Automotive mechanics (volume II)(Part 4, chapter24) automatic transmission service

Automatic transmission service 479 Maintenance 480 Checking and changing the fluid 480 Automatic transmission fluids 482 Fluid problems 484 Extra cooling and filtering 485 Transmission adjustments 486 Brake band adjustments 490 Fault diagnosis and checks 491 Road testing 493 Diagnosing problems 496 Transmission overhaul 499 Transmission construction 499 Technical terms 500 Review questions 501

Automatic transmission service

Chapter 24

Maintenance

Checking and changing the fluid

Automatic transmission fluids

Fluid problems

Extra cooling and filtering

Transmission adjustments

Brake band adjustments

Fault diagnosis and checks

There are many variations in the design of automatic

transmissions and transaxles, and this affects the

methods of dismantling and servicing However, there

are some common servicing requirements and these

will be covered in this chapter For particular

transmissions, reference to the detailed procedures in

the appropriate service manual will be necessary.

Maintenance

Automatic transmissions and transaxles require a

regular check of the fluid level They also need more

detailed service at regular intervals This will vary with

different transmissions.

Some manufacturers specify service on a time

and kilometre basis, for example, each three years or

60 000 kilometres, others have only a time period.

However, all manufacturers state that service should be

carried out more frequently if the vehicle is operating

in adverse conditions Where no definite service period

is specified, then it is reasonable to carry out a service

at every 50 000 kilometres This would be for a

passenger vehicle with normal use.

An automatic transmission service could include

draining and replacing the fluid in the transmission and

converter, removing and cleaning the oil pan, cleaning

or replacing the filter, carrying out adjustments to the

bands and linkage and, finally, road testing the vehicle

to check performance.

Increased power and increased under-bonnet

temperatures mean that automatic transmission fluids

in modern motor vehicles are subjected to increasingly

severe operating conditions Therefore, regular

replace-ment of transmission fluid is now more important than

in the past.

Changing the transmission fluid will:

1 remove dirt, metal particles and condensation

2 remove fluid contaminants formed as a result of

high fluid temperatures

3 restore the correct balance of additives such as

anti-corrosives and detergent/dispersants.

Cleanliness

When working on an automatic transmission,

clean-liness is essential The transmission contains a large

number of valves, seals and passages, and any dirt or

foreign material introduced into the transmission will

cause malfunction and possible damage The correct

type of fluid must be used, and all containers and

dis-pensing equipment must be perfectly clean to prevent

contamination.

Checking and changing the fluid

The level of fluid in the transmission is checked with a dipstick The dipstick has full-level and low-level marks and may have different levels marked for hot and cold fluid (Figure 24.1).

figure 24.1 Checking the level of the automatic

trans-mission fluid MAZDA

The level of the fluid is checked while the engine is idling and at normal operating temperature, and with the selector lever in the drive or park position.

■ Unless the fluid level is checked by the mended method, a false level will be indicated on the dipstick.

recom-Checking the fluid level

A typical method for checking the fluid level is as follows:

1 Have the vehicle on a level floor with the transmission at operating temperature.

2 Move the selector lever through all positions and then select P.

3 Allow the engine to idle for about two minutes.

4 With the engine idling, remove the dipstick and wipe it with a non-fluffy rag or clean paper.

5 Insert the dipstick, then withdraw it immediately and check the fluid level.

If the fluid level is low, add the correct type of fluid

to bring the level up to the full mark, but do not overfill If frequent topping up is necessary, then a leak

is indicated This must be located and rectified immediately, as a low fluid level will cause erratic operation and damage to the transmission.

Some transmissions do not have a dipstick In these cases a fill plug will need to be removed from the side of the transmission to allow the fluid level to

be checked and topped up if necessary.

■ When checking the dipstick, both the level and the

condition of the fluid should be checked See the

later section ‘Dipstick information’.

Fluid change

Some transmissions have a drain plug that enables the

fluid to be drained without removing the oil pan

(Figure 24.2(a)) Other transmissions have no drain

plug and the oil pan has to be removed to change the

fluid This allows additional servicing, such as filter

cleaning or replacement to be carried out.

When the transmission is drained, fluid will also

drain from the torque converter However, the

trans-mission will not drain completely because some fluid

will always remain in the lower part of the converter.

When removing an oil pan that has not been

drained, do not remove all the bolts – leave a bolt in

each corner Loosen the front bolts but unscrew the

rear bolts a number of turns The back of the oil pan can be lowered onto the bolts so that most of the fluid can be drained before it is completely removed.

The oil pan will probably be stuck to the mission case and may have to be hit with a rubber hammer to break the joint Hit the edges or corners so that the oil pan is not damaged.

trans-After the oil pan is removed and before it is cleaned, it should be examined for deposits (Figure 24.3) The colour of the fluid and the amount and types

of deposits in the oil pan and filter will indicate the condition of the transmission, particularly the con- dition of the bands and clutches.

Most oil pans are pressed sheet steel and can distort

as a result of uneven or excessive tightening If this has happened the oil pan sealing surface should be straightened before refitting Oil leaks can result if a distorted oil pan is refitted.

Internal filter

A fluid filter is located underneath the valve-body assembly (Figure 24.2(b)) This filters all the fluid before it enters the oil pump.

Some filters are discarded and a new filter fitted as part of the transmission service Other filters have a fine gauze screen which is cleaned, and the filter is refitted.

■ During normal operation, the fluid in an automatic transmission can reach temperatures in the vicinity

of 200°C Care should be taken to avoid being burnt by hot fluid when it is being drained.

External fluid filter

Some transmissions are provided with an external fluid filter A filter of this type is fitted to the transaxle in

figure 24.2 Oil pan and filter

(a) removing oil pan (b) filter bolted to the

valve-body assembly HYUNDAI

figure 24.3 Checking the deposits after removing the oil

pan HOLDEN LTD

Figure 24.4 This is similar to an engine oil filter.

When carrying out a service, the filter is removed and

discarded and a new filter is fitted.

Refilling the transmission

Figure 24.5 shows the different levels of fluid in a

transmission when it is being refilled.

After draining the fluid, the normal method for

refilling is to add an initial quantity of fluid

(approxi-mately two-thirds the capacity) and then start the

engine to allow the converter to be filled This reduces

the level in the transmission The final fill is made with

the engine idling, to bring the fluid level up to the

‘full’ mark.

Automatic transmission fluids

Automatic transmission fluid has to perform many

functions such as: power transmission; hydraulic

control; heat transfer; lubrication of bearing surfaces; epicyclic gear lubrication; and friction control It has to perform all these functions while operating at very low temperatures, or at temperatures as high as 200°C.

Additives in automatic transmission fluids

To enable an automatic transmission fluid to perform the above functions, the following additives are used:

1 Anti-oxidants – to lengthen fluid life, permit high temperature tolerance and prevent formation of sludge and varnish.

2 Detergent/dispersants – to maintain contaminants

in suspension and keep hydraulic control components and filter screens clean.

3 Corrosion inhibitors – to prevent oil degradation products from corroding metal components.

4 Anti-wear additives – to prevent seizure of metal components under load and provide maximum protection against wear.

5 Seal swell additives – to provide control of swelling

of the rubber seals to prevent loss of fluid Fluid loss can lead to overheating and transmission failure.

6 Viscosity index improvers – to maintain the correct viscosity of the fluid over a wide temperature range.

7 Pour point depressants – to permit fluid flow at extremely low temperatures.

8 Friction modifiers – to control the friction between the clutch surfaces, enabling smooth gear changes.

9 Anti-foam additives – to ensure rapid collapse of foam and rejection of any trapped air.

figure 24.4 Automatic transmission with an external

filter HYUNDAI

torque converter oil filter

dipstick

oil pan/cover

figure 24.5 Refilling a transmission

(a) initial quantity of fluid (overfull) (b) after starting (underfull) (c) after topping up and checking (correct level) (d) after engine has been stopped for a period (overfull)

Clutch packs, convertor clutches and brake bands,

which are responsible for power transmission and also

for the feel of the gear shifts, require fluid with these

additives.

■ Fluid with the correct properties is critical for

ensuring smooth shifts and long clutch and band life.

Friction modified fluids

Figure 24.6 shows the difference between a friction

modified automatic transmission fluid and a

non-friction modified fluid.

The diagram shows that, when the speed difference

between the plates is close to zero (that is, when the

plates are grabbing or releasing), the frictional

coefficients of these two fluids are very different The

non-friction modified oil grabs harshly and releases

quickly, producing a firm shift The friction modified

oil grabs softly and releases softly, producing a soft

shift.

Manufacturers design the frictional material in their

transmissions to suit the recommended oil Therefore,

if a type F fluid is used in a transmission which has

been designed for a Dexron type fluid this will produce

a bumpy or harsh shift.

If a Dexron type fluid is used in a transmission

which has been designed for a type F fluid, then a

different feel of shift could be expected This would be

a particularly soft shift with the possibility of clutch

plate and band slippage under full power conditions.

■ Some transmission specifications exclude the

addition of friction modifiers.

The frictional area of the transmission (mainly the bands and clutches) generates approximately 50% of the heat of the transmission fluid The other major heat source is the torque converter.

If excessive slip occurs in the bands and clutches, the transmission fluid will be subjected to greatly increased temperatures and these will degrade the fluid The diagram in Figure 24.7 illustrates the likely relationship between fluid operating temperature and the kilometres likely to be travelled before a trans- mission requires overhaul.

What the diagram does show is the advantage of maintaining cool fluid in the transmission.

figure 24.6 Comparison of friction-modified and

figure 24.7 Graph shows the possible relationship

between the operating temperature and the service life of an automatic transmission

Specific types of transmission fluids

There are five types of automatic transmission fluid in general use:

1 Type F Used in Ford and Ford-type 3-speed

transmissions It has no friction modifier and

therefore imparts a short, abrupt shift feel to the

transmission.

2 Dexron II Used in Holden and Holden-type

trans-missions without electronic control Gives a soft

shift feel characteristic to the transmission Should

not be used for extended service interval units.

3 Dexron III Supersedes all previous Holden Dexron

fluids Soft shift feel Suitable for non-serviceable

units and extended oil drain periods.

4 Ford 95 Fluid Specially formulated for the BTR

4-speed unit supplied for Ford Falcon and Fairlane

models Gives a very soft shift feel Transmission

durability is enhanced with the use of this fluid.

5 Mitsubishi M-SP Fluid Specially formulated for

Mitsubishi, Hyundai and Proton transmissions

fitted with a damper clutch Transmission, damper

clutch performance and durability could be affected

by not using this fluid.

■ Always consult the manufacturer’s

recommen-dations when selecting an automatic transmission

fluid.

Fluid problems

Some of the possible problems associated with

automatic transmissions can be related to the fluid For

this reason, the fluid should be one of the first things to

be checked.

Low fluid level

Low fluid level can produce many symptoms that can

be mistaken for more serious transmission problems.

Two of the most likely causes of low fluid level are

incorrect filling of the transmission during service, and

external leaks However, before deciding that the

transmission is low on fluid, ensure that the fluid level

is being checked in the correct manner If the

manu-facturer’s recommendation is not followed, a false

reading can easily be obtained.

■ If a fluid level problem is suspected be sure that the

check is being done correctly and the correct

dipstick is being used.

Fluid leaks

Oil seals and gaskets are possible sources of external

fluid leaks Other possible places are the connections

between the cooler pipes and the transmission, or the connections between the cooler pipes and the cooler Automatic transmission fluid has a distinctive colour, and this enables automatic transmission leaks

to be distinguished from engine-oil leaks.

Figure 24.8 identifies the location of possible engine oil and transmission fluid leaks in the torque converter area of an automatic transmission Some of the fluid leaks might be cured by tightening the housing bolts; other leaks can only be cured by renewing an oil seal, an O-ring or a gasket To do this, the transmission might have to be removed from the vehicle.

figure 24.8 Possible sources of oil leaks in the converter

area – leaks could originate from the engine

or from the automatic transmission FORD

Transaxles, although of different design, could have leaks from the area of the converter or from the oil pan Leaks are also possible from the joints between the parts of the gear casing.

The drive-shaft oil seals, which seal between the drive shafts and the final-drive housing, are also possible places for oil leaks.

Internal leaks are more difficult to locate, but leaks that cause problems usually result in a loss of pressure, and this affects transmission operation Pressure tests can be carried out.

Dipstick information

While the dipstick is provided to measure the level of

the fluid in the transmission, it can also be used to

sample the fluid and provide a guide to its condition

(Figure 24.9).

problem probably exists within the transmission, the likely one being that a clutch or band is slipping and causing overheating.

Particles in the fluid

Fluid that is dark in colour, has a strong burnt odour and is contaminated with small particles of foreign matter, indicates problems.

The particles are the result of band or clutch slip and wear The discoloration is due to overheating and degradation of the fluid as well as the particles that it contains A transmission with fluid in this condition would probably require an overhaul.

Water contamination

Water in the fluid will cause it to emulsify and turn a milky colour Water contamination is not usual, but could occur from some external source It could also occur if the cooler in the radiator was faulty.

The frictional material of the bands and clutch plates is attached with water-based glue If water enters a transmission it can dissolve the glue and cause the frictional material to become detached.

Extra cooling and filtering

Degradation of the transmission fluid will shorten the life of a transmission A vehicle that has severe or heavy duty service (high loads, towing, heavy vehicle, powerful engine) can have extra cooling and filtering units fitted These help to combat degradation and so maintain the quality of the fluid.

Degradation of the fluid will occur by chemical breakdown due to heat and by contamination from metal particles and particles of friction materials.

■ The term degradation refers to a reduction in the quality of the fluid.

Additional cooler

Automatic transmissions are fitted with a cooler (heat exchanger) in the lower radiator tank This transfers heat from the transmission fluid to the relatively cooler engine coolant.

A supplementary cooler can be fitted This extra cooler will help to extend the life of the transmission fluid by maintaining it at optimum fluid temperature The additional cooler is like a small radiator (Figure 24.10) It is fitted either in front of or behind the engine coolant radiator where it can get maximum air flow.

figure 24.9 The fluid on the dipstick is checked for level,

colour and odour

The fluid on the dipstick can be checked as follows.

Fluid colour

If the fluid has its original distinctive colour, with no

discolouration, it can be considered to be in good

condition Discolouration indicates that the fluid is in

poor condition Badly discoloured fluid indicates that

there is probably a transmission problem as well.

Fluid odour

If the fluid has a noticeable burnt odour, it will also be

discoloured This indicates that the fluid has

deteriorated due to overheating and should be changed.

If the transmission is operating properly, the fluid level

is correct, and the fluid is not badly discoloured, then a

change of fluid could be sufficient.

However, if the fluid is badly discoloured, a

Additional filter

Some transmissions are fitted with a replaceable fluid

filter, but a supplementary filter can be fitted in

conjunction with the cooler The additional filter is

fitted into the cooler line, usually in the flexible line

and close to the transmission cooler for easy access It

is an in-line filter so all the fluid passing to the cooler

first passes through the filter.

The filter construction is shown in Figure 24.11 It

consists of:

1 an outer container – with fins to dissipate heat

2 a powerful magnet – to remove fine steel particles

3 a filter element – to remove foreign particles from

Transmission adjustments

Automatic transmissions and their controls are provided with adjustments These vary considerably with different makes and models of vehicle.

The type and method of adjustment will depend on whether the transmission has full hydraulic control,

or whether it has electronic control It can also depend

on whether the engine has a carburettor or electronic fuel injection (EFI).

Because of these differences, and the importance of correct adjustment, it is essential that detailed infor- mation is obtained from the appropriate service manual before attempting any adjustment on a vehicle.

The importance of checking the basic settings and adjustments cannot be overstated – many service complaints can be rectified by attending to the items outlined below A transmission simply cannot function properly unless these are correct.

The following are the types of service adjustments that are generally provided on automatic transmissions:

1 engine idle speed

2 selector cable, or linkage

3 starter isolator (neutral switch) or range switch

4 throttle cable or vacuum control (for hydraulic control)

5 throttle-position sensor (for electronic control)

6 hydraulic pressure

7 brake bands.

Engine idle speed

Idle speed is checked with a tachometer and adjusted

to the rpm specified for the vehicle (usually in the vicinity of 800 rpm).

Adjustment is made with the selector lever in P, and is finally checked for smooth idling in D A slight drop in engine rpm will be noticed when the selector is moved to D This is caused by the increased load on the engine.

High idle speed will cause creep; that is, the vehicle will move slowly as soon as a gear position is selected, even with the engine idling Creep is normal, but it will

be excessive if the idle speed is too high.

High idle speed could also cause a band or brake to

figure 24.10 An automatic transmission cooler at the front

of the radiator HOLDEN LTD

filter element

by-pass valve inlet

polymer casing outlet

steel core

be applied harshly when the selector lever is moved to

a gear position Low idle speed will cause the engine

to stall.

There are different methods of adjusting the idle on

carburettor and EFI engines, but the effects of

incorrect adjustment are the same.

■ For safety reasons, engine idle or any other engine

adjustments should only be carried out with the

selector lever in the N or P position with the park

brake applied.

Selector cable

The selector cable connects the driver’s selector lever

to the manual valve in the transmission, so that

whenever a gear position is selected, the manual valve

will be moved to that position For the selector

adjustment to be correct, the selector lever indicator

should be in the N position with the manual valve also

in the N position.

The two ends of a selector cable are shown in

Figure 24.12 The selector lever and its end of the

cable are shown in Figure 24.12(a) and the transaxle

end of the cable and its lever arrangement are shown in

Figure 24.12(b) This is a push–pull cable which

enables the selector lever to move the lever on the

transaxle case in both directions.

A threaded adjustment is provided at the selector

lever This is adjusted so that the selector positions at

the selector lever correspond with the selector

positions at the transaxle In the design shown, a roller

and quadrant are used to locate the selector lever in the

various selector positions Inside the transaxle, a detent

locates the manual valve in its correct positions The

adjustment is made with the selector lever in N and the

manual valve also in N.

The starter isolator or neutral switch

This is usually located on the transmission case (Figure

24.13) This is a safety switch, arranged to prevent the

engine from starting in any gear position except N or

P Incorrect operation of the switch would allow the

engine to be started with the transmission in gear,

causing the vehicle to move out of the control of the

driver.

The isolator switch forms part of the starter

solenoid circuit It is operated by the gear-selector

mechanism to provide an open circuit in all positions

except N or P This prevents the starter from operating

in any other position.

■ The isolator switch is also called the neutral switch, inhibitor switch and sometimes the range selector switch.

Checking the isolator switch operation

To check that the switch isolates the starter, try to start the engine with the selector lever in all positions The starter should operate in N and P only Check this while moving the selector lever backwards and forwards (within the free play) from N and also from P.

figure 24.12 Selector lever

(a) lever and selector end of cable (b)

trans-axle end of cable FORD

As a safety precaution, firmly apply the parking

brake and foot brake during the check because the

vehicle could move if the switch is incorrectly

adjusted.

To check the switch itself, the harness connector

will have to be disconnected at the switch and an

ohmmeter used to check between the terminals at the

switch connector The ohmmeter is connected between

the terminals, and the switch moved to each of the gear

positions.

Checking the range switch operation

With electronic control, the isolator switch is also a

position switch It sends signals to the ECU to tell it

which position has been selected In some vehicles, the

isolator switch is called the range switch An example

is shown in Figure 24.14.

Some vehicles have the selector positions (or range

positions) shown as indicators on the instrument

cluster (Figure 24.15) With this arrangement, the

range selector switch is used to illuminate the position

that is selected.

When checking the operation of the switch, the

panel indicators should also be checked.

Throttle cable

There are two cables in the accelerator and throttle

cable arrangement One is the accelerator cable

between the accelerator pedal and the throttle valve in

the throttle body of the engine’s air intake system The

other is the throttle-valve cable, which connects the

throttle valve to the hydraulic throttle valve (TV valve)

in the transmission.

To avoid confusing the two valves, the throttle

valve in the air intake will be referred to simply as the

throttle valve, while the hydraulic valve in the mission will be referred to as the TV valve.

trans-figure 24.13 Starter isolator and position switch, rotary

type MITSUBISHI

figure 24.14 Manual control lever and range switch

(a) switch assembly (b) cable adjustment

HYUNDAI

adjusting nut

manual control lever

range switch

(b)

electrical connector

manual control lever

mounting bolts range switch

(a)

figure 24.15 Automatic transmission range indicators on

an instrument cluster HYUNDAI

(a) Range indicators

(b) Instrument cluster

malfunction indicator

indicators

An arrangement with two cables is shown

schem-atically in Figure 24.16 When the accelerator pedal is

depressed, the throttle valve is opened by the

acceler-ator cable movement The transmission downshift

valve and the TV valve are moved by the action of the

throttle cable.

Throttle linkage adjustment is correct when the

accelerator pedal height, the throttle valve opening,

and the position of the TV valve in the transmission

are in correct relationship.

In general terms, the relationship of these three is as

follows:

1 The pedal should be of sufficient height above the

floor to allow full movement of the linkage.

2 The throttle valve should be closed (at idle

position).

3 The TV valve in the transmission should be at or

near its stop.

The manufacturer’s method of adjustment should

always be referred to, as any errors will affect both the

fluid pressure and the road speeds at which gear

changes occur.

■ Incorrect adjustment, because of low line pressure,

could cause the transmission to slip, resulting in

erratic changes and damage to bands or clutches.

Cable adjustment

The throttle valve ends of both an accelerator cable

and a throttle cable are shown on Figure 24.17 Both

the inner cables operate around quadrants With this

arrangement, the accelerator inner cable shortens and

the TV valve inner cable lengthens when the

acceler-ator pedal is pressed.

To check this particular arrangement, the ator cable is checked first: depress the accelerator pedal and check that the throttle valve is fully open If necessary, adjust the length of the cable with the adjusting nuts until the throttle valve opens fully.

acceler-With the accelerator pedal fully depressed, check that the throttle inner cable has moved out far enough

to expose a stop which is crimped to the inner cable The stop should be about 1 mm beyond the end of the boot on the cable If necessary, adjust the nuts on the outer cable to obtain this dimension (A in Figure 24.17(b)).

As with all transmissions with hydraulic control, the throttle cable adjustment affects throttle pressure in the transmission and throttle pressure operates on the shift valves If the outer cable is lengthened (the inner cable effectively shortened), then throttle pressure in the transmission will increase and delay upshifts If the outer cable is shortened, this will have the opposite effect and upshifts will occur earlier.

In the transmission, throttle pressure operates on the regulator valves as well as on the shift valves This means that both line pressure and converter pressure can be affected by throttle cable adjustment If line pressure is too high, the shifts will be harsh as well as

figure 24.16 Throttle cable arrangement HOLDEN LTD

throttle body

accelerator cable

accelerator

throttle valve (TV valve) throttle

cable

lever plunger

figure 24.17 Throttle cable assembly and adjustment

TOYOTA

accelerator cable

throttle body outer

cable

boot

A

ferrule adjustment

(a)

(b)

late If the pressure is too low, upshifts will be early

and clutches or bands could slip.

Vacuum control

A vacuum control unit is shown in Figure 24.18 This

performs a similar function to the throttle linkage.

The unit, which is mounted on the transmission case,

consists of a diaphragm in a sealed container connected

by a pipe to the engine manifold The diaphragm is

under the influence of engine manifold vacuum and

therefore is sensitive to variations in engine load.

A pushrod from the diaphragm extends into the

transmission and operates the throttle valve (TV

valve) Changes in throttle butterfly opening and

engine load will therefore affect throttle pressure,

which will determine when gearshifts occur (Line

pressure will also be affected.)

Adjustment, if necessary, can be made either by

changing the length of the pushrod between the

diaphragm and the throttle valve or by turning a small

screw located inside the vacuum connection.

Kickdown

Where a throttle cable is used, the downshift valve is

operated by the cable and no adjustment is required.

Where vacuum control is used, forced downshifts

are obtained by means of a kickdown switch operated

by the accelerator linkage, and a solenoid which

controls a downshift valve in the transmission

(Figure 24.19).

When the accelerator pedal is depressed to the

floor, the switch contacts are closed, the solenoid is

energised and downshift pressure is provided to the

shift valves to force a downshift An adjustment is

provided on the switch or the linkage, and this can be

altered if the switch does not operate.

Throttle-position sensor

The throttle-position sensor, used with electronic control, is attached to the throttle body The sensor can usually be adjusted, within limits, by loosening the screws and altering its position.

There are two basic types of sensors: one is a rheostat and the other is a switch The rheostat pro- vides a voltage signal that varies with throttle opening.

It is basically a variable resistance, so can be checked with an ohmmeter (Figure 24.20) Its resistance should vary between closed-throttle and full-throttle positions The switch-type sensor provides signals in a number of steps It is actually a switch with a number

of contacts For checking purposes, it can be treated in the same way as any other switch.

Brake band adjustments

These are carried out to compensate for wear of the band lining A loosely adjusted band could slip, resulting in overheating and excessive wear A tightly adjusted band will cause binding, again resulting in overheating and wear Correct adjustment provides a small clearance between the brake band and the drum.

figure 24.18 Vacuum-operated throttle valve

1 line pressure, 2 throttle pressure, 3 throttle

pressure, 4 exhaust

figure 24.19 Kickdown (downshift) switch and solenoid

MAZDA

This enables the band to be applied smoothly and

firmly, with minimum wear occurring.

Some transmissions have two band adjustments

while others have only one Some automatic

trans-missions have disc brakes instead of bands, and these

have no adjustments.

■ The band adjuster may be internal or external.

Internal adjustments can only be carried out after

the oil pan has been removed.

Internal adjustment

Figure 24.21 shows a band adjustment being carried

out on a transmission; in this case, the adjustment is

inside the transmission The oil pan has been removed and a small tension wrench is being used on the screw adjuster In most transmissions the band is adjusted by tightening the adjusting screw to a specified torque and then backing the screw off a specified number of turns Gauge blocks are sometimes used.

External adjustment

The adjustment shown in Figure 24.22 is an external adjustment, which is accessible after removing the outside cover from the servo.

The following is an example of how a band is adjusted:

1 Loosen the locknut.

2 Holding the servo piston to prevent it from turning, tighten the adjusting screw to 10 Nm and then back

it off Repeat this twice This is done to seat the band snugly on the drum and so prevent a false reading when adjusting.

3 Tighten the adjusting screw to 5 Nm and then back

it off 3 1 / 2 turns.

4 Tighten the locknut to 15 to 20 Nm.

Fault diagnosis and checks

An understanding of the construction and operation of the particular type of transmission or transaxle is necessary to enable faults to be diagnosed and checks

to be made While the general operating principles of most transmissions are similar, the details vary greatly.

■ Vehicle manufacturers provide diagnosis guides, and these should be referred to whenever a fault is suspected.

figure 24.20 Checking the throttle-position sensor

HYUNDAI

ohmmeter

electrical connector

plenum

chamber

throttle body

figure 24.21 Servo and band with an internal adjustment

MITSUBISHI

figure 24.22 Servo and band with an external adjustment