the automobile has become so sophisticated and the automatic transmissions so reliable

8/6/2019 The Automobile Has Become So Sophisticated and the Automatic Transmissions So Reliable

1/73

The automobile has become so sophisticated and theautomatic transmissions so reliable; that automatictransmissions are the most popular option, or are evenstandard on many models. Over 85% of all new vehicles are

ordered with an automatic transmission. All the driver has todo is start the engine, select a gear and operate theaccelerator and brakes. It may not be as much fun asshifting gears, but it is far more efficient if you haul heavyloads or pull a trailer.

The automatic transmission anticipates the engines needsand selects gears in response to various inputs (enginevacuum, road speed, throttle position, etc.) to maintain the

best application of power. The operations usually performedby the clutch and manual transmission are accomplishedautomatically, through the use of the fluid coupling, whichallows a very slight, controlled slippage between the engineand transmission. Tiny hydraulic valves control theapplication of different gear ratios on demand by the driver(position of the accelerator pedal), or in a preset response toengine conditions and road speed.

How the automatic transmission worksSee Figures 1, 2 and 3

The automatic transmission allows engine torque and powerto be transmitted to the drive wheels within a narrow rangeof engine operating speeds. The transmission will allow theengine to turn fast enough to produce plenty of power andtorque at very low speeds, while keeping it at a sensible rpmat high vehicle speeds.

The transmission uses a light fluid as the medium for thetransmission of power. This fluid also operates the hydrauliccontrol circuits and acts as a lubricant. Because thetransmission fluid performs all of these three functions,trouble within the unit can easily travel from one part toanother.
http://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_autotrans1.asp#Figure1%23Figure1http://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_autotrans1.asp#Figure1%23Figure1


2/73

The automatic transmission operates on a principle thatfluids cannot be compressed, and that when put into motion,will cause a similar reaction upon any resisting force. Tounderstand this law of fluids, think of two fans placed

opposite each other. If one fan is turned on, it will begin toturn the opposite fan blades. This principle is applied to theoperation of the fluid coupling and torque converter by usingdriving and driven members in place of fan blades.

Every type of automatic transmission has two sections. Thefront section contains the fluid coupling or torque converterand takes the place of the driver operated clutch. The rearsection contains the valve body assembly and the

hydraulically controlled gear units, which take the place ofthe manually shifted standard transmission.

Figure 1 Basic components of a automatictransmission.


3/73

Figure 2 Cutaway view of a typical 3-speed automatictransmission showing the basic components.

Figure 3 Cutaway view of a typical 3-speedautomatic transaxle showing the basic

components.
http://www.icarumba.com/includes/content/resourcecenter/encyclopedia/ch21/21Fig3.htmlhttp://www.icarumba.com/includes/content/resourcecenter/encyclopedia/ch21/21Fig2.html


4/73

Electronic transmission controls

See Figure 4

Numerous changes have occurred in transaxles andtransmissions in the last decade. The demand for lighter,smaller and more fuel efficient vehicles has resulted in theuse of electronics to control both the engine andtransmission to achieve the fuel efficient results that arerequired by law. The transaxle/transmission assembly is apart of the electronic controls, by sending signals of vehiclespeed to an on-board computer which, in turn, relates thesesignals, along with others from the engine assembly, to

determine gear selection for the best performance.

Sensors are used for engine and road speeds, engine load,gear selector lever position, and the kickdown switchoperation. In addition, the driving program, set by thefactory, is used to send signals to the microcomputer todetermine the optimum gear selection, according to a presetprogram. The shifting is accomplished by solenoid valves inthe hydraulic system. The electronics also control themodulated hydraulic pressure during shifting, along withregulating engine torque to provide smooth shifts betweengear ratio changes. This type of system can be designed fordifferent driving programs, such as giving the operator thechoice of operating the vehicle for either economy orperformance.

The transmission's sensors also let the operator of thevehicle know if there are any problems with the system. Ifthe transmission control computer detects a problem it will

store a trouble code in memory and it will light or flash atransmission warning lamp (or engine service light) on thedash to alert the operator something is wrong. Using theproper scan tools or techniques, a technician can retrievethe code (depending on the manufacturer) in order to helpdiagnose the trouble. To get a better understanding of


5/73

engine and transmission trouble codes, see a repair manualfor your year/make/model car.

Figure 4 Electronic controlled transmissions use solenoids forgear selection with microcomputer control.

Brake shift interlock system

As a safety feature on some vehicles the transmission willnot allow the operator to shift into drive or start the car untilthey place their foot onto the brake pedal. This system isusually controlled by a cable from the brake pedal to thetransmission or the transmission computer and sensors. Onsome vehicles the driver will hear a clicking which is thesensor operation

Automatic transmission components


6/73

TorqueconverterSee Figures 5 and 6

The front section is called the torque converter. In replacing

the traditional clutch, it performs three functions:

It acts as a hydraulic clutch (fluid coupling), allowingthe engine to idle even with the transmission in gear.

It allows the transmission to shift from gear to gearsmoothly, without requiring that the driver close thethrottle during the shift.

It multiplies engine torque making the transmissionmore responsive and reducing the amount of shiftingrequired.

The torque converter is a metal case that is shaped like asphere that has been flattened on opposite sides and isbolted to the rear of the engine's crankshaft. Generally, theentire metal case rotates at engine speed and serves as theengine's flywheel.

The case contains three sets of blades. One set is attacheddirectly to the case forming the impeller or pump. Another

set is directly connected to the output shaft, and forms theturbine. The third set (stator) is mounted on a hub which, inturn, is mounted on a stationary shaft through a one-wayclutch. Rollers are wedged into slots, preventing backwardrotation. When the rollers are not in the slots, the statorturns in the same direction as the impeller. The pump, whichis driven by the converter hub at engine speed, keeps thetorque converter full of transmission fluid at all times. Fluidflows continuously through the unit to provide cooling.

A fluid coupling will only transmit the torque the enginedevelops; it cannot increase the torque. This is one job ofthe torque converter. The impeller drive member is driven atengine speed by the engine's crankshaft and pumps fluid, toits center, which is flung outward by centrifugal force as itturns. Since the outer edge of the converter spins faster


7/73

than the center, the fluid gains speed. Fluid is directedtoward the turbine driven member by curved impellerblades, causing the turbine to rotate in the same direction asthe impeller. The turbine blades are curved in the opposite

direction of the impeller blades.

In flowing through the pump and turbine, the fluid flows intwo separate directions. It flows through the turbine blades,and it spins with the engine. The stator, whose blades arestationary when the vehicle is being accelerated at lowspeeds, converts one type of flow into another. Instead ofallowing the fluid to flow straight back into the pump, thestator's curved blades turn the fluid almost 90 toward the

direction of rotation of the engine. Thus the fluid does notflow as fast toward the pump, but is already spinning whenthe pump picks it up. This has the effect of allowing thepump to turn much faster than the turbine. This difference inspeed may be compared to the difference in speed betweenthe smaller and larger gears in any gear train. The result isthat engine power output is higher, and engine torque ismultiplied.

As the speed of the turbine increases, the fluid spins fasterand faster in the direction of engine rotation. Therefore, theability of the stator to redirect the fluid flow is reduced.Under cruising conditions, the stator is eventually forced torotate on its one-way clutch and the torque converter beginsto behave almost like a solid shaft, with the pump andturbine speeds being almost equal.

In the late 70's, Chrysler Corporation introduced anautomatic transmission, featuring what is called a "lock-up"

clutch in the transmission's torque converter. The lock-up isa fully automatic clutch that engages only when thetransmission shifts into top gear or when needed based on apredetermined demand factor.


8/73

The lock-up clutch is activated by a piston. When engaged,the lock-up clutch gives the benefits of a manualtransmission, eliminating torque converter slippage. In theengaged position, engine torque is delivered mechanically,

rather than hydrodynamically (through fluid). This givesimproved fuel economy and cooler transmission operatingtemperatures.

In the early 80's, Ford introduced what is known as theAutomatic Overdrive Transmission (AOT). Essentially, thistransmission uses a lock-up torque converter, by offering anadditional refinement. The transmission is a four-speed unit,with fourth gear as an overdrive (0.67:1). Torque is

transmitted via a full mechanical lock-up from the engine,completely bypassing the torque converter and eliminatinghydraulic slippage.

In third gear (1:1 ratio), engine power follows a "split-torque" path, in which there is a 60% lock-up. Sixty percentof the power is transmitted through solid connections and40% of the engine power is delivered through the torqueconverter.

Throughout the 90's, Subaru introduced an ElectronicContinuously Variable Transmission (ECVT) and Hondaintroduced their version (CVT). This transmission uses ametal belt and two variable-diameter pulleys to keepsmooth, uninterrupted range of gearing. Size of the pulleysis controlled through the use of hydraulics. This unitproduces a miles per gallon closer to a manual transmission,while attaining a smoother shift than an automatictransmission.


9/73

Figure 5 The torque converter housing is rotated by theengine crankshaft and turns the impeller. The impeller spinsthe turbine, which gives motion to the turbine (output) shaftto drive the gears.

Figure 6 Sectional view of operations of a typical "lock-up"clutch and torque converter.
http://www.icarumba.com/includes/content/resourcecenter/encyclopedia/ch21/21Fig6.html


10/73

Torque converter clutch controlSee Figures 7 and 8

Electrical and vacuum controls

The torque converter clutch should apply when the enginehas reached near normal operating temperature in order tohandle the slight extra load and when the vehicle speed ishigh enough to allow the operation of the clutch to besmooth and the vehicle to be free of engine pulses.

When the converter clutch is coupled to the engine, theengine pulses can be felt through the vehicle in the samemanner as if equipped with a clutch and standard

transmission. Engine condition, engine load and enginespeed determine the severity of the pulsation.

The converter clutch should release when torquemultiplication is needed in the converter, when coming to astop, or when the mechanical connection would affectexhaust emissions during a coasting condition.

The typical electrical control components consist of the

brake release switch, the low vacuum switch and thegovernor switch. Some vehicle models have a thermalvacuum switch, a relay valve and a delay valve. Dieselengines use a high vacuum switch in addition to certainabove listed components. These various components controlthe flow of current to the apply valve solenoid. By controllingthe current flow, these components activate or deactivatethe solenoid, which in turn engages or disengages thetransmission converter clutch, depending upon the driving

conditions as mentioned previously. The components havethe two basic circuits, electrical and vacuum.


11/73

Figure 7Using electrical and vacuum controls to operate thetorque converter clutch.

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

Electrical current flow

All of the components in the electrical circuit must be closedor grounded before the solenoid can open the hydrauliccircuit to engage the converter clutch. The circuit begins atthe fuse panel and flows to the brake switch and as long asthe brake pedal is not depressed, the current will flow to the


12/73

low vacuum switch on the gasoline engines and to the highvacuum switch on the diesel engines. These two switchesopen or close the circuit path to the solenoid, dependentupon the engine or pump vacuum. If the low vacuum switch

is closed (high vacuum switch on diesel engines), thecurrent continues to flow to the transmission case connector,into the solenoid and to the governor pressure switch. Whenthe vehicle speed is approximately 35-50 mph (56-80 kph) ,the governor switch grounds to activate the solenoid. Thesolenoid, in turn, opens a hydraulic circuit to the converterclutch assembly, engaging the unit.

It should be noted that external vacuum controls include the

thermal vacuum valve, the relay valve, the delay valve, thelow vacuum switch and a high vacuum switch (used ondiesel engines). Keep in mind that all of the electrical orvacuum components may not be used on all engines at thesame time.

Vacuum flow

The vacuum relay valve works with the thermal vacuumvalve 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 stillwarming up. The delay valve slows the response of the lowvacuum switch to changes in engine vacuum. This actionprevents the low vacuum switch from causing the converterclutch to engage and disengage too rapidly. The low vacuumswitch deactivates the converter clutch when engine vacuumdrops to a specific low level during moderate accelerationjust before a part-throttle transmission downshift. The low

vacuum switch also deactivates the clutch while the vehicleis coasting because it receives no vacuum from its portedvacuum source.

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


13/73

coasting. The low vacuum switch on the diesel modelsdeactivates the converter clutch only during moderateacceleration, just prior to a part-throttle downshift. Becausethe diesel engine's vacuum source is a rotary pump, rather

than taken from a carburetor port, diesel models requireboth the high and the low vacuum switch to achieve thesame results as the low vacuum switch on the gasolinemodels.

Computer controlled converter clutchSee Figure 9

With the use of microcomputers governing the engine fueland spark delivery, most manufacturers change theconverter clutch electronic control to provide the groundingcircuit for the solenoid valve through the microcomputer,rather than the governor pressure switch. Sensors are usedin place of the formerly used switches and send signals backto the microcomputer to indicate if the engine is in its propermode to accept the mechanical lock-up of the converterclutch.

Normally a coolant sensor, a throttle position sensor, an

engine vacuum sensor and a vehicle speed sensor are usedto signal the microcomputer when the converter clutch canbe applied. Should a sensor indicate the need for theconverter clutch to be deactivated, the grounding circuit tothe transmission solenoid valve would be interrupted andthe converter clutch would be released.

Figure 9 Typical computer controlled clutch.


14/73

Hydraulic converter clutch

Numerous automatic transmissions rely upon hydraulicpressures to sense and determine when to apply theconverter clutch assembly. This type of automatictransmission unit is considered to be a self-contained unitwith only the shift linkage, throttle cable or modulator valvebeing external. Specific valves, located within the valve bodyor oil pump housing, are caused to be moved when asequence of events occur within the unit. For example, toengage the converter clutch, most all automatictransmissions require the gear ratio to be in the top gearbefore the converter clutch control valves can be placed inoperation. The governor and throttle pressures mustmaintain specific fluid pressures at various points within thehydraulic circuits to aid in the engagement ordisengagement of the converter clutch. In addition, checkvalves must properly seal and move to exhaust pressuredfluid at the correct time to avoid "shudders" or "chuggles"during the initial application and engagement of theconverter clutch.


15/73

Centrifugal torque converter clutchSee Figure 10

A torque converter was used that locks up mechanically

without the use of electronics or hydraulic pressure. Atspecific input shaft speeds, brake-like shoes move outwardfrom the rim of the turbine assembly, to engage theconverter housing, locking the converter unit mechanicallytogether for a 1:1 ratio. Slight slippage can occur at the lowend of the rpm scale, but the greater the rpm, the tighterthe lock-up. Again, it must be mentioned, that when theconverter has locked-up, the vehicle may respond in thesame manner as driving with a clutch and standard

transmission. This is considered normal and does notindicate converter clutch or transmission problems. Keep inmind if engines are in need of tune-ups or repairs, the lock-up "shudder" or "chuggle" feeling may be greater.

Figure 10 Exploded view of the centrifugal lock-up converter.

Mechanical converter lock-up

Another type of converter lock-up is the Ford MotorCompany's AOD Automatic Overdrive transmission, whichuses a direct drive input shaft splined to the damper
http://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_autotrans2.asp#Figure10%23Figure10http://www.icarumba.com/includes/content/resourcecenter/encyclopedia/ch21/21Fig10.htmlhttp://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_autotrans2.asp#Figure10%23Figure10


16/73

assembly of the torque converter cover to the direct clutch,bypassing the torque converter reduction components. Asecond shaft encloses the direct drive input shaft and iscoupled between the converter turbine and the reverse

clutch or forward clutch, depending upon their appliedphase. With this type of unit, when in third gear, the inputshaft torque is split, 30% hydraulic and 70% mechanical.When in the overdrive or fourth gear, the input torque iscompletely mechanical and the transmission is lockedmechanically to the engine.

Overdrive unitsSee Figure 11

When the need for greater fuel economy stirred the world'sautomakers into action, the automatictransmission/transaxles were among the many vehiclecomponents that were modified to aid in this quest. Internalchanges have been made and in some cases, additions of afourth gear to provide the over direct or overdrive gearratio. The reasoning for adding the overdrive capability isthat an overdrive ratio enables the output speed of thetransmission/transaxle to be greater than the input speed,allowing the vehicle to maintain a given road speed with lessengine speed. This results in better fuel economy and aslower running engine.

The overdrive unit usually consists of an overdrive planetarygear set, a roller one-way clutch assembly and two frictionclutch assemblies, one as an internal clutch pack and thesecond for a brake clutch pack. The overdrive carrier issplined to the turbine shaft, which in turn, is splined into the

converter turbine.

Another type of overdrive assembly is a separation of theoverdrive components by having them at various pointsalong the gear train assembly and utilizing them for othergear ranges. Instead of having a brake clutch pack, an


17/73

overdrive band is used to lock the planetary sun gear. Inthis type of transmission, the converter cover drives thedirect drive shaft clockwise at engine speed, which in turndrives the direct clutch. The direct clutch then drives the

planetary carrier assembly at engine speed in a clockwisedirection. The pinion gears of the planetary gear assembly"walk around" the stationary reverse sun gear, again in aclockwise rotation. The ring gear and output shafts aretherefore driven at a faster speed by the rotation of theplanetary pinions. Because the input is 100% mechanicaldrive, the converter can be classified as a lock-up converterin the overdrive position.

Figure 11 Exploded and sectional views of direct driveand overdrive power flows.


18/73

The planetary gearboxSee Figures 12, 13 and 14

The rear section of the transmission is the gearbox,containing the gear train and valve body to shift the gears.

The ability of the torque converter to multiply engine torqueis limited, so the unit tends to be more efficient when the

turbine is rotating at relatively high speeds. A planetarygearbox is used to carry the power output from the turbineto the driveshaft to make the most efficient use of theconverter.

Planetary gears function very similarly to conventionaltransmission gears. Their construction is different in thatthree elements make up one gear system, and in that thethree elements are different from one another. The three

elements are:

An outer gear that is shaped like a hoop, with teeth cutinto the inner surface.

A sun gear mounted on a shaft and located at the verycenter of the outer gear.

A set of three planet gears, held by pins in a ring-likeplanet carrier and meshing with both the sun gear andthe outer gear.

Either the outer gear or the sun gear may be heldstationary, providing more than one possible torquemultiplication factor for each set of gears. If all three gearsare forced to rotate at the same speed, the gear set forms,in effect, a solid shaft.


19/73

Bands and clutches are used to hold various portions of thegear-sets to the transmission case or to the shaft on whichthey are mounted.

Figure 12 Planetary gears are similar to manualtransmission gears, but are composed of three parts.

Figure 13 Planetary gears in maximum reduction (low).

The ring gear is held and a lower gear ratio is obtained.

Figure 14 Planetary gears in the minimum reduction


20/73

(Drive). The ring gear is allowed to revolve, providing ahigher gear ratio.

Shifting gearsSee Figures 15 and 16

Shifting is accomplished by changing the portion of eachplanetary gear set that is held to the transmission case orshaft.

A valve body contains small hydraulic pistons and cylinders.

Fluid enters the cylinder under pressure and forces thepistons to move to engage the bands or clutches.

The hydraulic fluid used to operate the valve body comesfrom the main transmission oil pump. This fluid is channeledto the various pistons through the shift valves. There isgenerally a manual shift valve that is operated by the


21/73

transmission selector lever and an automatic shift valve foreach automatic upshift the transmission provides. Two-speed automatics have a low-high shift valve; while three-speeds will have a 1-2 shift valve, and a 2-3 shift valve;

whereas four-speeds have a 1-2 shift valve, a 2-3 shiftvalve, and a 3-4 shift valve.

Two pressures affect the operation of these valves. One(governor pressure) is determined by vehicle speed, whilethe other (modulator pressure) is determined by intakemanifold vacuum or throttle position. Governor pressurerises with an increase in vehicle speed, and modulatorpressure rises as the throttle is opened wider. By responding

to these two pressures, the shift valves cause the upshiftpoints to be delayed with increased throttle opening to makethe best use of the engine's power output. If the acceleratoris pushed further to the floor the upshift will be delayedlonger, (the vehicle will stay in gear).

The transmission modulator also governs line pressure, usedto actuate the servos. In this way, the clutches and bandswill be actuated with a force matching the torque output ofthe engine.

Most transmissions also make use of an auxiliary circuit fordownshifting. This circuit may be actuated by the throttlelinkage or the vacuum line that actuates the modulator or bya cable or solenoid. It applies pressure to a special downshiftsurface on the shift valve or valves, to shift back to low gearas vehicle speed decreases.


22/73

Figure 15Servos, operated by pressure, are used toapply or release the bands, either holding the ringgear or allowing it to rotate.

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.


23/73

Transaxles

When the transmission and the drive axle are combined inone unit, it is called a "transaxle." The transaxle is bolted tothe engine and has the advantage of being an extremelyrigid unit of engine and driveline components. The completeengine transaxle unit may be located at the front of thevehicle (front wheel drive) or at the rear of the vehicle (rearwheel drive).

The power flow through the transmission section of thetransaxle is the same as through a conventionaltransmission.

Automatic transmission maintenance

Automatic transmission fluid

Automatic transmission fluids can be broken down into twotypes, Dexron III and Ford type F. These fluids are specificto the transmission using them. Don't assume that all Fordvehicles use type F, they don't!


24/73

There are the following types of fluids.

Dexron III, sometime referred to as multi purposeATF. This replaces the old Type A, Suffix A, which was

recommended by GM, Chrysler and AMC between1956-1967. It also supercedes Dexron and Dexron II fluids. Ford vehicles 1977 and later with the C6transmission or the Jatco transmission in the Granadaand Monarch also use this fluid. Ford refers to this fluidas Mercon, or on older models as type H or CJ whererecommended.

Type F fluid is recommended by Ford Motor Co. and afew imported manufacturers, and contains certain

frictional compounds required for proper operation inthese transmissions.

There is not much of a problem here, since the bottles areclearly marked to indicate the type of fluid. If you are indoubt, check your owner's manual. Also, some transmissiondipsticks are labeled or stamped with the recommended fluidtype.

Checking fluid level

See Figures 17, 18, 19 and 20

Check the transmission fluid level at least every 6,000 miles(9,654 km) or 6 months, whichever comes first. Underextreme usage the fluid should be checked at shorterintervals or if a problem exists.

In most cases the vehicle should be on a level surface,transmission in Park, and the engine running. The fluid

should be at normal operating temperature. If the vehiclehas been used to haul a trailer or has been on an extendedtrip, wait half an hour before checking so a correct readingcan be measured.

1. Park the vehicle on a level surface, with the parkingbrake on. Start the engine and allow to idle for about


25/73

15 minutes. Move the transmission through the gearsand then back to park.

2. Remove the dipstick and carefully touch the wet end ofthe dipstick to see if fluid is cool, warm, or hot. Wipe it

clean and then reinsert it firmly. Be sure that it ispushed all the way in. Remove the dipstick again whileholding it horizontally.

a. If fluid is cool (room temperature), the levelshould be about 1/8 to 3/8 inches (3-10mm)below the add/cold mark.

b. If fluid is warm, the level should be close to theadd mark, either above or below.

c. If fluid is hot, the level should be at the full/hot

mark.3. If the level is low, add the appropriate fluid through the

dipstick tube. This is easily done with the aid of afunnel. Check the level often as you are filling thetransmission. Be extremely careful not to overfill it.Overfilling may cause slippage, seal damage andoverheating. Typically, 1 pint (0.473L) of ATF will raisethe fluid level from one notch/line to the other.

If the fluid on the dipstick appears discolored (brownor black), or smells burnt, serious transmissiontroubles (probably due to overheating) should besuspected. The transmission should be inspected by a


26/73

qualified technician to locate the cause of the burntfluid.

Figure 17 Remove the dipstick and wipeclean. Reinsert the dipstick all the way.Remove it again and check the fluid level.


27/73

Figure 19 The fluid level should be between the ADD andFULL marks depending upon transmission temperature,cold (A) or hot (B). Also, check the appearance of thefluid.

Figure 20 If the level is low, add fluid through thedipstick tube, using a long funnel. Do not mix fluid typesand do not overfill.


28/73

Fluid temperatureSee Figure 21

Transmission fluid is designed to last many thousands ofmiles under normal conditions. However, one of the mostimportant factors affecting the life of the fluid and thetransmission is the temperature of the fluid. Overheatedfluid forms sludge and particles of carbon that can block the

minute passages and lines that circulate the fluid throughoutthe transmission. This causes the transmission to overheateven more and will lead to eventual failure of thetransmission.

Some cars come from the factory with coolers that help withthe temperature. The transmission oil flows through thecooler as air flows across the cooler to lower thetemperature of the transmission fluid. The coolers can bepurchased at any after-market store for most cars. Somecars have warning lights for the transmission that will alertthe owner of any maintenance intervals or overheatingproblems.

Anything that puts a load on the engine can cause thetransmission to heat up and speed the deterioration of the


29/73

fluid. Towing a trailer, idling in traffic and climbing long hillsis all hard on a transmission. The accompanying graphillustrates just how much transmission temperature affectsthe life of transmission components. Fluid that lasts 50,000

miles (80,450 km) at a temperature of 220F (104C), willonly last half that long if the temperature is consistently 20higher.

The secret to long transmission life is regular fluid changesand keeping an eye on the condition of the fluid-bothtemperature and color.

Transmission fluid indications

The appearance and odor of the transmission fluid can givevaluable clues to the overall condition of the transmission.Always note the appearance of the fluid when you check thefluid level or change the fluid. Rub a small amount of fluidbetween your fingers to feel for grit and smell the fluid onthe dipstick.

If the fluid appears: It indicates:


30/73

Clear and red colored Normal operation

Discolored (extremely dark redor brownish)or smells burnt

Band or clutch pack failure,usually caused by anoverheated transmission.

Hauling very heavy loadswith insufficient power orfailure to change the fluidoften result in overheating.

Do not confuse thisappearance with newerfluids that have a darker redcolor and a strong odor

(though not a burnt odor).Foamy or aerated (light in colorand full of bubbles)

The level of fluid is too high(gear train is churningfluid).

An internal air leak (air ismixing with the fluid). Havethe transmission checkedprofessionally

Solid residue in the fluid

Defective bands, clutch packor bearings. Bits of bandmaterial or metal abrasivesare clinging to the dipstick.Have the transmissionchecked professionally.

Varnish coating on the dipstick The transmission isoverheating.

Figure 21 Transmission temperature indications.


31/73

Checking for leaks

If the fluid level is consistently low, suspect a leak. Theeasiest way is to slip a piece of clean newspaper under thevehicle overnight, but this is not always an accurateindication, since some leaks will occur only when thetransmission is operating.

Other leaks can be located by driving the vehicle. Wipe theunderside of the transmission clean and drive the vehicle forseveral miles to bring the fluid temperature to normal. Stopthe vehicle, shut OFF the engine and look for leakage.

Remember, however, that where the fluid is locatedmay not be the source of the leak. Airflow around thetransmission while the vehicle is moving may carrythe fluid to some other point.

Servicing the transmission

Aside from changing the fluid and filter and tightening nutsand bolts, repair or overhaul of the automatic transmissionshould be left to a trained technician. This is because thereare special tools needed for inspection (such as hydraulic


32/73

pressure testers, electronic scan tools for retrieving troublecodes, etc.) and for the repair of the transmissions.Transmissions are heavy and special jacks or hydraulictables are needed to remove them from the vehicle. In some

cases special tools are used to separate the transmissionfrom the engine.


33/73

CLUTCH

Introduction

To overcome inertia and start the vehicle moving, theautomobile engine develops power that is transmitted as atwisting force (torque) from the engine crankshaft to therear wheels. A smooth and gradual transfer of power andtorque is accomplished using a clutch friction unit to engageand disengage the power flow. A transmission is used tovary the gear ratio for the best speed and power, and toprovide for vehicle movement under the different conditionsof starting, stopping, accelerating, maintaining speed andreversing. The various components necessary to deliverpower to the drive wheels are the flywheel, pressure plate,clutch plate, release bearing, control linkages and thetransmission.

The clutch-driven disc may contain asbestos, whichhas been determined to be a cancer-causing agent.Never clean clutch surfaces with compressed air!

Avoid inhaling any dust from any clutch surface! Whencleaning clutch surfaces, use a commercially availablebrake cleaning fluid.

How the clutch worksSee Figure 1

The clutch is a device to engage and disengage power fromthe engine, allowing the vehicle to be stopped and started.

A pressure plate or "driving member" is bolted to the engineflywheel and a clutch plate or "driven member" is locatedbetween the flywheel and the pressure plate. The clutchplate is splined to the shaft extending from the transmissionto the flywheel, commonly called a clutch shaft or inputshaft.
http://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_manualtran.asp#Figure1%23Figure1http://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_manualtran.asp#Figure1%23Figure1


34/73

When the clutch and pressure plates are locked together byfriction, the clutch shaft rotates with the engine crankshaft.Power is transferred from the engine to the transmission,where it is routed through different gear ratios to obtain the

best speed and power to start and keep the vehicle moving.

Figure 1 Clutch engagement and disengagement.

The flywheel

See Figure 2

The flywheel is located at the rear of the engine and isbolted to the crankshaft. It helps absorb power impulses,resulting in a smoothly-idling engine and providesmomentum to carry the engine through its operating cycle.The rear surface of the flywheel is machined flat and theclutch components are attached to it.


35/73

Figure 2 Typical clutch components.

The pressureplateSee Figures 3, 4, 5 and 6

The driving member is commonly called the pressure plate.It is bolted to the engine flywheel and its main purpose is toexert pressure against the clutch plate, holding the platetight against the flywheel and allowing the power to flow

from the engine to the transmission. It must also be capableof interrupting the power flow by releasing the pressure onthe clutch plate. This allows the clutch plate to stop rotatingwhile the flywheel and pressure plate continues to rotate.

The pressure plate consists of a heavy metal plate, coilsprings or a diaphragm spring, release levers (fingers), anda cover.

When coil springs are used, they are evenly spaced aroundthe metal plate and located between the plate and the metalcover. This places an even pressure against the plate, whichin turn presses the clutch plate tight against the flywheel.The cover is bolted tightly to the flywheel and the metalplate is movable, due to internal linkages. The coil springsare arranged to exert direct or indirect tension on the metal


36/73

plate, depending upon the manufacturer's design. Threerelease levers (fingers), evenly spaced around the cover, areused on most pressure plates to release the holdingpressure of the springs on the clutch plate, allowing it to

disengage the power flow.

When a diaphragm spring is used instead of coil springs, theinternal linkage is necessarily different to provide an "over-center" action to release the clutch plate from the flywheel.Its operation can be compared to the operation of an oilcan.When depressing the slightly curved metal on the bottom ofthe can, it goes over-center and gives out a loud "clicking"noise; when released the noise is again heard as the metal

returns to its original position. A click is not heard in theclutch operation, but the action of the diaphragm spring isthe same as the oil can.

Figure 3 The operation of a diaphragm spring-typepressure plate can be compared to the effect of pressingthe bottom of a can of oil.


37/73

Figure 4 Typical clutch pressure plate assembly.

Figure 5The underside of the pressure plate contains thefriction surface, which is compressed against the clutchdisc.


38/73

Figure 6 The clutch pressure plate assembly is bolted tothe flywheel on the back of the engine.

The clutch plateSee Figures 7 and 8

The clutch plate or driven member consists of a round metal

plate attached to a splined hub. The outer portion of theround plate is covered with a friction material of molded orwoven asbestos and is riveted or bonded to the plate. Thethickness of the clutch plate and/or facings may be warpedto give a softer clutch engagement. Coil springs are ofteninstalled in the hub to help provide a cushion against thetwisting force of clutch engagement. The splined hub ismated to (and turns) a splined transmission shaft when theclutch is engaged.


39/73

Figure 7 Typical clutch-driven disc plate.

Figure 8 The clutch disc is installed between the pressureplate assembly and the flywheel.

The release bearing

The release (throw out) bearing is usually a ball bearingunit, mounted on a sleeve, and attached to the release orthrowout lever. Its purpose is to apply pressure to thediaphragm spring or the release levers in the pressure plate.


40/73

When the clutch pedal is depressed, the pressure of therelease bearing or lever actuates the internal linkages of thepressure plate, releasing the clutch plate and interruptingthe power flow. The release bearing is not in constant

contact with the pressure plate. A linkage adjustmentclearance should be maintained.

Power flow disengagement

Mechanical clutch activationSee Figure 9

The clutch pedal provides mechanical means for the driverto control the engagement and disengagement of the clutch.

The pedal is connected mechanically to either a cable orrods, which are directly connected to the release bearinglever.

When the clutch pedal is depressed, the linkage moves therelease bearing lever. The release lever is attached at theopposite end to a release bearing which straddles thetransmission clutch shaft, and presses inward on thepressure plate fingers or the diaphragm spring. This inward

pressure acts upon the fingers and internal linkage of thepressure plate and allows the clutch plate to move awayfrom the flywheel, interrupting the flow of power.


41/73

Figure 9 Cross-sectional view of a typical clutch. Note themechanical clutch linkage.

Hydraulic clutch activationSee Figure 10

Hydraulic clutch activation systems consist of a master and a

slave cylinder. When pressure is applied to the clutch pedal(the pedal is depressed), the pushrod contacts the plungerand pushes it up the bore of the master cylinder. During thefirst 1/32 in. (0.8 mm) of movement, the center valve sealcloses the port to the fluid reservoir tank and as the plungercontinues to move up the bore of the cylinder, the fluid isforced through the outlet line to the slave cylinder mountedon the clutch housing. As fluid is pushed down the pipe fromthe master cylinder, this in turn forces the piston in the

slave cylinder outward. A pushrod is connected to the slavecylinder and rides in the pocket of the clutch fork. As theslave cylinder piston moves rearward the pushrod forces theclutch fork and the release bearing to disengage thepressure plate from the clutch disc. On the return stroke(pedal released), the plunger moves back as a result of thereturn pressure of the clutch. Fluid returns to the master


42/73

cylinder and the final movement of the plunger lifts thevalve seal off the seat, allowing an unrestricted flow of fluidbetween the system and the reservoir.

A piston return spring in the slave cylinder preloads theclutch linkage and assures contact of the release bearingwith the clutch release fingers at all times. As the driven discwears, the diaphragm spring fingers move rearward forcingthe release bearing, fork and pushrod to move. Thismovement forces the slave cylinder piston forward in itsbore, displacing hydraulic fluid up into the master cylinderreservoir, thereby providing the self-adjusting feature of thehydraulic clutch linkage system.

Figure 10 Typical clutch hydraulic actuating systemcomponents.

Power flow engagement

While the clutch pedal is depressed and the power flowinterrupted, the transmission can be shifted into any gear.The clutch pedal is slowly released to gradually move theclutch plate toward the flywheels under pressure of thepressure plate springs. The friction between the clutch plate


43/73

and flywheel becomes greater as the pedal is released andthe engine speed increased. Once the vehicle is moving, theneed for clutch slippage is lessened, and the clutch pedalcan be fully released.

Coordination between the clutch pedal and accelerator isimportant to avoid engine stalling, shock to the drivelinecomponents and excessive clutch slippage and overheating.

How the manual transmission works

The internal combustion engine creates a twisting motion ortorque, which is transferred to the drive wheels. However,the engine cannot develop much torque at low speeds; it will

only develop maximum torque at higher speeds. Thetransmission, with its varied gear ratios, provides a meansof providing this low torque to move the vehicle.

The transmission gear ratios allow the engine to be operatedmost efficiently under a variety of driving and loadconditions. Using gear ratios, the need for extremely highengine rpm at high road speeds is avoided.

The modern transmission provides both speed and powerthrough selected gear sizes that are engineered for the bestall-around performance. A power (lower) gear ratio startsthe vehicle moving and speed gear ratios keep the vehiclemoving. By shifting to gears of different ratios, the drivercan match engine speed to road conditions.

Gear ratiosSee Figure 11

To obtain maximum performance and efficiency, gear ratiosare engineered to each type of vehicle, dependent uponsuch items as the size of the engine, the vehicle weight andexpected loaded weight, etc.


44/73

The gear ratio can be determined by counting the teeth onboth gears. For example, if the driving gear has 20 teethand the driven gear has 40 teeth, the gear ratio is 2 to 1.The driven gear makes one revolution for every two

revolutions of the drive gear. If the driving gear has 40teeth and the driven gear 20 teeth, the gear ratio is 1 to 2.The driven gear revolves twice, while the drive gear revolvesonce.

The transmissions used today may have four, five or sixspeeds forward, but all have one speed in reverse. Thereverse gear is necessary because the engine rotates only inone direction and cannot be reversed. The reversing

procedure must be accomplished inside the transmission.

By comparing gear ratios, you can see which transmissiontransmits more power to the drive wheels at the sameengine rpm. The five-speed transmission's low or first gearwith a ratio of 3.61 to 1 means that for 3.61 revolutions ofthe input or clutch shaft (coupled to the engine by theclutch), the output shaft of the transmission will rotate once.This provides more power to the drive wheels, compared tothe four-speed transmission's low gear of 2.33 to 1.

When the transmission is shifted into the high gear in thefour-speed transmission, and fourth gear in the five-speedtransmission, the gear ratio is usually 1 to 1 (direct drive).For every rotation of the engine and input shaft, the outputshaft is rotating one turn.

Fifth gear in a five-speed transmission is usually anoverdrive. This gear is used for higher speed driving where

very little load is placed on the engine. This gear ratioprovides better economy by lowering the engine RPM tomaintain a specific speed. The input shaft rotates only 0.87of a turn, while the output shaft rotates one revolution,resulting in the output shaft rotating faster than the inputshaft.


45/73

The overdrive is normally incorporated in the transmission.

Figure 11 Example of determining gear ratio. Gear ratiocan be found by dividing the number of teeth on the smaller

gear into the number of the teeth on the larger gear.

Syncromesh transmissionSee Figure 12

The power flow illustrated in a typical four-speed is aconventional, spur-geared transmission. To obtain a quietoperation and gear engagement, synchronizing clutches areadded to the main-shaft gears. The addition of synchronizersallows the gears to be in constant mesh with the clustergears (gears that provide a connection between input andoutput shafts), and the synchronizing clutch mechanismlocks the gears together.

The main purpose of the synchronizer is to speed up or slowdown the rotation speeds of the shaft and gear, until bothare rotating at the same speeds so that both can be lockedtogether without a gear clash.


46/73

Since the vehicle is normally standing still when shifted intoreverse gear, a synchronizing clutch is ordinarily not used onreverse gear.

Figure 12 Power flow through a four-speed, fullysynchronized transmission.

Five and six-speed transmissions

The power flow through the five-speed transmission can becharted in the same manner as the four-speed transmission.

As a rule, the power flow in high gear is usually straightthrough the transmission-input shaft to the mainshaft, whichwould be locked together. When in the reduction gears, thepower flow is through the input shaft, to the cluster gearunit, and through the reduction gear to the mainshaft.

Transaxles

See Figures 13 and 14

When the transmission and the drive axle are combined inone unit, it is called a "transaxle." The transaxle is bolted tothe engine and has the advantage of being an extremelyrigid unit of engine and driveline components. The complete
http://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_manualtran.asp#Figure13%23Figure13http://www.icarumba.com/includes/content/resourcecenter/encyclopedia/ch20/20Fig12.htmlhttp://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_manualtran.asp#Figure13%23Figure13


47/73

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

The power flow through the transmission section of thetransaxle is the same as through a conventionaltransmission.

Figure 13 Transaxles combine the transmission anddifferential into one unit.


48/73

Figure 14 This simple illustration shows the power flowthrough a front wheel drive transaxle. Note how the directionof rotation of the engine crankcase and the axle shafts(attached to the wheels) are the same. This is made possible

using an intermediate shaft.

Figure 16 Clutch and transmission maintenance intervals.

TRAMISSION SYSTEM
http://www.icarumba.com/includes/content/resourcecenter/encyclopedia/ch20/20Fig16.htmlhttp://www.icarumba.com/includes/content/resourcecenter/encyclopedia/ch20/20Fig14.html


49/73

DriveshaftsSee Figures 1 and 2

In a conventional longitudinally mounted front-engine/rear

wheel drive vehicle, a driveshaft is used to transfer thetorque from the engine, through the transmission outputshaft, to the differential in the axle, which in turn transmitstorque to the wheels. The driveshaft can be made out ofsteel or aluminum and can be either solid or hollow(tubular).

A splined slip yoke assembly, either as an integral part ofthe shaft or utilizing a splined transmission output shaft,permits the driveshaft to move forward and rearward as theaxle moves up and down. This provides smooth performanceduring vehicle operation.

Figure 1 Cut-away view of a typical solid driveshaft andrelated components.

On some four wheel drive vehicles, a front driveshaftconnects the power flow from the transfer case to the frontdrive axle.
http://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_driveshaft1.asp#Figure1%23Figure1http://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_driveshaft1.asp#Figure1%23Figure1


50/73

The driveshaft uses flexible joints, called Universal joints (U-joints) or Constant Velocity joints (CV-joints) to couple thetransmission/transaxle to the drive axle/drive wheels. Referto the Universal and Constant Velocity joints section for

more information.

Front wheel drive vehicles also utilize driveshafts, althoughthey are usually referred to as halfshafts. The halfshafts areusually equipped with CV-joints on each end which allow thewheels to turn as well as move up and down while stillsmoothly transferring engine power to the wheels. Frontwheel drive vehicles typically use a transaxle (a combinationTRANSmission and drive AXLE)

Figure 2 Exploded view of a typical front wheel drivehalfshaft assembly using CV joint components on bothends.

Some rear and four wheel drive vehicles use halfshafts.These vehicles will usually have a rigidly mounteddifferential and an independent suspension with halfshaftslinking the differential to the drive wheels. For example, the1998 Chevrolet Corvette--not only does it use halfshafts todrive the rear wheels, the rigidly mounted transaxle isactually in the rear of the vehicle with a driveshaft


51/73

connecting the front mounted engine to the transaxle! Asanother example, the four wheel drive Subaru models use amodified front wheel drive transaxle assembly with anadditional power output. A driveshaft couples the front

transaxle to the rear differential with four halfshafts drivingthe front and rear wheels.

Universal and constant velocity jointsSee Figures 3, 4, 5, 6 and 7

Because of changes in the angle between the driveshaft orhalfshaft and the axle housing or driven wheel, U-joints andCV-joints are used to provide flexibility. The engine ismounted rigidly to the vehicle frame (or sub-frame), whilethe driven wheels are free to move up and down in relationto the vehicle frame. The angle between the driveshaft orhalfshaft and the axle housing or driven wheels changesconstantly as the vehicle responds to various roadconditions.

Figure 3 U-joints are necessary to compensate forchanges in the angle between the driveshaft and the driveaxle.

To give flexibility and still transmit power as smoothly aspossible, several types of U-joints or CV-joints are used.


52/73

The most common type of universal joint is the cross andyoke type. Yokes are used on the ends of the driveshaft withthe yoke arms opposite each other. Another yoke is usedopposite the driveshaft and, when placed together, both

yokes engage a center member, or cross, with four armsspaced 90 apart. A bearing cup is used on each arm of thecross to accommodate movement as the driveshaft rotates.

Figure 4 Exploded view of a typical cross and yoke universalassembly.

The second type is the ball and trunnion universal, a T-shaped shaft that is enclosed in the body of the joint. The

trunnion ends are each equipped with a ball mounted onneedle bearings that move freely in grooves in the outerbody of the joint, in effect creating a slip-joint. This type ofjoint is always enclosed.


53/73

Figure 5 Cut-away view of a typical enclosed ball andtrunnion type U-joint.

A conventional universal joint will cause the driveshaft tospeed up or slow through each revolution and cause acorresponding change in the velocity of the driven shaft.This change in speed causes natural vibrations to occurthrough the driveline necessitating a third type of universal joint -- the double cardan joint. A rolling ball moves in acurved groove, located between two yoke-and-crossuniversal joints, connected to each other by a coupling yoke.The result is uniform motion as the driveshaft rotates,avoiding the fluctuations in driveshaft speeds.


54/73

Figure 6 Exploded view of a typical double cardan U-jointassembly.

The CV-joints, which are most commonly associated withfront wheel drive vehicles, include the Rzeppa, the doubleoffset, Tri-pod and Birfield joint.

The Rzeppa and double offset are similar in construction.They use a multi-grooved cross which is attached to theshaft. Balls ride in the cross grooves and are retained to thecross by a cage. The entire assembly then slides into anouter housing which has matching grooves for the balls toride in.


55/73

Figure 7Exploded view of a CV-joint equipped halfshaft.CV-joints shown are the Rzeppa/double offset style andtheTri-pod.

The Tri-pod design is similar to the ball and trunnion design,except it has three needle bearing mounted balls inside thehousing spaced evenly apart (thus its name).

The newest of the CV-joints is called the Birfield. This joint is

primarily found on import vehicles although some domesticvehicles are starting to use it as well. This joint is notserviceable and the manufacturers give no pictures ordescriptions of its construction.

Front-wheel drive.See Figure 8

Front-wheel-drive vehicles are the more commonarrangement for most cars and mini-vans these days. Thesevehicles do not have conventional transmissions, drive axlesor driveshafts. Instead, power is transmitted from theengine to a transaxle, or combination of transmission anddrive axle, in one unit. Refer to the Automatic or ManualTransmission/Transaxle Section for more information on thetransaxle.
http://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_driveshaft1.asp#Figure8%23Figure8http://www.icarumba.com/includes/content/resourcecenter/encyclopedia/ch22/22Fig7.htmlhttp://www.icarumba.com/icarumba/resourcecenter/encyclopedia/icar_resourcecenter_encyclopedia_driveshaft1.asp#Figure8%23Figure8


56/73

A single transaxle accomplishes the same functions as atransmission and drive axle in a front-engine/rear-drive axledesign. The difference is in the location of components.

In place of a conventional driveshaft, a front wheel drivedesign uses two driveshafts, usually called halfshafts, whichcouple the drive axle portion of the transaxle to the wheels.Universal or constant velocity joints are used just as theywould be in a rear wheel drive design.

Figure 8 Example of a typical transverse engine, front-wheel drive system. Notice that the components are similarto the rear-wheel drive systems, except for location.

Rear-wheel driveSee Figure 9

Rear-wheel-drive vehicles are mostly trucks, very largesedans and many sports car and coupe models. The typicalrear wheel drive vehicle uses a front mounted engine andtransmission assemblies with a driveshaft coupling thetransmission to the rear drive axle. The rear axle assemblyis usually a solid (or live) axle, although some import and/orperformance models have used a rigidly mounted center


57/73

differential with halfshafts coupling the wheels to thedifferential.

Figure 9 View of the typical rear-wheel drive axle systemwith leaf springs.

Some vehicles do not follow this typical example. Such asthe older Porsche or Volkswagen vehicles which were rearengine, rear drive. These vehicles use a rear mountedtransaxle with halfshafts connected to the drive wheels.Also, some vehicles were produced with a front engine, reartransaxle setup with a driveshaft connecting the engine tothe transaxle, and halfshafts linking the transaxle to thedrive wheels.

Four-wheel driveSee Figure 10

When the vehicle is driven by both the front and rearwheels, two complete axle assemblies are used and powerfrom the engine is directed to both drive axles at the sametime. A transfer case may be attached to, or mounted near,the rear of the transmission/transaxle and directs the power


58/73

flow to the rear and/or front axles through two driveshafts.Since the angles between the front and rear driveshaftschange constantly, slip joints are used on the shafts toaccommodate the changes in distance between axles and

transfer case.

Another form of four or All Wheel Drive (AWD) design mayuse a front mounted engine and modified front wheel drivetransaxle assembly with an additional power output. Twohalfshafts connect the front wheels to the transaxle. Somemodels may have a transfer case connected to thetransaxle's additional power output. A driveshaft couples thefront transaxle or transfer case to the rear differential with

two halfshafts driving the rear wheels.

Shifting devices attached to transfer cases disengage thefront drive axle when four wheel drive capability is notneeded. However, some newer transfer cases are inconstant mesh and cannot be totally disengaged. These areknown as "full-time" four wheel drive and are just what thename says, four wheel drive operating all the time. This ismade possible by either a differential in the transfer case orthrough the use of a hydraulic viscous coupling.

Jeep& vehicles use a full-time system called Quadra-Trac,which is full-time four wheel drive with a limited slipdifferential in the transfer case. All you have to do is drive.


59/73

Figure 10 Typical transmission and transfer case design fourwheel drive system. The shaded area represents the powerflow.

Viscous coupling transfer case

Back in the early '80s, American Motors created a full-timefour wheel drive system that requires no action by the driverto activate the system, and take advantage of the improved

traction and handling of four wheel drive.

Since this time other similar systems have beenimplemented both in domestic and import vehicles.

The heart of these systems is a transfer case, whichdistributes the torque between front and rear axles bymeans of a viscous or fluid coupling. The coupling provides aslip-limiting action and absorbs minor driveline vibrations,giving smoother and quieter operation.

When the front and rear driveshafts turn at the same speed,as they do when the vehicle drives straight down the road,there is no differential action. In a turn or other maneuverswhere front and rear wheels must travel slightly differentdistances, differential action is required because the


60/73

driveshafts must be able to rotate at slightly differentspeeds. When this happens, the fluid in the coupling-a liquidsilicone-permits normal differential action.

Greater variations in speed between the driveshafts, such asoccur when a wheel or pair of wheels encounter reducedtraction and tend to spin, bring the viscous coupling's slip-limiting characteristics into action. The action of the viscouscoupling is velocity-sensitive, permitting the comparativelyslow movements typical of normal differential action butquickly building up resistance and effectively transmittingavailable torque to the axle with the best traction.

The action of the fluid between the plates in the couplingcould be compared to the action of water against a bodywhen wading across a pool. In waist-deep water, a personcan walk with comparatively little effort as long as he movesslowly and gently. However, when he tries to hurry, theadditional effort that is required is proportionate to theincrease in speed he attempts to achieve. Therefore, it iswith the viscous coupling. However, instead of water, thereis liquid silicone with a viscosity nearly the consistency ofhoney.

This four wheel drive system is more efficient than otherautomatic four wheel drive systems because there is no"open" differential (as opposed to a limited-slip differential)between the driveshafts. In the "open" differential system,the loss of traction at one wheel results in no torque beingdelivered to the other wheels, since it is the nature of theopen differential to deliver motion to the "easy" shaft-theone that is slipping. When using a viscous coupling, the loss

of traction at one wheel on the rear axle brings the slip-limiting character of the viscous coupling into action, causingdrive torque to be transferred to the front axle.

In addition to the differential function, the viscous couplingalso improves braking effectiveness. It acts as a skid


61/73

deterrent, tending to equalize drive-shaft speeds when thewheels at one end or the other want to lock and slide.

Drive axle/differential

All vehicles have some type of drive axle/differentialassembly incorporated into the driveline. Whether it is front,rear or four wheel drive, differentials are necessary for thesmooth application of engine power to the road.

PowerflowSee Figure 11

The drive axle must transmit power through a 90 angle.

The flow of power in conventional front engine/rear wheeldrive vehicles moves from the engine to the drive axle inapproximately a straight line. However, at the drive axle,the power must be turned at right angles (from the line ofthe driveshaft) and directed to the drive wheels.

This is accomplished by a pinion drive gear, which turns acircular ring gear. The ring gear is attached to a differentialhousing, containing a set of smaller gears that are splined to

the inner end of each axle shaft. As the housing is rotated,the internal differential gears turn the axle shafts, which arealso attached to the drive wheels.

Figure 11 Component parts of a typical driven axle


62/73

assembly.

Differential operationSee Figure 12

The differential is an arrangement of gears with twofunctions: to permit the rear wheels to turn at differentspeeds when cornering and to divide the power flowbetween both rear wheels.

The accompanying illustration has been provided to helpunderstand how this occurs. The drive pinion, which isturned by the driveshaft, turns the ring gear (1).

The ring gear, which is attached to the differential case,turns the case (2).

The pinion shaft, located in a bore in the differential case, isat right angles to the axle shafts and turns with the case

(3).

The differential pinion (drive) gears are mounted on thepinion shaft and rotate with the shaft (4).


63/73

Differential side gears (driven gears) are meshed with thepinion gears and turn with the differential housing and ringgear as a unit (5).

The side gears are splined to the inner ends of the axleshafts and rotate the shafts as the housing turns (6).

When both wheels have equal traction, the pinion gears donot rotate on the pinion shaft, since the input force of thepinion gears is divided equally between the two side gears(7).

When it is necessary to turn a corner, the differentialgearing becomes effective and allows the axle shafts to

rotate at different speeds (8).

As the inner wheel slows down, the side gear splined to theinner wheel axle shaft also slows. The pinion gears act asbalancing levers by maintaining equal tooth loads to bothgears, while allowing unequal speeds of rotation at the axleshafts. If the vehicle speed remains constant, and the innerwheel slows down to 90 percent of vehicle speed, the outerwheel will speed up to 110 percent. However, because this

system is known as an open differential, if one wheel shouldbecome stuck (as in mud or snow), all of the engine powercan be transferred to only one wheel.

Figure 12 Overview of differential gear operating


64/73

principles.

Limited-slip and locking differential operationSee Figure 13

Limited-slip and locking differentials provide the drivingforce to the wheel with the best traction before the otherwheel begins to spin. This is accomplished through clutchplates, cones or locking pawls.

The clutch plates or cones are located between the sidegears and the inner walls of the differential case. When theyare squeezed together through spring tension and outwardforce from the side gears, three reactions occur. Resistanceon the side gears causes more torque to be exerted on theclutch packs or clutch cones. Rapid one wheel spin cannotoccur, because the side gear is forced to turn at the samespeed as the case. So most importantly, with the side gearand the differential case turning at the same speed, theother wheel is forced to rotate in the same direction and atthe same speed as the differential case. Thus, driving forceis applied to the wheel with the better traction.

Locking differentials work nearly the same as the clutch andcone type of limited slip, except that when tire speed


65/73

differential occurs, the unit will physically lock both axlestogether and spin them as if they were a solid shaft.

Figure 13 Limited-slip differentials transmit power

through the clutches or cones to drive the wheel havingthe best traction.

Identifying a limited-slip drive axle

Metal tags are normally attached to the axle assembly at thefiller plug or to a bolt on the cover. During the life of thevehicle, these tags can become lost and other means mustbe used to identify the drive axle.

To determine whether a vehicle has a limited-slip or aconventional drive axle by tire movement, raise the rearwheels off the ground. Place the transmission in PARK(automatic) or LOW (manual), and attempt to turn a drivewheel by hand. If the drive axle is a limited-slip type, it willbe very difficult (or impossible) to turn the wheel. If thedrive axle is the conventional (open) type, the wheel willturn easily, and the opposing wheel will rotate in the reversedirection.


66/73

Place the transmission in neutral and again rotate a rearwheel. If the axle is a limited-slip type, the opposite wheelwill rotate in the same direction. If the axle is a conventionaltype, the opposite wheel will rotate in the opposite direction,

if it rotates at all.

Gear ratioSee Figure 14

The drive axle of a vehicle is said to have a certain axleratio. This number (usually a whole number and a decimalfraction) is actually a comparison of the number of gearteeth on the ring gear and the pinion gear. For example, a4.11 rear means that theoretically, there are 4.11 teeth onthe ring gear for each tooth on the pinion gear or, putanother way, the driveshaft must turn 4.11 times to turn thewheels once. Actually, with a 4.11 ratio, there might be 37teeth on the ring gear and 9 teeth on the pinion gear. Bydividing the number of teeth on the pinion gear into thenumber of teeth on the ring gear, the numerical axle ratio(4.11) is obtained. This also provides a good method ofascertaining exactly which axle ratio one is dealing with.

Another method of determining gear ratio is to jack up andsupport the vehicle so that both drive wheels are off theground. Make a chalk mark on the drive wheel and thedriveshaft. Put the transmission in neutral. Turn the wheelone complete turn and count the number of turns that thedriveshaft/halfshaft makes. The number of turns that thedriveshaft makes in one complete revolution of the drivewheel approximates the axle ratio.


67/73

Figure 14 The numerical ratio of the drive axle is thenumber of the teeth on the ring gear divided by the numberof the teeth on the pinion gear.

Driveline maintenanceSee Figures 15 and 16

Maintenance includes inspecting the level of and changingthe gear lubricant, and lubricating the universal joints if they

are equipped with zerk-type grease fittings. Apply hightemperature chassis grease to the U-joints. CV-joints requirespecial grease, which usually comes in a kit along with anew rubber boot.

Most modern universal joints are of the "extended life"design, meaning that they are sealed and require no periodiclubrication. However, it is wise to inspect the joints forhidden grease plugs or fittings, initially.

Also, inspect the driveline for abnormal looseness, wheneverthe vehicle is serviced.


68/73

Figure 15 Some U-joints are equipped with grease (zerk)fittings. Lubricate these using a grease gun.

Figure 16 Recommended driveshaft and differential servicelocations for rear-wheel drives.
http://www.icarumba.com/includes/content/resourcecenter/encyclopedia/ch22/22fig16.html


69/73

CV boot inspectionSee Figures 23 and 24

It is vitally important during any service procedures

requiring boot handling, that care be taken not to punctureor tear the boot by over tightening clamps, misuse of tool(s)or pinching the boot. Pinching can occur by rotating the CVjoints (especially the tripod) beyond normal working angles.

The driveshaft boots are not compatible with oil, gasoline, orcleaning solvents. Care must be taken that the boots neverencounter any of these liquids.

The ONLY acceptable cleaning agent for driveshaft

boots is soap and water. After washing, the boot mustbe thoroughly rinsed and dried before reusing.

Many manufacturers recommend inspecting the CV boots atevery oil change (every 3,000 miles or 4,800 km). However,a good rule of thumb is that, if the vehicle needs to beraised for any procedure, check the CV boots. Noticeableamounts of grease on areas adjacent to or on the exterior ofthe CV joint boot is the first indication that a boot is

punctured, torn or that a clamp has loosened. When a CVjoint is removed for servicing of the joint, the boot should beproperly cleaned and inspected for cracks, tears and scuffedareas on the interior surfaces. If any of these conditionsexist, boot replacement is recommended.


70/73

Figure 23 Inspect CV boots periodically for damage.

Figure 24 A torn boot should be replaced immediately.

Basic drive axle problems

Drive axle problems frequently give warnings in the form ofabnormal noises. Unfortunately, they are often confusedwith noise produced by other parts.

First, determine when the noise is most noticeable.

Drive noise: Produced during vehicle acceleration.


71/73

Coast noise: Produced while the vehicle coasts with aclosed throttle.

Float noise: Occurs while maintaining constant vehiclespeed on a level road.

Second, make a thorough check to be sure the noises arecoming from the drive axle, and not from some other part ofthe car.

Road noise

Brick or rough concrete roads produce noises that seem tocome from the drive axle. Road noise is usually identicalwhether driving or coasting. Driving on a different type of

road will tell whether the road is the problem.

Tire noise

Tire noises are often mistaken for drive axle problems. Snowtreads or unevenly worn tires produce vibrations seeming tooriginate elsewhere. Temporarily inflating the tires to 40 psiwill significantly alter tire noise, but will have no effect ondrive axle noises (which normally cease below about 30

mph).

Engine or transmission noise

Determine at what speed the noise is most pronounced andthen stop the vehicle in a quiet place. With the transmissionin Neutral, run the engine through speeds corresponding toroad speeds where the noise was noticed. Noises producedwith the vehicle standing still are coming from the engine or

transmission.

Front wheel bearings

While holding the vehicle speed steady, lightly apply the footbrake; this will often decrease bearing noise, as some of theload is taken from the bearing.


72/73

Drive axle no

the automobile has become so sophisticated and the automatic transmissions so reliable

Documents