Advanced Automotive Fault Diagnosis

gear stick! All manual gearboxes have a neutral
position; three, four, or five forward gears; and a
reverse gear. A few even have six forward gears
now! The driver puts the gearbox into neutral as
the engine is being started, or when a car is
parked with the engine left running.
Power travels in to the gearbox via the input
shaft. A gear at the end of this shaft drives a gear on
another shaft called the countershaft or layshaft. A
number of gears of various sizes are mounted on
the layshaft. These gears drive other gears on a
third motion shaft also known as the output shaft.
The gearbox produces various gear ratios by
engaging different combinations of gears. For
reverse, an extra gear called an idler operates
between the countershaft and the output shaft. It
turns the output shaft in the opposite direction to
the input shaft. Figure 10.2 shows a front wheel
drive gearbox.
Figure 10.3 shows the power flows through
this box in each of the different gears. Note how
in each case (with the exception of reverse) the
gears do not move. This is why this type of gear-
box has become known as constant mesh. In other
words the gears are running in mesh with each
other at all times. Dog clutches are used to select
which gears will be locked to the output shaft.
These clutches which are moved by selector
levers, incorporate synchromesh mechanisms.
A synchromesh mechanism is needed because
the teeth of the dog clutches would clash if they
met at different speeds. The system works like a
friction type cone clutch. The collar is in two parts
and contains an outer toothed ring that is spring-
loaded to sit centrally on the synchromesh hub.
When the outer ring (synchroniser sleeve) is made
to move by the action of the selector mechanism
the cone clutch is also moved because of the lock-
ing keys. The gear speeds up as the cones touch,
thus allowing the dog clutches to engage smoothly.
A baulking ring is fitted between the cone on the
gear wheel and the synchroniser hub. This is to pre-
vent engagement until the speeds are synchronised.
A detent mechanism is used to hold the selected
gear in mesh. In most cases this is just a simple
ball and spring acting on the selector shaft(s).
Figure 10.4 shows a rear wheel drive gearbox with
the detent mechanism marked. Gear selection
interlocks are a vital part of a gearbox. These are to
prevent more than one gear from being engaged at
any one time. On the single rail (one rod to change
the gears) gearbox shown in the figure, the inter-
lock mechanism is shown at the rear. As the rod is
turned (side to side movement of the gear stick)
towards first–second, third–fourth or fifth gear
positions, the interlock will only engage with
either the first–second, third–fourth or fifth gear
selectors as appropriate. Equally when any selec-
tor clutch is in mesh the interlock will not allow
the remaining selectors to change position.

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