Final drive fault diagnosis wheel. The other half, called the turbine, is con-
10.2.8 Final drive fault diagnosis wheel. The other half, called the turbine, is con-
nected to the gearbox-input shaft. Each half is
table
lined with vanes or blades. The pump and the tur- bine face each other in a case filled with oil. A
Symptom Possible cause
bladed wheel called a stator is fitted between them.
Oil leaks Gaskets split
The engine causes the pump to rotate and
Driveshaft oil seals
throw oil against the vanes of the turbine. The
Final drive output bearings worn
force of the oil makes the turbine rotate and send
(drive-shafts drop and cause leaks)
power to the transmission. After striking the tur-
Noisy operation Low oil level
bine vanes, the oil passes through the stator and
Incorrect preload adjustment
returns to the pump. When the pump reaches a
Bearings worn
specific rate of rotation, a reaction between the oil
Whining noise Low oil level Worn differential gears
and the stator increases the torque. In a fluid fly- wheel oil returning to the impeller tends to slow it
254 Advanced automotive fault diagnosis ● an internal gear or annulus is a ring with
internal teeth, which surrounds the planet gears and meshes with them.
Any part of a set of planetary gears can be held stationary or locked to one of the others. This will produce different gear ratios. Most of the auto- matic gearboxes of the type shown earlier have two sets of planetary gears that are arranged in line as shown in Figure 10.8. This provides the necessary number of gear ratios. The appropriate elements in the gear train are held stationary by a system of hydraulically operated brake bands and clutches. These are worked by a series of hydraulically operated valves in the lower part of the gearbox. Oil pressure to operate the clutches
Figure 10.7 This cut-away gearbox shows the torque converter
and brake bands is supplied by a pump. The sup-
components
ply for this is the oil in the sump of the gearbox. Unless the driver moves the gear selector to operate the valves, automatic gear changes are
down. In a torque converter the stator or reactor made depending on just two factors: diverts the oil towards the centre of the impeller for extra thrust. Figure 10.7 shows a gearbox with ● throttle opening a cable is connected from
a cut-away torque converter. the throttle to the gearbox; When the engine is running slowly, the oil may ● road speed when the vehicle reaches a set
not have enough force to rotate the turbine. But speed a governor allows pump pressure to when the driver presses the accelerator pedal, the
take over from the throttle. engine runs faster and so does the impeller. The
The cable from the throttle also allows a facility action of the impeller increases the force of the oil.
known as ‘kick down’. This allows the driver to This force gradually becomes strong enough to
change down a gear, such as for overtaking, by rotate the turbine and move the vehicle. A torque
pressing the throttle all the way down. converter can double the applied torque when
Many modern automatic gearboxes now use moving off from rest. As engine speed increases
gears the same as in manual boxes. The changing the torque multiplication tapers off until at cruis-
of ratios is similar to the manual operation except ing speed there is no increase in torque. The reac-
that hydraulic clutches and valves are used. tor or stator then freewheels on its one-way clutch at the same speed as the turbine.
10.3.4 Constantly variable
The fluid flywheel action reduces efficiency
transmission
because the pump tends to rotate faster than the turbine. In other words some slip will occur
Figure 10.9 shows the Ford CTX (Constantly vari- (about 2%). To improve efficiency, many trans-
able TransaXle) transmission. This kind of auto- missions now include a lock-up clutch. When the
matic transmission called a continuously variable pump reaches a specific rate of rotation, this
transmission uses two pairs of cone-shaped pulleys clutch locks the pump and turbine together,
connected by a metal belt. The key to this system is allowing them to rotate as one.
the high friction drive belt. The belt is made from high performance steel and transmits drive by thrust rather than tension. The ratio of the rota-
10.3.3 Epicyclic gearbox
tions, often called the gear ratio, is determined by
operation
how far the belt rides from the centres of the pul- leys. The transmission can produce an unlimited
Epicyclic gears are a special set of gears that are number of ratios. As the car changes speed the part of most automatic gearboxes. They consist ratio is continuously adjusted. Cars with this sys- of three elements: tem are said to use fuel more efficiently than cars
a sun gear, located in the centre; with set gear ratios. Within the gearbox hydraulic ● the carrier that holds two, three, or four planet
control is used to move the pulleys and hence gears, which mesh with the sun gear and
change the drive ratio. An epicyclic gear set is used
Transmission systems 255
Forward Planet First annulus Brake
Forward Sun
Brake Second
clutch carrier applied
clutch
wheel applied annulus
First gear: the line shows power
Second gear: one gear reduction is
flow through the two epicyclic
achieved by driving the first annulus
gears. Driving the front annulus
and braking the common sun wheel
and braking the rear planet carrier gives two gear reductions
Reverse-high clutch
Brake applied
Top gear: all gears are held
Reverse: braking the second planet
stationary in relation to each other,
carrier causes the rear annulus and
and the complete assembly rotates
Figure 10.8 Two epicyclic gear sets can produce to provide direct drive
output shaft to turn in the opposite
direction
three forward gears and reverse
256 Advanced automotive fault diagnosis
10.3.5 Electronic control of
from formula 1 and top rally cars. The electronic
transmission
clutch was developed for these racing vehicles to improve the get away performance and speed of
The main aim of electronically controlled auto-
gear changes.
matic transmission (ECAT) is to improve on con- For production vehicles a system has been ventional automatic transmission in the following
developed which can interpret the driver’s inten- ways:
tion. With greater throttle openings the clutch ● gear changes should be smoother and quieter;
operation changes to prevent abuse and driveline ● improved performance;
damage. Electrical control of the clutch release ● reduced fuel consumption;
bearing position is by a solenoid actuator or elec- ● reduction of characteristic changes over sys-
tric motor with worm gear reduction. This allows tem life;
the time to reach the ideal take off position to be ● increased reliability.
reduced and the ability of the clutch to transmit torque to be improved.
The important points to remember are that This technique has now been developed to the gear changes and lock-up of the torque converter
stage where it will operate the clutch automat- are controlled by hydraulic pressure. In an ECAT
ically as the stick is moved to change gear. Sensors system electrically controlled solenoid valves can
are fitted to monitor engine and road speed as well influence this hydraulic pressure. Most ECAT
as gear position. The Ferrari Mondiale uses a sys- systems now have a transmission ECU that is in
tem similar to the one described here. communication with the engine control ECU. With an ECAT system the actual point of gearshift is determined from pre-programmed memory within the ECU. Data from other sen-
10.4 Diagnostics –
sors is also taken into consideration. Actual gearshifts are initiated by changes in hydraulic
automatic transmission
pressure, which is controlled by solenoid valves. The two main control functions of this system
10.4.1 Systematic testing
are hydraulic pressure and engine torque. A tem- If the reported fault is that the kick down does porary reduction in engine torque during gear not operate proceed as follows. shifting allows smooth operation. This is because
the peak of gearbox output torque which causes
1. Road test to confirm the problem. the characteristic surge during gear changes on
2. Is the problem worse when the engine is hot? conventional automatics is suppressed. Because
Check the transmission fluid level! Has work of these control functions smooth gearshifts are
been done to the engine? possible and, due to the learning ability of some
3. If fluid level is correct then you must investi- ECUs, the characteristics remain throughout the
gate further. Work on the engine may have life of the system.
disturbed the kick down cable. The ability to lock-up the torque converter has
4. Check the adjustment/fitting of the kick down been used for some time even on vehicles with
cable.
more conventional automatic transmission. This
5. Adjust if incorrect.
6. Run and repeat road test. driveability. Lock-up is carried out using a hydraulic valve, which can be operated gradually to produce a smooth transition. The timing of lock-
gives better fuel economy, quietness and improved
10.4.2 Test equipment
up is determined from ECU memory in terms of the vehicle speed and acceleration.
Note: You should always refer to the manufac- turer’s instructions appropriate to the equip-
10.3.6 Semi-automatic
ment you are using.
transmission
A number of different types of semi-automatic
Revcounter
transmission are either in use or under develop-
A revcounter may be used during a stall test to ment. An interesting system is the electronically
check the operation of the torque converter and controlled clutch. This technology is a spin off
the automatic gearbox.
Transmission systems 257
Discoloured and/
Low fluid level
or burnt smell
Slipping clutches and/or brake
to fluid
bands in the gearbox Fluid requires changing
Gear selection
Incorrect selector adjustment
fault
Low fluid level Incorrect kick down cable adjustment Load sensor fault (maybe vacuum pipe, etc.)
No kick down
Incorrect kick down cable adjustment Kick down cable broken Low fluid level
Figure 10.10 Transmission pressure testing
Engine will not
Inhibitor switch adjustment incorrect
start or starts
Faulty inhibitor switch
Pressure gauge
in gear
Incorrect selector adjustment
This is a standard type of gauge but with suitable
Transmission slip,
Low fluid level
adapters for connection to a gearbox. Figure 10.10
no drive or poor
Internal automatic gearbox faults
shows where various tests can be carried out on
quality shifts
often require the attention of a
an automatic gearbox. specialist