LV27 manual transmissions systems (2)

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Student Workbook

LV27 Manual Transmission Systems (2)

kap all phase 2 & 3 6/11/03 11:36 am Page 23

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV27 MANUAL TRANSMISSION

SYSTEMS (2)

Contents

Page Page

Control mechanisms wear 13

Clutch pedal height – hydraulic 4 Incorrect gear thrust clearance 14 Clutch push rod free play – hydraulic 4 Shift detent wear 14 Clutch release fork free play – Gear and shift hub sleeve spline wear 15

Clutch free play – cable 6

Clutch trouble shooting 6

Gearbox troubleshooting 10

Synchroniser ring wear 10

Shift key and spring wear 11

Gearbox troubleshooting 12

Introduction

On completion of this module you will be expected to effectively:

• explain the routine maintenance requirements for manual gearboxes

• explain the routine maintenance requirements for clutches

• identify common faults with clutch operation

• identify common faults with manual gearbox operation

Routine Maintenance

Clutches are designed to require minimal routine maintenance However the following can be carried out during regular scheduled servicing:

• check clutch operating mechanism (cable or levers) for correct adjustment

• lubricate, where required, exposed cable and lever joints

• check degree of clutch disc (or plate), wear can often be judged by the position and feel of the pedal

• check clutch pedal height and free play

• check hydraulic clutch fluid level

• check hydraulic clutch fluid for contamination

In this example the clutch master cylinder shares a reservoir with the brake system A baffle inside the reservoir creates two chambers to keep the

circuits separate The fluid used in the clutch system is usually identical to that normally used in the brake system It should be changed and the system bled periodically as indicated by the manufacturer or when contamination of the fluid is present

Care should be taken to ensure all air is removed from the circuit by thorough bleeding Routine maintenance is limited to inspection of the system to

identify fluid leaks, visual damage or wear and maintaining fluid level

Clutch pedal height – hydraulic

Clutch pedal height is adjusted by slackening the lock nut and turning the stop bolt in the appropriate direction before securing the lock nut again

It is measured as indicated and compared with the manufacturers’

specification If no specification is available it is usually set to match the brake pedal Pedal play must be checked and adjusted if necessary after pedal height adjustment

Clutch push rod free play – hydraulic

Gently pushing the pedal will establish clutch pedal free play It can be

adjusted by slackening the lock nut and turning the push rod in the

appropriate direction Pushing the pedal harder will establish total pedal free play Total pedal play is push rod free play plus release fork free play

Clutch release fork free play – hydraulic

Release fork free play can be adjusted to specification by slackening the lock nut and turning the push rod in the appropriate direction

Clutch pedal height – cable

The cable operated system pedal height can be adjusted in the same way as the hydraulic clutch

Clutch free play – cable

The cable operated clutch pedal free play is measured in the same manner as the hydraulic system The cable-operated clutch often incorporates a self-adjusting mechanism Release fork free play can be adjusted on some

models This usually requires the E-ring or clip to be removed from the cable

as indicated

Clutch troubleshooting

Clutch disengagement operation can be investigated in the following way:

• apply hand brake and chock the wheels

• depress clutch pedal and start the engine

• release clutch pedal with transmission in neutral

• without depressing the clutch, very carefully and slowly move the gear stick towards the reverse position

• as soon as the gears contact fully depress the clutch pedal

• if reverse gear then selects smoothly you can conclude that the clutch is disengaging correctly

A clutch disc or plate slipping under load between the flywheel and pressure plate will soon lead to total clutch failure If the vehicle does not accelerate on

a hill climb, despite heavy throttle use, and is accompanied by a strong

burning smell the following procedure will determine clutch condition:

• apply hand brake and chock the wheels

• depress clutch pedal and start the engine

• select top gear

• gradually increase engine rpm and attempt to drive off

• if the engine stalls you can assume the clutch is not slipping

A clutch grabbing or juddering prevents the vehicle from starting off smoothly The symptoms can usually be exaggerated, for the purpose of diagnosis, by starting off on a hill or with the handbrake partially applied Clutch judder can also be caused by incompatible materials or an overheated and damaged clutch Overheating can increase pressure plate or flywheel run out

Transmission Routine Maintenance

Modern transmission systems are designed to require very little routine

maintenance Visual inspection of the casing for fluid leaks or damage and the inspection of gear linkage is generally all that is required. Few

manufacturers specify transmission oil change as part of the periodic servicing

of the vehicle

Nevertheless, when it is carried out it is vital that the correct oil is used

Manufacturers specify oils of very different types for use in manual

transmission/transaxle These vary from standard API class transmissions oils to Dexron automatic fluids

Oil drained from the plug indicated, should be checked for particles of the internal components of the gearbox This can give an indication of wear taking place within the transmission The oil filler plug often doubles as the level indicator to help prevent over filling Care should be taken not to remove the securing bolts indicated, by mistake This error often requires the

transmission to be rebuilt to relocate items within

Gearbox troubleshooting

Synchroniser ring wear

Grooves are provided on the inner surface of the synchroniser ring They have the effect of cutting through the oil film to increase friction between the ring and the cone If the grooves become worn, friction can be lost This will lead to poor synchronisation of the mashing gear speeds and gear crunching during selection

Synchroniser ring wear can be assessed by checking the clearance when fitted onto the gear Attempting to turn the ring on the gear by hand will also give an indication of excessive wear

Shift key and spring wear

The shifting keys are kept under tension by the shift key spring If the spring becomes weak the key will not force the synchroniser ring against the gear correctly The projecting part of the key may be worn excessively Both problems can be the cause of gear crunching

Gearbox troubleshooting

Gear Shifting

Control mechanisms wear

Remote gear selection mechanisms can be a source of stiffness and wear Floor mounted mechanisms have cables or rods that can be exposed to road dirt and damage Cables may become worn or stiff

Gearbox troubleshooting

Incorrect gear thrust clearance

Incorrect gear thrust clearance due to wear or damage can mean that the gears no longer mesh accurately This can cause the transmission to jump out of gear In this illustration thrust clearance is being measured on a

partially dissembled shaft

Shift detent wear

The fork shafts have grooves into which detent balls are forced by the springs This prevents the transmission from jumping out of gear and provides a

positive feedback to the driver If the springs become weak the transmission will jump out of gear more easily

Gear and shift hub sleeve spline wear

The hub sleeve splines have been chamfered on this reverse gear

mechanism A corresponding taper on the gear spline helps prevent the transmission from jumping out of gear The transmission will jump out of gear more easily if the splines become worn

Abnormal transmission noise

Abnormal noise or selection crunching that affects all gears is often a fault with the clutch Crunching or noise that affects only one gear is most likely to

be a fault within the transmission gearbox Therefore it is important to check the operation of the clutch prior to carrying out investigation of the gearbox

If depressing the clutch pedal with the engine running in neutral stops an abnormal noise it is most likely to originate in the gearbox However abnormal transmission noise often only occurs when the vehicle is moving and the transmission is under load A suitable road test is then required to attempt to locate the problem