Manual transmission fault

10.2.6 Clutch faults diagnosis
table
Symptom
Possible cause
No pedal 
Broken cable
resistance
Air in hydraulic system
Hydraulic seals worn
Release bearing or fork broken
Diaphragm spring broken
Clutch does 
As above
not disengage
Disc sticking in gearbox splines
Disc sticking to flywheel
Faulty pressure plate
Clutch slip
Incorrect adjustment
Worn disc linings
Contaminated linings (oil or grease)
Faulty pressure plate
Judder when 
Contaminated linings (oil or grease)
engaging
Worn disc linings
Distorted or worn pressure plate
Engine mountings worn, loose or broken
Clutch disc hub splines worn
Noisy Broken 
components
operation
Release bearing seized
Disc cushioning springs broken
Snatching
Disc cushioning springs broken
Operating mechanism sticking (lubrication?)
10.2.7 Drive shafts fault 
diagnosis table
Symptom
Possible cause
Vibration
Incorrect alignment of propshaft joints
Worn universal or CV joints
Bent shaft
Mountings worn
Grease leaking
Gaiters split or clips loose
Knocking noises
Dry joints
Worn CV joints (gets worse on tight
corners)
10.2.8 Final drive fault diagnosis
table
Symptom
Possible cause
Oil leaks
Gaskets split
Driveshaft oil seals
Final drive output bearings worn 
(drive-shafts drop and cause leaks)
Noisy operation
Low oil level
Incorrect preload adjustment
Bearings worn
Whining noise
Low oil level
Worn differential gears
10.3 Automatic 
transmission
10.3.1 Introduction
An automatic gearbox contains special devices
that automatically provide various gear ratios, as
they are needed. Most automatic gearboxes have
three or four forward gears and reverse. Instead 
of a gearstick, the driver moves a lever called a
selector. Most automatic gearboxes now have
selector positions for park, neutral, reverse, drive, 2
and 1 (or 3, 2 and 1 in some cases). The engine will
only start if the selector is in either the park or neu-
tral position. In park, the drive shaft is locked so
that the drive wheels cannot move. It is now quite
common when the engine is running to be able to
move the selector out of park only if you are press-
ing the brake pedal. This is a very good safety fea-
ture as it prevents sudden movement of the vehicle.
For ordinary driving, the driver moves the
selector to the drive position. The transmission
starts out in the lowest gear and automatically
shifts into higher gears as the car picks up speed.
The driver can use the lower positions of the gear-
box for going up or down steep hills or driving
through mud or snow. When in position 3, 2, or 1,
the gearbox will not change above the lowest gear
specified.
10.3.2 Torque converter 
operation
The torque converter is a device that almost all
automatic transmissions now use. It delivers power
from the engine to the gearbox like a basic fluid
flywheel but also increases the torque when the car
begins to move. The torque converter resembles a
large doughnut sliced in half. One half, called the
pump impeller, is bolted to the drive plate or fly-
wheel. The other half, called the turbine, is con-
nected to the gearbox-input shaft. Each half is
lined with vanes or blades. The pump and the tur-
bine face each other in a case filled with oil. A
bladed wheel called a stator is fitted between them.
The engine causes the pump to rotate and
throw oil against the vanes of the turbine. The
force of the oil makes the turbine rotate and send
power to the transmission. After striking the tur-
bine vanes, the oil passes through the stator and
returns to the pump. When the pump reaches a
specific rate of rotation, a reaction between the oil
and the stator increases the torque. In a fluid fly-
wheel oil returning to the impeller tends to slow it
Transmission systems
253

down. In a torque converter the stator or reactor
diverts the oil towards the centre of the impeller
for extra thrust. Figure 10.7 shows a gearbox with
a cut-away torque converter.
When the engine is running slowly, the oil may
not have enough force to rotate the turbine. But
when the driver presses the accelerator pedal, the
engine runs faster and so does the impeller. The
action of the impeller increases the force of the oil.
This force gradually becomes strong enough to
rotate the turbine and move the vehicle. A torque
converter can double the applied torque when
moving off from rest. As engine speed increases
the torque multiplication tapers off until at cruis-
ing speed there is no increase in torque. The reac-
tor or stator then freewheels on its one-way clutch
at the same speed as the turbine.
The fluid flywheel action reduces efficiency
because the pump tends to rotate faster than the
turbine. In other words some slip will occur
(about 2%). To improve efficiency, many trans-
missions now include a lock-up clutch. When the
pump reaches a specific rate of rotation, this
clutch locks the pump and turbine together,
allowing them to rotate as one.

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