Clutch Assembly section 13

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 94, the reader should be
able to:

•
•
•

Prepare for ASE Manual Drive Train and Axles
(A3) certification test content area “A” (Clutch
Diagnosis and Repair).

List the parts that are included in the clutch
system.
Describe how the clutch works.

Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 94, the reader should be
able to:

•

•
•

Explain the importance of proper clutch pedal
freeplay.
List the steps necessary to replace a clutch.
Describe the characteristic noise each clutch
component part makes when defective.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
Bushing

candlestick • clutch disc • coil spring clutch • cushioning
spring
front bearing retainer
lever style (pressure plate)
marcel spring
pilot bearing • pressure plate
Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
quill • quill shaft
release bearing

slave cylinder
torsional dampers • throwout bearing

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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PURPOSE AND FUNCTION OF A CLUTCH
The clutch assembly is located between the engine and the
transmission/transaxle. The purpose and function of a clutch include
the following:

To disconnect the engine from the transmission/transaxle to permit
the engine to remain running when the vehicle is stopped and to
permit the transmission/transaxle to be shifted

Connect and transmit engine torque to the transmission/ transaxle
To dampen and absorb engine impulses and drivetrain vibration
To provide a smooth engagement and disengagement between the
engine and the transmission/transaxle

Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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NOTE: The term transmission refers to rear-wheel-drive vehicles and the
term transaxle usually refers to front-wheel-drive vehicles that have
a differential built into the unit. A separate differential is used with a
transmission.

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COMPONENT PARTS AND OPERATION
OF A CLUTCH ASSEMBLY
A clutch assembly consists of a clutch disc that is splined to the input
shaft of the transmission/transaxle. When the driver depresses the
clutch pedal, a throwout bearing (release bearing) is forced against
the fingers of the pressure plate, which is bolted to and rotates with
the flywheel.
When force is exerted on the center of the pressure plate, the pressure
is released from the clutch disc that has been forced against the engine
flywheel. With the pressure removed from the clutch disc, the engine

can be operated without transferring torque to the transmission/
transaxle.
See Figure 94–1.
Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 94–1 Typical automotive clutch assembly showing all related parts. (Courtesy of LUK)

Continued

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When the driver reduces force on the clutch pedal, the pedal return spring and the pressure plate spring
combine to return the clutch pedal to its at-rest position (clutch-engaged position).
When the clutch pedal moves up, the pressure on the throwout bearing is released and the force against
the pressure plate spring(s) is released allowing the force of the pressure plate to clamp the clutch tightly
between the flywheel and the pressure plate.

See Figure 94–2.

To summarize:
When the clutch pedal is up, the clutch is engaged.
When the clutch pedal is down, the clutch is disengaged.

Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 94–2 (a) When the clutch is in the released position (clutch pedal depressed), the clutch
fork is applying a force to the throwout (release) bearing, which pushes on the diaphragm
spring, releasing the pressure on the friction disc.

Continued

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Figure 94–2 (b) When the clutch is in the engaged position (clutch pedal up), the diaphragm
spring exerts force on the clutch disc, holding it between the flywheel and the pressure plate.

Continued

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Additional related parts include the pilot bearing (or bushing) that
supports the front of the transmission input shaft.

Figure 94–3 The transmission has just been
removed. Note that this type of transmission
incorporates the bell housing, which was
therefore removed at the same time as the
transmission. The clutch fork and throwout
(release) bearing also came off together. All
that remained attached to the engine was the
flywheel, clutch disc, and pressure plate.

The throwout bearing is often supported and rides on the
transmission/transaxle front bearing retainer (called the quill).
NOTE: Most front-wheel-drive transaxles do not use a pilot bearing.
Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Clutch Pedal Linkage
Levers and rods Through a series of levers and rods, the
release fork is forced against the throwout bearing. This method
was commonly used on many older vehicles.
Cable operation A cable is used similar to a brake cable used
on a bicycle.

Figure 94–4
A typical cableoperated clutch.

Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Clutch Pedal Linkage
Hydraulic A small master cylinder and a slave cylinder
located near the throwout bearing is a very common method
of connecting the clutch pedal to the release fork on vehicles
equipped with manual transmission.

Figure 94–6 A typical clutch master cylinder
and reservoir mounted on the bulkhead on
the driver’s side of the vehicle. Brake fluid is
used in the hydraulic system to operate the
slave cylinder located on the bell housing.

See also Figure 94–5 following.
Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 94–5 A hydraulic clutch linkage uses a master cylinder and a slave cylinder.

Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Clutch Disc The clutch disc is round with a splined center hole
that slips over the splines of the input shaft of the transmission/
transaxle. Friction material is riveted to both sides of the clutch
disc—one side touches the flywheel of the engine and the other
side touches the friction surface of the pressure plate.
Friction material is either woven or molded from a mixture of
other materials. Woven materials are softer and help cushion clutch
engagement, but they may not last as long as moldedmaterials.
Friction materials operate in high heat and pressure. For many
years, asbestos was the most common material used in both clutch
and brake linings. Today, clutch friction material may contain
paper, cotton, and bits of copper or brass wire with resin holding
the mixture together.
Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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CAUTION: Always take precautions when working around clutch lining
material and assume that it contains asbestos. These precautions should
include wetting the friction disc before removal and preventing any dust
from the lining from becoming airborne.

High-Performance Clutch Disc Another type of friction material is
a ceramic and metallic mixture. This creates a hard, long-lasting
lining, but is more expensive and does not cushion clutch engagement
as much as a softer lining.
Instead of a full circle of softer friction material, the disc may have
only a few segments or buttons of this ceramic-metallic material.
Clutches that use these discs, which are sometimes called “button
clutches,” are found in racing applications where strength and

durability are a greater concern than smooth engagement.
See Figure 94–7.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 94–7 A racing or high-performance clutch disc lacks the features of a stock clutch disc
that help provide smooth engagement.

Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Stock Clutch Disc Around the center hub of the clutch disc are
torsional dampers that absorb the initial shock of engagement
and help dampen engine-firing in pulses being transmitted into
and through the transmission/transaxle.
See Figure 94–8.
The torsional dampers are either coil springs or made of rubber.
In the space between the friction surfaces is a wavy spring steel
material called a cushioning spring or marcel spring.
The marcel spring also helps to absorb the initial shock of
engagement and allows for a smooth engagement of the clutch.
See Figure 94–9.
Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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