as clutches apply, if timing is off, flare or slip occurs. Methods to smooth shifts include wavy separator plates, restricting orifices, accumulator pistons (divert pressure to cushion), and electronic pulsing of solenoids. Shift timing is determined by throttle pressure (engine load) and governor pressure (vehicle speed) acting on opposite ends of shift valves. When governor pressure overcomes throttle pressure and spring tension, the valve moves and an upshift occurs.

Hydraulic Circuits Example (Three‑Speed Transaxle):

A detailed circuit example illustrates valve body operation.

Neutral/Park:

Pump pressure goes to pressure regulator and manual valve. Regulated line pressure goes to converter via switch valve; converter pressure returns as cooling/lube. Line pressure flows to manual valve, seats check balls, and stops at throttle valve (closed). Accumulator is charged for cushioning when D or R is selected.

D first gear:

Line pressure from manual valve seats check ball 9 and passes ball 8 to throttle valve, creating throttle pressure. It also pressurizes the rear clutch and governor. Throttle pressure acts on kickdown valve, regulator throttle plug, and shift valves. Governor pressure is low, so no upshift. Converter pressure keeps TCC unlocked.

D second gear:

Line pressure from manual valve outlet 1 creates throttle pressure; outlet 2 feeds the 1‑2 shift valve, which directs line pressure to rear clutch and governor. The 1-2 valve is upshifted, sending pressure to front servo to engage the kickdown band. Throttle pressure boosts line pressure. Governor pressure overcomes throttle pressure at 1‑2 valve, causing upshift.

D third gear:

Line pressure from outlet 1 goes to throttle valve; outlet 2 feeds forward circuit, rear clutch, governor. Line pressure also passes the 1‑2 and 2‑3 valves to the shuttle valve, then to the release side of the front servo and front clutch, engaging front clutch for direct drive. Governor pressure moves 1‑2 and 2‑3 valves to upshifted positions.

TCC lockup in third:

PCM receives coolant temp, VSS and vacuum signals, energizes lockup relay and solenoid, seating check ball, increasing line pressure to move switch valve, directing fluid to fill converter and push clutch piston for lockup.

Reverse:

Line pressure from manual valve bypass goes to low‑reverse servo and front clutch. Check balls seat. Throttle pressure boosts line pressure to 200‑300 psi because the front clutch lacks a Belleville spring higher pressure prevents slipping. Switch valve stays in unlocked position for torque multiplication. TCC never engages in reverse.