speeds up, but the vehicle speed does not increase as it
should. The clutch disc is not being gripped firmly, and it
slips between the flywheel and pressure plate as the
members rotate.
Note: In this section, we are referring to
abnormal clutch slippage. For smooth
shifts, some degree of slippage is desir-
able. The clutch pedal should not be released too
quickly. A controlled release will permit some
slippage. As a result, you will not ruin the clutch or
other parts of the drive train. In addition, it makes the
ride easier on the vehicle occupants.
Clutch slippage begins as a minor problem. At first, it
will occur on initial and hard accelerations. As the
problem progresses, slippage will occur on upshifts, down-
shifts, and on any kind of acceleration. Left uncorrected,
slippage only becomes worse. Eventually, the clutch disc
becomes so badly worn that there is not enough friction
present to move the vehicle.
A slipping clutch gets very hot. The extreme heat
damages the contact surfaces of the pressure plate and
flywheel, causing them to lose their proper finish. This
happens because the heat changes the chemical makeup
of the metal, reducing its strength and heat absorption
capability. A slipping clutch should be repaired before the
pressure plate or flywheel is damaged by excessive heat.
A badly slipping clutch is usually too glazed and worn
down to be fixed by adjustment. A clutch disc that is too
glazed cannot develop any driving friction between itself
and the flywheel and pressure plate. If the clutch disc is
completely worn down, the apply springs will not be able
to apply enough pressure to the pressure plate to hold it
tight against the flywheel. The clutch must be replaced.
A clutch disc that is soaked with oil, such as from a
leaking rear main bearing seal, will also slip. In this case,
the clutch may not be worn down. If the clutch disc is
soaked with oil, however, it must be replaced.
A clutch often slips if the vehicle is driven through
deep water. This is because water enters the clutch hous-
ing and soaks the clutch disc. The clutch will usually work
normally after the disc dries.
Slippage can also be caused by disc, pressure plate,
or flywheel contact surfaces that are warped. If the surface
is not perfectly flat and straight, the flywheel or pressure
plate will not make good contact with the disc. As a result,
the disc may slip (or chatter). Warped surfaces may be
caused by improper manufacturing processes or by exces-
sive heat.
Sometimes, a clutch slips because the pressure plate
apply springs are weak. Weak springs may have been
installed in the pressure plate assembly originally, or the
assembly may have been intended for a smaller engine or
vehicle. The springs may have become weak from over-
heating, or under normal circumstances, the springs may
have become weak because of long usage. A clutch pedal
that depresses with almost no effort may be a sign of weak
clutch apply springs.
Clutch slippage can be caused by improper clutch
linkage adjustment. If the clutch linkage is adjusted to
where there is no free play and the throwout bearing is
applying pressure to the release levers or fingers, the clutch
will not be fully applied, and it will slip.
Note that if the clutch fork return spring is discon-
nected, the pedal will have no free play. However, this
condition will not cause slippage. When the clutch pedal
is released after being applied, the apply springs will fully
re-engage the clutch. The throwout bearing will ride on the
release levers or fingers but will not apply pressure on
them to cause slippage. Therefore, do not confuse this con-
dition with the lack of free play caused by misadjustment.
Grabbing Clutch
A grabbing clutch engages with a jerk or shudder no
matter how slowly and carefully it is applied. The effect
may be a series of jerks, which is often described as buck-
ing. This problem may be severe enough to cause damage
to universal joints or other drive train parts.
If you suspect a grabbing clutch, always check the
engine and transmission or transaxle mounts before trou-
bleshooting the clutch. Loose mounts can cause a sensa-
tion similar to a grabbing clutch. Also, make sure all clutch
housing and transmission bolts are tight.
A grabbing clutch is usually caused by oil deposits
disbursed on the friction facing of the clutch disc or by an
overly hot or glazed facing. Less common causes are worn
splines on the clutch disc hub flange or transmission input
shaft, a warped clutch disc, or warped pressure plate or
flywheel machined surfaces. Another possible cause is a
worn or misaligned clutch fork. This can cause the release
bearing to become crooked or tip on its shaft. The result is
an uneven application of pressure on the pressure plate
assembly. The bearing collar may begin to wear because of
the uneven pressure from the fork.
Clutch Chatter
Clutch chatter is a specific type of vibration that is
accompanied by a rapid clunking or rattling noise. It
occurs while the clutch is being engaged, as opposed to
while the clutch is fully engaged. It is closely related to
clutch grabbing. It differs, however, in that it is heard more
than it is felt. The actual noise is caused not by the clutch,
but by the vibration set up in the clutch linkage and drive
train. Chatter can be caused by loose or misaligned drive
train components. For example, the transmission and
clutch housing may be seriously misaligned.
Often, what seems like clutch chatter is really a worn
out constant-velocity joint or other universal joint. Other
causes of chatter include a misaligned flywheel, a worn pilot
bearing, or both. It is possible that release levers are
unevenly adjusted or worn. Sometimes, dust from a worn
clutch disc facing clogs the disc segments and cause chatter.
Chapter 7 Clutch Problems, Troubleshooting, and Service 129