against your body when the brake is applied.
The retarding force of the brake may not al-
ways be noticed, but it is actually preventing
the vehicle from going much faster.
Shutting Off the Engine
Make sure the exhaust brake is turned off before
shutting off the engine.
Clutches
General Information
The major reason why clutches wear out too soon is
excessive heat. Clutches are designed to absorb and
dissipate more heat than encountered in typical op-
eration. The temperatures developed in typical op-
eration will not break down the clutch friction sur-
faces. However, if a clutch is slipped excessively or
asked to do the job of a fluid coupling, high tempera-
tures develop quickly and destroy the clutch. Tem-
peratures generated between the flywheel, driven
discs, and pressure plates can be high enough to
cause the metal to flow and the friction facing mate-
rial to char and burn.
Heat and wear are practically nonexistent when a
clutch is fully engaged. But during the moment of
engagement, when the clutch is picking up the load,
it generates considerable heat. An improperly ad-
justed or slipping clutch will rapidly generate suffi-
cient heat to destroy itself.
The most important items that a driver should be
aware of to ensure long service life of the clutch in-
clude: starting in the right gear, clutch malfunctions,
and when to adjust a clutch.
Clutch Operation
Starting the Vehicle in the Proper Gear
An empty vehicle can be started in a higher trans-
mission gear ratio than a partially or fully loaded ve-
hicle. A good rule of thumb for the driver to follow is:
select the gear combination that allows you to start
moving with an idling engine, or if necessary, just
enough throttle to prevent stalling the engine. After
the clutch is fully engaged, the engine should be ac-
celerated to the correct rpm for the upshift into the
next higher gear.
Gear Shifting Techniques
Many drivers upshift into the next gear, or even skip-
shift into a higher gear, before the vehicle has
reached the proper speed. This type of shifting is
almost as bad as starting off in a gear that is too
high, since the engine rpm and vehicle speeds are
too far apart, requiring the clutch to absorb the speed
difference with friction, creating heat. For transmis-
sion operating instructions, refer to
Chapter 8
in this
manual.
The clutch brake is applied by fully depressing the
clutch pedal. Its purpose is to stop the transmission
gears from rotating in order to engage the transmis-
sion gears quickly in making an initial start.
CAUTION
Never apply the clutch brake when making down-
shifts or upshifts. The clutch pedal should never
be fully depressed before the transmission is put
in neutral. If the clutch brake is applied with the
transmission still in gear, a reverse load will be
put on the gear. At the same time, it will have the
effect of trying to stop or decelerate the vehicle
with the clutch brake. Rapid wear of the friction
discs will take place necessitating frequent re-
placement. Considerable heat will be generated,
which will be detrimental to the release bearings
and transmission front bearings.
Excessive Vehicle Overload, or
Overloading the Clutch
Clutches are designed for specific vehicle applica-
tions and loads. These limitations should not be ex-
ceeded.
CAUTION
Overloading will not only result in damage to the
clutch, but also to the entire powertrain.
Riding the Clutch Pedal
Riding the clutch pedal is very destructive to the
clutch, since partial clutch engagement permits slip-
page, generating excessive heat. Riding the clutch
pedal will also put a constant thrust load on the re-
lease bearing, which can thin out the lubricant. Re-
lease bearing failures can be attributed to this type of
misuse.
Engines and Clutches
7.23
