Automatic Transmission Components

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AUTOMATIC TRANSMISSION COMPONENTS

Torque converter

See Figures 5 and 6

The front section is called the torque converter. In replacing the traditional clutch, itperforms three functions:

It acts as a hydraulic clutch (fluid coupling), allowing the engine to idle even with

the transmission in gear.

It allows the transmission to shift from gear to gear smoothly, without requiring

that the driver close the throttle during the shift.

It multiplies engine torque making the transmission more responsive and reducing

the amount of shifting required.

The torque converter is a metal case that is shaped like a sphere that has been flattened on

opposite sides and is bolted to the rear of the engines crankshaft. !enerally, the entiremetal case rotates at engine speed and serves as the engines flywheel.

The case contains three sets of blades. "ne set is attached directly to the case forming the

impeller or pump. #nother set is directly connected to the output shaft, and forms theturbine. The third set (stator) is mounted on a hub which, in turn, is mounted on a

stationary shaft through a one$way clutch. %ollers are wedged into slots, preventing

backward rotation. &hen the rollers are not in the slots, the stator turns in the samedirection as the impeller. The pump, which is driven by the converter hub at engine

speed, keeps the torque converter full of transmission fluid at all times. 'luid flows

continuously through the unit to provide cooling.

# fluid coupling will only transmit the torque the engine develops it cannot increase thetorque. This is one ob of the torque converter. The impeller drive member is driven at

engine speed by the engines crankshaft and pumps fluid, to its center, which is flung

outward by centrifugal force as it turns. *ince the outer edge of the converter spins fasterthan the center, the fluid gains speed. 'luid is directed toward the turbine driven member

by curved impeller blades, causing the turbine to rotate in the same direction as the

impeller. The turbine blades are curved in the opposite direction of the impeller blades.

In flowing through the pump and turbine, the fluid flows in two separate directions. Itflows through the turbine blades, and it spins with the engine. The stator, whose blades

are stationary when the vehicle is being accelerated at low speeds, converts one type offlow into another. Instead of allowing the fluid to flow straight back into the pump, thestators curved blades turn the fluid almost +- toward the direction of rotation of the

engine. Thus the fluid does not flow as fast toward the pump, but is already spinning

when the pump picks it up. This has the effect of allowing the pump to turn much fasterthan the turbine. This difference in speed may be compared to the difference in speed

between the smaller and larger gears in any gear train. The result is that engine power

output is higher, and engine torque is multiplied.
http://www.procarcare.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_autotrans2.asp#Figure5%23Figure5http://www.procarcare.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_autotrans2.asp#Figure5%23Figure5


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#s the speed of the turbine increases, the fluid spins faster and faster in the direction of

engine rotation. Therefore, the ability of the stator to redirect the fluid flow is reduced.

nder cruising conditions, the stator is eventually forced to rotate on its one$way clutchand the torque converter begins to behave almost like a solid shaft, with the pump and

turbine speeds being almost equal.

In the late /s, 0hrysler 0orporation introduced an automatic transmission, featuring

what is called a 1lock$up1 clutch in the transmissions torque converter. The lock$up is afully automatic clutch that engages only when the transmission shifts into top gear or

when needed based on a predetermined demand factor.

The lock$up clutch is activated by a piston. &hen engaged, the lock$up clutch gives thebenefits of a manual transmission, eliminating torque converter slippage. In the engaged

position, engine torque is delivered mechanically, rather than hydrodynamically (through

fluid). This gives improved fuel economy and cooler transmission operating

temperatures.

In the early 2s, 'ord introduced what is known as the #utomatic "verdrive

Transmission (#"T). 3ssentially, this transmission uses a lock$up torque converter, by

offering an additional refinement. The transmission is a four$speed unit, with fourth gear

as an overdrive (.4/:5). Torque is transmitted via a full mechanical lock$up from theengine, completely bypassing the torque converter and eliminating hydraulic slippage.

In third gear (5:5 ratio), engine power follows a 1split$torque1 path, in which there is a

46 lock$up. *i7ty percent of the power is transmitted through solid connections and86 of the engine power is delivered through the torque converter.

Throughout the +s, *ubaru introduced an 3lectronic 0ontinuously 9ariableTransmission (309T) and onda introduced their version (09T). This transmission uses

a metal belt and two variable$diameter pulleys to keep smooth, uninterrupted range ofgearing. *i;e of the pulleys is controlled through the use of hydraulics. This unit produces

a miles per gallon closer to a manual transmission, while attaining a smoother shift than

an automatic transmission.

Figure 5 The torque converter housing is rotated by the engine crankshaft and turns the impeller. Theimpeller spins the turbine, which gives motion to the turbine (output) shaft to drive the gears.


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Figure 6 *ectional view of operations of a typical 1lock$up1 clutch and torque converter

Torque converter clutch control

See Figures 7 and 8

Electrical and vacuum controls

The torque converter clutch should apply when the engine has reached near normal

operating temperature in order to handle the slight e7tra load and when the vehicle speed

is high enough to allow the operation of the clutch to be smooth and the vehicle to be freeof engine pulses.

&hen the converter clutch is coupled to the engine, the engine pulses can be felt throughthe vehicle in the same manner as if equipped with a clutch and standard transmission.

3ngine condition, engine load and engine speed determine the severity of the pulsation.

The converter clutch should release when torque multiplication is needed in the

converter, when coming to a stop, or when the mechanical connection would affect

e7haust emissions during a coasting condition.

The typical electrical control components consist of the brake release switch, the lowvacuum switch and the governor switch. *ome vehicle models have a thermal vacuum

switch, a relay valve and a delay valve.


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Figure 7 sing electrical and vacuum controls to operate the torque converter clutch.

Figure 8Typical diesel engine vacuum and electrical schematic for the torque converter clutch.

Electrical current flow

#ll of the components in the electrical circuit must be closed or grounded before the

solenoid can open the hydraulic circuit to engage the converter clutch. The circuit begins

at the fuse panel and flows to the brake switch and as long as the brake pedal is not

depressed, the current will flow to the low vacuum switch on the gasoline engines and tothe high vacuum switch on the diesel engines. These two switches open or close the

circuit path to the solenoid, dependent upon the engine or pump vacuum. If the lowvacuum switch is closed (high vacuum switch on diesel engines), the current continues toflow to the transmission case connector, into the solenoid and to the governor pressure

switch. &hen the vehicle speed is appro7imately >?$? mph (?4$2 kph) , the governor

switch grounds to activate the solenoid. The solenoid, in turn, opens a hydraulic circuit tothe converter clutch assembly, engaging the unit.


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It should be noted that e7ternal vacuum controls include the thermal vacuum valve, the

relay valve, the delay valve, the low vacuum switch and a high vacuum switch (used on

diesel engines). @eep in mind that all of the electrical or vacuum components may not beused on all engines at the same time.

acuum flow

The vacuum relay valve works with the thermal vacuum valve to keep engine vacuum

from reaching the low vacuum valve switch at low engine temperatures. This actionprevents the clutch from engaging while the engine is still warming up. The delay valve

slows the response of the low vacuum switch to changes in engine vacuum. This action

prevents the low vacuum switch from causing the converter clutch to engage anddisengage too rapidly. The low vacuum switch deactivates the converter clutch when

engine vacuum drops to a specific low level during moderate acceleration ust before a

part$throttle transmission downshift. The low vacuum switch also deactivates the clutch

while the vehicle is coasting because it receives no vacuum from its ported vacuum

source.

The high vacuum switch, when on diesel engines, deactivates the converter clutch while

the vehicle is coasting. The low vacuum switch on the diesel models deactivates the

converter clutch only during moderate acceleration, ust prior to a part$throttle downshift.=ecause the diesel engines vacuum source is a rotary pump, rather than taken from a

carburetor port, diesel models require both the high and the low vacuum switch to

achieve the same results as the low vacuum switch on the gasoline models.

Com!uter controlled converter clutch

See Figure 9

&ith the use of microcomputers governing the engine fuel and spark delivery, most

manufacturers change the converter clutch electronic control to provide the groundingcircuit for the solenoid valve through the microcomputer, rather than the governor

pressure switch. *ensors are used in place of the formerly used switches and send signals

back to the microcomputer to indicate if the engine is in its proper mode to accept the

mechanical lock$up of the converter clutch.

Aormally a coolant sensor, a throttle position sensor, an engine vacuum sensor and a

vehicle speed sensor are used to signal the microcomputer when the converter clutch can

be applied. *hould a sensor indicate the need for the converter clutch to be deactivated,

the grounding circuit to the transmission solenoid valve would be interrupted and theconverter clutch would be released.


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Figure 9 Typical computer controlled clutch.

"#draulic converter clutch

Aumerous automatic transmissions rely upon hydraulic pressures to sense and determine

when to apply the converter clutch assembly. This type of automatic transmission unit is

considered to be a self$contained unit with only the shift linkage, throttle cable ormodulator valve being e7ternal. *pecific valves, located within the valve body or oilpump housing, are caused to be moved when a sequence of events occur within the unit.

'or e7ample, to engage the converter clutch, most all automatic transmissions require the

gear ratio to be in the top gear before the converter clutch control valves can be placed inoperation. The governor and throttle pressures must maintain specific fluid pressures at

various points within the hydraulic circuits to aid in the engagement or disengagement of

the converter clutch. In addition, check valves must properly seal and move to e7haustpressured fluid at the correct time to avoid 1shudders1 or 1chuggles1 during the initial

application and engagement of the converter clutch.

Centrifu$al torque converter clutchSee Figure 10

# torque converter was used that locks up mechanically without the use of electronics orhydraulic pressure. #t specific input shaft speeds, brake$like shoes move outward from

the rim of the turbine assembly, to engage the converter housing, locking the converter

unit mechanically together for a 5:5 ratio. *light slippage can occur at the low end of therpm scale, but the greater the rpm, the tighter the lock$up. #gain, it must be mentioned,

that when the converter has locked$up, the vehicle may respond in the same manner as

driving with a clutch and standard transmission. This is considered normal and does notindicate converter clutch or transmission problems. @eep in mind if engines are in need

of tune$ups or repairs, the lock$up 1shudder1 or 1chuggle1 feeling may be greater.


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igure 10 37ploded view of the centrifugal lock$up converter.

Mechanical converter loc%&u!

#nother type of converter lock$up is the 'ord Botor 0ompanys #"< #utomatic"verdrive transmission, which uses a direct drive input shaft splined to the damper

assembly of the torque converter cover to the direct clutch, bypassing the torque

converter reduction components. # second shaft encloses the direct drive input shaft andis coupled between the converter turbine and the reverse clutch or forward clutch,

depending upon their applied phase. &ith this type of unit, when in third gear, the input

shaft torque is split, >6 hydraulic and /6 mechanical. &hen in the overdrive or fourthgear, the input torque is completely mechanical and the transmission is locked

mechanically to the engine.

Overdrive units

See Figure 11

&hen the need for greater fuel economy stirred the worlds automakers into action, theautomatic transmissionCtransa7les were among the many vehicle components that were

modified to aid in this quest. Internal changes have been made and in some cases,

additions of a fourth gear to provide the over direct or overdrive gear ratio. The reasoningfor adding the overdrive capability is that an overdrive ratio enables the output speed of

the transmissionCtransa7le to be greater than the input speed, allowing the vehicle to


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maintain a given road speed with less engine speed. This results in better fuel economy

and a slower running engine.

The overdrive unit usually consists of an overdrive planetary gear set, a roller one$wayclutch assembly and two friction clutch assemblies, one as an internal clutch pack and the

second for a brake clutch pack. The overdrive carrier is splined to the turbine shaft, whichin turn, is splined into the converter turbine.

#nother type of overdrive assembly is a separation of the overdrive components byhaving them at various points along the gear train assembly and utili;ing them for other

gear ranges. Instead of having a brake clutch pack, an overdrive band is used to lock the

planetary sun gear. In this type of transmission, the converter cover drives the direct driveshaft clockwise at engine speed, which in turn drives the direct clutch. The direct clutch

then drives the planetary carrier assembly at engine speed in a clockwise direction. The

pinion gears of the planetary gear assembly 1walk around1 the stationary reverse sun

gear, again in a clockwise rotation. The ring gear and output shafts are therefore driven at

a faster speed by the rotation of the planetary pinions. =ecause the input is 56mechanical drive, the converter can be classified as a lock$up converter in the overdrive

position.

Figure 11 37ploded and sectional views of direct drive and overdrive power flows


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The !lanetar# $ear'o(

See Figures 12, 13 and 14

The rear section of the transmission is the gearbo7, containing the gear train and valvebody to shift the gears.

The ability of the torque converter to multiply engine torque is limited, so the unit tends

to be more efficient when the turbine is rotating at relatively high speeds. # planetary

gearbo7 is used to carry the power output from the turbine to the driveshaft to make themost efficient use of the converter.

Dlanetary gears function very similarly to conventional transmission gears. Their

construction is different in that three elements make up one gear system, and in that the

three elements are different from one another. The three elements are:

#n outer gear that is shaped like a hoop, with teeth cut into the inner surface.

# sun gear mounted on a shaft and located at the very center of the outer gear. # set of three planet gears, held by pins in a ring$like planet carrier and meshing

with both the sun gear and the outer gear.

3ither the outer gear or the sun gear may be held stationary, providing more than onepossible torque multiplication factor for each set of gears. If all three gears are forced to

rotate at the same speed, the gear set forms, in effect, a solid shaft.

=ands and clutches are used to hold various portions of the gear$sets to the transmission

case or to the shaft on which they are mounted.

Figure 12 Dlanetary gears are similar to manual transmission gears, but are composed of three parts.


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Figure 13 Dlanetary gears in ma7imum reduction (low). The ring gear is held and a lower gear ratio is

obtained.

Figure 14 Dlanetary gears in the minimum reduction (


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# valve body contains small hydraulic pistons and cylinders. 'luid enters the cylinder

under pressure and forces the pistons to move to engage the bands or clutches.

The hydraulic fluid used to operate the valve body comes from the main transmission oilpump. This fluid is channeled to the various pistons through the shift valves. There is

generally a manual shift valve that is operated by the transmission selector lever and anautomatic shift valve for each automatic upshift the transmission provides. Two$speed

automatics have a low$high shift valve while three$speeds will have a 5$E shift valve,and a E$> shift valve whereas four$speeds have a 5$E shift valve, a E$> shift valve, and a

>$8 shift valve.

Two pressures affect the operation of these valves. "ne (governor pressure) is determinedby vehicle speed, while the other (modulator pressure) is determined by intake manifold

vacuum or throttle position. !overnor pressure rises with an increase in vehicle speed,

and modulator pressure rises as the throttle is opened wider. =y responding to these two

pressures, the shift valves cause the upshift points to be delayed with increased throttle

opening to make the best use of the engines power output. If the accelerator is pushedfurther to the floor the upshift will be delayed longer, (the vehicle will stay in gear).

The transmission modulator also governs line pressure, used to actuate the servos. In this

way, the clutches and bands will be actuated with a force matching the torque output ofthe engine.

Bost transmissions also make use of an au7iliary circuit for downshifting. This circuit

may be actuated by the throttle linkage or the vacuum line that actuates the modulator orby a cable or solenoid. It applies pressure to a special downshift surface on the shift valve

or valves, to shift back to low gear as vehicle speed decreases.

Figure 15 *ervos, operated by pressure, are used to apply or release the bands, either holding the ring gear

or allowing it to rotate.


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Figure 16 The valve body, containing the shift valves, is normally located at the bottom of the

transmission. The shift valves (there are many more than shown) are operated by hydraulic pressure.

Transa(les

&hen the transmission and the drive a7le are combined in one unit, it is called a

1transa7le.1 The transa7le is bolted to the engine and has the advantage of being an

e7tremely rigid unit of engine and driveline components. The complete engine transa7le

unit may be located at the front of the vehicle (front wheel drive) or at the rear of thevehicle (rear wheel drive).

The power flow through the transmission section of the transa7le is the same as through a

conventional transmission

Automatic Transmission Components

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