11.1 INTERNAL COMBUSTION ENGINE
An engine (Fig. 11-1) is a machine that converts heat energy into
mechanical energy. The heat from burning a fuel produces power which
moves the vehicle.Sometimes the engine is called the power plant.
Automotive engines are internal-combustion(IC) engines because the fuel
that runs them is burned internally, or inside the engine. There are two types:
reciprocating and rotary (Fig. 11-2). Reciprocating means moving up and
down, or back and forth. Most automotive engines are reciprocating. They
have piston that move up and down, or reciprocate, in cylinder (Fig.11-3).
These are piston engines.
Rotary engines have rotors that spin, or rotate. The only such engine now
used in automobiles is the Wankel engine (12-7).
PISTON ENGINE BASICS
11-2 TWO KINDS OF PISTON ENGINES
The two kinds of piston engines are the spark-ignition engine and the
compression-ignition(diesel) engine. The differences between them are:
• The type of fuel used.
• The way the fuel gets into the cylinders.
• The way the fuel is ignited.
The spark-ignition engine usually runs on a liquid fuel such as gasoline
or alcohol blend. The fuel must be highly volatile so that it vaporizes quickly.
The fuel vapor mixes with air before entering the engine cylinders. This forms
the highly combustible air-fuel mixture that burns easily. The mixture then
enters the cylinders and is compressed. Heat from an electric spark produced
by the ignition system sets fire to, or ignites, the air-fuel mixture. As the
mixture burns (combustion), high temperature and pressure are produced in
the cylinder (9-9). This high pressure, applied to the top of the piston, forces it
to move down the cylinder. The motion is carried by gears and shafts to the
wheels that drive the car. The wheels turn and the car moves.
In the diesel or compression-engine, the fuel mixes with air after it
enters the engine cylinders. The piston compresses the air to as 1/22 of its

control valves. They are either open or closed .The fuel pump
sends fuel under constant pressure to the injectors . On the
system shown in Fig 11-20, each cylinder receives fuel from its
own injector.This is a port injection system . At the proper time for
fuel delivery, the ECM turns on each injector. This opens the valve
in the end of the injector. The pressurized fuel then sprays out into
the air entering the cylinder.
Fuel delivery continues as long as the valve is open. The
time is computed and controlled by the ECM. When the proper
amount of fuel has sprayed out , the ECM turns off the injector .
The valve closes and fuel delivery stops.
Another fuel-injection system uses one or two injectors
located above the throttle valve (Fig 1-13). They feed the proper
amount of fuel to the air entering the intake manifold. This is
throttle-body injection (TBI)
In the past, carburetors (chap.21) were part of most fuel
systems. Carburetors are mixing devices. Air passing through the
carburetor picks up and mixes with the fuel to provide a
combustible mixture. Most vehicles now have fuel-injection
systems.
11-17 Electric ignition system
The fuel system delivers a combustible mixture to each cylinder. The
upward movement of the piston compresses the mixture. Then the ignition
system (Fig 11-21) delivers an electric spark to the spark plug in that cylinder.
The spark ignites the compressed air-fuel mixture and combustion follows
The ignition system takes the low voltage of the battery (12 volts) and
steps up the voltage as high as 47000 volts ( or higher ) in some systems.
This high voltage produces sparks that jump the gaps in the spark plugs. The
hot sparks ignite the compressed air-fuel mixture.
11-18 Lubricating system

The exhaust system reduces the noise of the burned gases leaving the
cylinders. Also, it carries the exhaust gases and excess heat safely away from
the passenger compartment.
The emission-control system reduces the air pollution from the vehicle
and the engine. The starting system cranks and starts the engine. A battery
provides the electric power to operate the starting motor and the ignition
system during cranking. Later chapters describe these systems .
12-11 Firing order
The firing order is the sequence in which the cylinders deliver their
power strokes. It is designed into the engine. The crankpin and camshaft
arrangement determine the firing other. In most engines, the firing order
evenly distributes the power strokes along the crankshaft ( Fig 12-20). Most
engine designs avoid firing two cylinders, one after the other , at the same
end of the crankshaft .
Firing orders for the same type of engine may differ. Two firing orders
for in-line four-cylinder engines are 1-3-4-2 and 1-2-4-3. In-line six-cylinder
engines use 1-5-3-6-2-4 (fig 12-20). A Chrysler V-6 and two General Motors
V-6 engines (fig 12-19) all have the same firing order of 1-2-3-4-5-6. Ford V-6
engines have fired 1-4-2-5-3-6 and 1-4-2-3-5-6. A firing order used on V-8
engines by Chrysler and General Motors is 1-8-4-3-6-5-7-2 ( fig 12-20). Ford
V-8 engines use 1-5-4-2-6-3-7-8 and 1-3-7-2-6-5-4-8 .
Many engine service jobs require that you know the cylinder numbering
and firing order. Some engines have cylinder numbering identification, firing
order, and direction of ignition-distributor rotation cast into or imprinted on the
intake manifold. The information is also in the manufacturer’s service manual.
The complete firing order of a four-cycle engine represents two
complete revolutions of the crankshaft. This is 720 degrees of crankshaft
rotation. Most engines are “even firing “. This means, for example, that is an
in-line six-cylinder engine a firing impulse occurs every 120 degrees of
crankshaft rotation (720 ÷ 6 = 120). The firing order of this engine is 1-5-3-6-

pintle back down onto its seat. This stops the fuel spray. Each opening and
closing of the injector pintle is an injector pulse.
Note : some injectors use a ball valve instead of a needle valve.
Operation of the ball-type injector is basically the same as described above.
19-3 Electronic fuel injection
Figure 10-19 shows the components of an electronic fuel injection (EFI)
system. Most fuel-injection systems are electronically controlled. The
controller is an electronic control module (ECM) or electronic control unit
(ECU). It is also called an “ on-board computer“ because it is “on-board“ the
car.
Various components of the engine and fuel system send electric signals
to the ECM (fig 19-5). The ECM continuously calculates how much fuel to
inject. It then opens the fuel injectors so the proper amount of fuel sprays out
to produce the desired air-fuel ratio.
19-6 Air and fuel metering
The fuel system must accurately measure or meter the air and fuel
entering the engine. This produces the proper air-fuel ratio to make a
combustible mixture. A mixture that is too lean (not enough fuel in it) will not
burn and produces excessive pollutants. A mixture that is too rich (excess fuel
in it) will also produce excess pollutants. Figure 19-8 shows how mixture
richness affects engine power. As the mixture becomes leaner, power falls off.
The electronic engine control system includes the ECM and various
sensing devices or sensors that report to it. A sensor is a device that receives
and reacts to a signal. This may be a change in pressure, temperature, or
voltage. Some sensors report the amount of air entering. The ECM then
calculates for how long to open the injectors.
19-7 Operaion of fuel-injection systems
Sensors that report to the ECM include ( fig 19-5)
• Engine speed.
• Throttle position.

measured by a sensor that changes vacuum (or absolute pressure) into a
varying voltage signal. The ECM combines this with the TPS signal to
determine how much air entering. Inputs from other sensor may cause the
ECM to modify this calculation (fig 19-5 ). Engine speed (instead of throttle
position) and intake-manifold vacuum can also tell the ECM how much air is
entering the engine .
19-13 Measuring intake-manifold vacuum (manifold absolute
pressure)
Intake-manifold vacuum is measured in two ways ( fig 19-19 ):
1. With a vacuum gauge.
2. With a manifold absolute pressure (MAP) gauge.
The two gauges are basically the same. Both have a flexible diaphragm
that separates the two chambers in the gauge. The difference is that one
chamber of the vacuum gauge is open to the atmosphere. One chamber of
the absolute-pressure gauge contains a vacuum (fig 19-19). The vacuum
gauge compares atmospheric pressure with intake-manifold pressure. In a
naturally-aspirated engine, manifold pressure is less than atmospheric
pressure. A vacuum gauge measures this partial vacuum in the intake-
manifold .
The manifold absolute-pressure (MAP) gauge compares the actual
pressure in the intake manifold with a vacuum. This is more accurate than the
vacuum gauge which compares intake manifold vacuum with atmospheric
pressure. The vacuum gauge is less accurate because atmospheric pressure
varies .
Vacuum and pressure sensor are not constructed exactly like the
gauges described above. But their operation is basically the same. Most
electronic engine control systems include a manifold-absolute pressure (MAP)
sensor (figs 10-19 and 19-20 ). It senses the pressure (vacuum) changes in
the intake manifold. This information is sent as a varying voltage signal to the
ECM .

4. Heated film :The heated film consists of metal foil or nickel grid
coated with a high-temperature material (fig19-22). Current
flowing through the film heats it. Air flowing past the film cools it.
Like the heated wire, the system maintains the film at a specific
temperature. The amount of current required is a measure of air
flow .
19-15 Atmospheric-pressure and air-temperature sensors
Changing atmospheric pressure and air temperature change the density
of the air. Air that is hot and at low atmospheric pressure is less dense. It
contains less oxygen than an equal volume of cooler air under higher
atmospheric pressure. When the amount of oxygen entering the engine
varies, so does the amount of fuel that can be burned .
Some systems include an atmospheric-pressure sensor. It is also called
the barometric-pressure sensor or BARO sensor. It is similar to the MAP
sensor. However, the barometric-pressure sensor reads atmospheric
pressure. The air-temperature sensor (fig 19-23) is a thermistor. Its electrical
resistance decreases as its temperature increases. Figure 19-21 shows its
location in the vane-type air-flow meter. Both types of sensors send varying
voltage signals to the ECM so it knows the atmospheric pressure and air
temperature .
21-1 Purpose and types of carburetors:
The carburetor (fig 21-1) is a mixing device that supplies the engine with
a combustible air-fuel mixture. Figure 21-2 shows the three basic parts of a
fixed-venturi carburetor.These are the air horn, the float bowl, and the throttle
body .
The venturi is a restricted space through which the air entering the
engine must pass. The air movement produces a partial vacuum in the
venturi. This is called venturi vacuum. The resulting pressure differential
causes fuel to discharge from the fuel nozzle into the intake air (fig 21-3).This
produces the air-fuel mixture for the engine.

2. Compresses only air on the compression stroke.
3. Heat of compression ignites fuel as it sprays into the engine
cylinders.
4. Has a high compression ratio of 16:1 to 22:1.
5. Controls engine power and speed only by the amount of fuel
sprayed into the cylinders. More fuel equals more power.
6. Has glow plugs or an electric intake-manifold heater to make
starting easier.
25-1 Heat in the engine
The burning air-fuel mixture in the engine cylinders may reach 4000
o
F
(2200
o
C) or higher. This means engine parts get hot. However, cylinder walls
must not get hotter than about 500
o
F (260
o
C). Higher temperatures cause
lubricating oil to break down and lose its lubricating ability. Other engine parts
are also damaged. To prevent over-heating, the cooling system removes the
excess heat (fig 15-14). This is about one-third of the heat produced in the
combustion chambers by the burning air-fuel mixtrure .
25-2 Purpose of cooling system
The cooling system (figs 11-23 and 25-1) keeps the engine at its most
efficient temperature at all speeds and operating conditions. Burning fuel in
the engine produces heat. Some of this heat must be taken away before it
damages engine parts. This is one of the three jobs performed by the cooling
system. It also helps bring the engine up to normal operating temperature as

31-1 Purpose of ignition system
The purpose of the ignition system (figs 11-21 and 31-1) is to ignite the
compressed air-fuel mixture in the engine combustion chambers. This should
occur at the proper time for combustion to begin. To start combustion, the
ignition system delivers an electric spark that jumps a gap at the combustion-
chamber ends of the spark plugs. The heat from this arc ignites the
compressed air-fuel mixture. The mixture burns, creating pressure that
pushes the piston down the cylinders so the engine runs.
The ignition system may be either a contact-point ignition system or an
electronic ignition system. This chapter describes the contact-point ignition
system. Chapter 32 covers electronic ignition systems. Ignition system
trouble-diagnosis and service are covered in chap. 33.
31-3 Producing the spark
The ignition system consists of two separate but related circuits: the
low-voltage primary circuit and the high-voltage secondary circuit. The ignition
coil (fig 31-1) has two windings. The primary winding of few hundred turns of
heavy wire is part of the primary circuit. The secondary winding of many
thousand turns of fine wire is part of the secondary circuit. When the ignition
key is ON and the contact points closed, current flows through the primary
winding(fig 31-7). This produces a magnetic field around the primary windings
in the coil .
When the contact points open, current flow stops and the magnetic field
collapses. As it collapses, it cuts across the thousands of turns of wire in the
coil secondary winding. This produces a voltage in each turn. These add
together to produce the high voltage delivered through the secondary circuit to
the spark plug (fig 31-5).
31-7 Advancing the spark
When the engine is idling, the spark is timed to reach the spark plug just
before the piston reaches TDC on the compression stroke. At higher speeds,
the spark must occur earlier. If it does not, the piston will be past TDC and

diaphragm. This rotates the breaker plate so the contact-points open and
close earlier (fig 31-14). Any vacuum port above the throttle valve provides
ported vacuum .
32.1 Types of electronic ignition systems
By the early 1970s, most automotive engines using a contact-point
distributor ( Chap. 31) could not meet exhaust-emission standards.
Federal regulations required the ignition system to operate for 50.000
miles [ 80.465km] with little or no maintenance. Contact points cannot do
this. They burn and wear during normal operation. This changes the
point gap, which changes ignition timing and reduces spark energy.
Misfiring and increased exhaust emissions result.
Most 1975 and later automotive engines have an electronic
ignition system (Fig. 32-1). It does not use contact points. Instead,
transistors and other semiconductor devices (Chap. 10) act as an
electronic switch that turns the coil primary current on and off.
There are four basic types of electronic ignition systems:
1. Distributor type with mechanical centrifugal and vacuum advance
(Figs. 1-19 and 32-2).
2. Distributor type with electronic spark advance ( Figs. 1-27 and 1-28).
3. Distributor type with multiple ignition coils ( Figs. 1-8 and 1-13) .
4. Distributor type with direct capactior-discharge (CD) ignition for
each spark plug.
42.1 Purpose of the clutch
The automotive drive train or power train ( 1-11) carries power from
the engine to the drive wheels. In vehicles with a manual transmission
or manual transaxle ( Chap. 43), the power flows through a clutch ( Figs.
1-19 and 42-1). This device couples and uncouples the manual
transmission or transaxle and the engine. The clutch is usually operated
by the driver’s foot. Some clutches have a power-assist device to
reduce driver effort. Various electronic devices may be used so that the

4. When engaged ( clutch pedal up ), the clutch transmits power from the
engine to the transmission. All slipping has stopped.
43-1 Purpose of transmission or transaxle :
There are three reasons for having a transmission or transaxle in
the automotive power train or drive train. The transmission or transaxle can:
1. Provide the torque needed to move the vehicle under a variety of road
and load conditions. It does this by changing the gear ratio between the
engine crankshaft and vehicle drive wheels.
2. Be shifted into reverse so the vehicle can move backward .
3. Be shifted into neutral for starting the engine and running it without
turning the drive wheels.
There are two basic types of transmissions and transaxles: manual
and automatic. Manual transmissions and transaxles are shifted manually, or
by hand. Automatic transmissions and transaxles shift automatically, with no
help from the driver .
43-2 Difference between transmissions and transaxles
The manual transmissions (Figs , 42.1and 43.1) is an assembly of
gears, shafts, and related parts. There are contained in a metal case or
housing filled with lubricant (43.16). A manual transmissions is used in some
front–wheel-drive vechicles (Fig , 42.1) and in front-engine rear- wheel-drive
vehicles (Fif,43.2). It is positioned between the clutch (Chap , 42 ) and
the driveshaft ( Chap , 45) that carries engine power to the drive wheels. The
engine, clutch, transmission, and driveshaft are all in a single line .
The manual transaxle ( Figs,42.2 and 43.3 ) is also an assembly
of gears and shafts. It attaches to a front-mounted tranverse engine and
drives the front wheels (Fig , 43.4). Rear-engine cars use engine-mounted
transaxle to drive the rear wheels. A few front-engine cars drive the rear
wheels through a rear-mounted transaxle.
The transaxle includes a final drive and a differential (front differential
in Fig . 43.3). There devices are not found in the transmission.The final drive

Moving the gearshift lever (Fig , 42.3, and 43.1) makes the shift which
changes the gear ratio( 42.3). In some vehicles, the gearshift lever is on the
steering column ( 43.13). In others, it is on the floor or in a center console( Fig
34.29).
45-3 Universal joints
A universal joint allows driving torque to be carried through two shafts
that are at an angle with each other. Figure 45.3 shows a simple cardan
universal joint. It is a double-hinged joint made of two Y-shaped yokes and a
cross-shaped member or spider. One yoke is on the driving shaft, and the
other is on the driven shaft. The four arms of the spider or trunnions are
assembled in needle bearings in the two yokes (Fig , 45.4).
The driving-shaft-and-yoke force the spider to rotate. The other two
trunnions of the spider then cause the driven yoke to rotate. When the two
shafts are at an angle with each other, the needle bearings permit the yokes
to swing around on the trunnions with each revolution.
There are several types of universal joints. The simplest is the spider-
and-two-yoke design (Fig.45-3 and 45-4). However, this is not a constant-
velocity universal joint. If the two shafts are at an angle, the driven shaft
speeds up and slows down slightly, twice per revolution. The greater the
angle, the geater the speed varies. This can cause a pulsating load that wears
the bearings and gears in the drive axle. Contant-velocity universal joints or
CV joints eliminate this unwanted speed change.
Figure 45-1 shows a two-piece drive line with the sections connected
through a double-cardan universal joint at the center. The double-cardan joint
is one type of constant-velocity universal joint. It basically is two simple
universal joints assembled together (Fig.45-5). They are linked by a centering
ball and socket which splits the angle between the two shafts. This cancels
any speed variation because the two joints operate at the same angle (half
the total). The acceleration of one joint is canceled by the deceleration of the
second joint. Later sections describe other types of universal joints.

the differential case (45-18).
NOTE: The final-drive gears described above are bevel or hypiod gears
(Figs. 45-6). They change the direction of power flow by 90 degrees so
rotation of the driveshaft rotates the axle shafts (Figs 45-14). In a transxale,
the final-drive gears are usually hellcal gears (Figs. 43-3 and 43-15). These
are used because the pinion gear and ring gear are on parallel shafts. Figure
43-6 shows both types of gears.
45-19 Differential operation
Figure 45-17 shows the basic parts of a differential. Figure 45-13 shows
an assembled differential. When the car is on a straight, level road and both
tires have equal traction, there is no differential action. (Traction is the
adhesive or pulling friction of a tire on the road). The ring gear, differential
case, differential pinion gears, and differential side gears all turn as a unit.
The pinion gears do not rotate on the pinion shaft, but rather turn both side
gears and axle shafts at the same speed.
When the vehicle enters a curve, the resistance of the inner tire to
turning begins to increase. It now has a shorter distance to travel (Fig. 45-18).
The outer tire must travel a greater distance. The differential pinion gears are
applying the same torque to each side gear. However, the unequal loads from
the tires cause the pinion gears to begin rotating on the pinion shaft. They
walk around the slower-turning inner-wheel side gear. This increases the
speed of the outer-wheel side gear by the same amount.
Figure 45-18 shows differential action in a typical turn. The differential
case speed is 100 percent. The rotating pinion gears carry 90 percent of this
speed to the slower-turning inner wheel. The rotating pinion gears carry 110
percent of the speed to the faster-turning outer wheel.
The differential described above is a standard or open differential. It
delivers the same torque to each wheel. If one tire begins to slip and spin, the
open differential divides the rotary speed unequally. The tire with good
traction slows and stops. This may also stop the vehicle or prevent it from

auxiliary axle built into the transmission or transaxle.
46-3 Purpose of the transfer case
The typical transfer case attaches to the rear of the transmission in
place of the extension housing (Figs 46-1 and 46-3). Engine power flows
through the transmission output shaft to the transfer-case input shaft. If the
vehicle has part-time four-wheel drive, the driver selects either two-wheel or
four-wheel drive. Gearing in the transfer case then sends power to only the
rear axle (two-wheel drive) or to both front and rear axles (four-wheel drive).
Some vehicles have full-time four-wheel drive. The transfer case remains in
four-wheel drive and the front axle engages automatically as soon as the rear
wheels begin to spin.
Automotive transfer cases are classified as single-speed or two-speed.
The single-speed transfer case can divide the power and deliver it to either
axle or both axles. In addition, the two-speed transfer case has a low range
and a high range. The driver can select either two-wheel drive or four-wheel
drive in high range. Neutral, or low range with four-wheel drive (Fig.46-4).
Figure 46-5 shows the power-flow through a two-speed transfer case as
the shift lever is moved to the different positions. The four modes of transfer
case operation are obtained by moving two sliding gears. These are splined to
the transfer-case output shafts for the front and rear axles.
High range in the transfer case provides direct drive, or a gear ratio of
1:1. Low range usually produces a gear reduction of about 2.5:1. This reduces
vehicle speed while greatly increasing the low-speed torque available. A
single-speed transfer case usually has s 1:1 ratio .
49-1 Purpose of the suspension system
The suspension system (Fig. 49-1) is located between the wheel axles
and the vehicle body or frame. Its purpose is to:
1. Support the weight of the vehicle.
2. Cushion bumps and holes in the road.
3. Maintain traction between the tires and the road.

from a tapered rod (Fig. 49-3). This gives the springs a variable spring
rate (49-5). As the spring is compressed, its resistance to further
compression increases.
2. LEAF SPRING Two types of leaf springs are single-leaf and multileaf
springs (Fig. 49-4). These have several flexible steel plates of graduated
length, stacked and held together by clips. In operation, the spring
bends to absorb road shocks. The plates bend and slide on each other
to permit this action. Single-leaf springs are described in 49-13
3. TORTION BAR The torsion bar is a straight rod of spring steel, rigidly
fastened at one end to the vehicle frame or body. The other end
attaches to an upper or lower control arm (Fig. 49-5). As the control arm
swings up and down in response to wheel movement, the torsion bar
twists to provide spring action.
4. AIR SPRING The air spring (Fig 49-6) is a rubber cylinder or air bag
filled with compressed air. A plastic piston on the lower control arm
moves up and down with the lower control arm. This causes the
compressed air to provide spring action. If the load in the vehicle
changes, a valve at the top of the air bag opens to add or release air.
An air compressor connected to the valve keeps the air springs inflated.
49-4 Sprung and unsprung weight
The total weight of the vehicle includes the sprung weight and the
unsprung weight. The sprung weight is the weight supported by springs. The
unsprung weight is the part not supported by springs. This includes the weight
of drive axles, axle shafts, wheels, and tires.
The unsprung weight is kept as low as possible. The roughness of the
ride increases as unsprung weight increases. To take an extreme example,
suppose the unsprung weight equals the sprung weight. As the unsprung
weight moves up and down, due to the wheels meeting road bumps and
holes, the sprung weight would move up and down the same amount. For this
reason, the unsprung weight should be only a small part of the total weight of

Figure 52-1 shows the brake system in an automobile. It has two types of
brakes:
1. The service brakes, operated by a food pedal, which slow or stop the
vehicle.
2. The parking brakes, operated by a food pedal or hand lever, which hold
the vehicle stationary when applied.
Most automotive services brakes are hydraulic brakes. They operate
hydraulically by pressure applied through a liquid. The service or
foundation brakes on many medium and heavy-duty trucks and buses
are oprated by air pressure (pneumatic). These are air brakes. Many
boat and camping trailers have electric brakes. All these braking system
depend on friction( 52-2) between moving parts and stationary parts for
their stopping force.
52-15 Types of disc brakes
The disc brake (fig 52-17) has a metal disc or rotor instead of a drum. It
uses a pair of flat, lined shoes or pads that are forced against the rotating disc
to produce braking. The pads are held in a caliper (figs 52-17 and 52-18) that
straddles the disc. The caliper has one or more pistons, with a seal and dust
boot for each. During braking, hydraulic pressure behind each piston in fig.
52-17 pushes it outward. This forces the pad into contact with the disc. The
resulting frictional contact slows and stops the disc and wheel.
There are three types of disc brakes. Figure 52-17 shows a fixed-caliper
disc brake. The other two are the floating-caliper and sliding-caliper. Each
differs in how the caliper mounts and operates.
Note: All three types of disc brakes work in the same general way. However,
vehicle manufacturers have used many variations of each. Typical examples
are described below. Refer to the vehicle service manual for information about
the brakes on a specific vehicle.
1. fixed-caliper disc brake: A fixed caliper (figs 52-17 and 52-19A) has
pistons on both sides of the disc. Some use two pistons, one on each

pressure to the brake at each wheel. This “pumping the brakes” keeps the
rate of wheel deceleration below the speed at which the wheels can lock.
53-2 Operation of the antilock-braking system
Figure 53-1 shows a vehicle equipped with a vacuum brake booster (52-
32) and four-wheel antilock brakes.The brake lines from the master cylinder
connect to a hydraulic unit or actuator. Lines from the actuator connect to the
wheel brakes. The actuator is controlled by the ABS control module.
Wheel-speed sensors (fig.53-1 and 53-2) at each wheel continuosly
send wheel-speed information to the ABS control module. There is ABS
action until the stoplight switch signals the control module that the brake pedal
has been depressed. When the control module senses a rapid drop in wheel
speed, it signals the actuator to adjust or modulate the brake pressure to that
wheel.This prevents wheel lockup.
53-9 Purpose of traction control
Any time a tire is given more torque than it can transfer to the road, the
tire loses traction and spins. This usually occurs during acceleration. To
prevent unwanted wheelspin, some vehicles with ABS also have a traction-
control system (TCS). When a wheel is about to spin. The traction-control
system (Fig.53-10) applies the brake at that wheel. This slows the wheel until
the chance of wheel spin has passed.
53-10 Operation of traction-control system
The antilock-braking system and traction-control system share many
parts. The wheel-speed sensors report wheel speed to the ABS/TCS control
module (Fig. 53-10). When a wheel slows so quickly that it is about to skid,
the ABS holds or releases the brake pressure at that wheel. If wheel speed
increases so quickly that the wheel is about to spin, the TCS applies the brake
at that wheel. This slows the wheel and prevent wheel spin.
The TCS can also reduce engine speed and torque if braking alone
does not prevent wheelspin. When this is necessary, the ABS/TCS control
module signals the engine control module. It then retards the spark and