chap81 pps Automotive technology at University of Cambridge

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

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OBJECTIVES:
After studying Chapter 81, the reader should
be able to:

•
•
•

Prepare for ASE Brakes (A5) certification test
content area “F” (Antilock Brake System
Diagnosis and Repair).

Explain the reason for ABS.
Describe the purpose and function of the ABS
components, such as wheel speed sensors,
electrohydraulic unit, and electronic
controller.
Continued

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

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OBJECTIVES:
After studying Chapter 81, the reader should
be able to:

•
•

Discuss how the ABS components control
wheel slippage.
Explain how the ABS components control
acceleration traction control.

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

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KEY TERMS:
accumulator • active sensor • air gap • antilock braking
systems (ABS)

channel • control module
Electronic Stability Control (ESC) • flash codes
integral ABS • isolation solenoid • nonintegral ABS
pressure decay stage • pressure dump stage • pressure
holding stage • pressure increase stage • pressure
reduction stage • pressure release stage
Continued
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KEY TERMS:
Rear Antilock Braking System (RABS) • Rear Wheel AntiLock (RWAL) • release solenoid
select low principle • solenoid valves
tire slip • tone ring • traction • traction control

wheel speed sensors (WSS)

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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ABS CHARACTERISTICS
Antilock braking systems (ABS) help prevent the wheels from 
locking during sudden braking, especially on slippery surfaces. 
They eliminate lockup and minimize the danger of skidding, 
allowing the vehicle to stop in a straight line. 
ABS can optimize braking when road conditions are less than 
ideal, as when making a sudden panic stop or when braking on a 
wet or slick road.
ABS does this by monitoring the relative speed of the wheels to 

one another. It uses this information to modulate brake pressure as 
needed to control slippage and maintain traction when the brakes 
are applied.
Continued
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ABS and Tire Traction  Preventing brake lockup is important 
because of the adverse effect a locked wheel has on tire traction.
Figure 81–1 Maximum braking traction occurs when
tire slip is between 10% and 20%. A rotating tire has
0% slip and a locked-up wheel has 100% slip.

The brakes slow rotation of the wheels; 

friction between tire and road stops the 
vehicle and allows it to be steered. 
If tire traction is reduced, stopping 
distances increase, and directional
stability of the vehicle suffers.
A free­rolling wheel has nearly zero
tire slip, while a locked wheel has
100% tire slip.
See Figure 81–1.
Continued
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Tire Slip and Braking Distance On dry or wet pavement, 

maximum braking traction occurs when tire slip is held between 
approximately 15% and 30%. 
On snow­ or ice­covered pavement, the optimum slip range is 20% 
to 50%. In each case, if tire slip increases beyond these levels, the 
amount of traction decreases. 
Figure 81–2 Traction is determined by
pavement conditions and tire slip.

Shortest stopping distances
are obtained when the brakes
are applied with just enough 
force to keep the tire slip in
the range where traction is 
greatest.
Continued
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Tire Slip and Vehicle Stability  A tire’s contact patch with the 
road can provide only a certain amount of traction. When a 
vehicle is stopped in a straight line, nearly all available traction 
can be used to provide braking force. If a vehicle has to stop and 
turn at the same time, the available traction must be divided to 
provide both cornering (lateral) and braking force.
No tire can provide full cornering and full braking power at the 
same time. When a brake is locked and the tire has 100% slip, all 
of available traction is used for braking; none is left for steering.
A skidding tire follows the path of least resistance. If the rear 
brakes lock, the back end of the vehicle will swing around toward 
the front. If the front brakes lock, steering control will be lost and 
the vehicle will slide forward in a straight line.
Continued
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ABS and Base Brakes  An antilock braking system is an “add­on” 
to the existing base brake system, and only comes into play when 
traction conditions are marginal or during stops when the tires lose 
traction and slip excessively. The rest of the time ABS has no 
effect on normal driving, handling, or braking.
A vehicle with ABS brakes uses the same brake linings, calipers, 
wheel cylinders, and other system components as a vehicle without 
ABS brakes. The only exception being the master cylinder.
All ABS are also designed to be as “fail­safe” as possible. Should a 
failure occur that affects the operation of the ABS, the system will 
deactivate itself and the vehicle will revert to normal braking.
ABS failure will not prevent the vehicle from stopping.
Continued
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ABS Limitations  An antilock brake system will not provide the 
shortest stopping distances in straight stops on smooth, dry pavement 
by an expert driver. 
Figure 81–3 A good driver can control tire
slip more accurately than an ABS if the
vehicle is traveling on a smooth, dry
road surface.

This is possible because  
antilock braking systems 
may allow tire slip to drop 
as low as 5%, below the 
point where maximum tire 
traction is achieved. 
For the average driver, antilock
brakes will stop the vehicle in
a shorter distance.
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Antilock brakes will not provide shortest stops when braking on 
loose gravel or dirt, or in deep, fluffy snow. A locked wheel will 
stop the vehicle faster because loose debris builds up and forms a 
wedge that helps stop the vehicle. 
Figure 81–4 A wedge of gravel or snow in the front of a locked wheel can help stop a vehicle
faster than would occur if the wheel brakes were pulsed on and off by an antilock braking
system.

Continued
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An antilock braking system will prevent this wedge from forming, 
so some vehicles with antilock brakes have a switch on the 
instrument panel that allows the system to be deactivated when 
driving on these kinds of road surfaces.
ABS can’t overcome physics. The weight and speed of a moving 
vehicle give it kinetic energy, and only so much of that energy can 
be converted into braking or cornering force at any given time.
The limiting factor in this conversion is the traction between the 
tires and road.

Continued
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Figure 81–5 Being able to steer and control the
vehicle during rapid braking is one major advantage
of an antilock braking system.

Another situation occurs 
when a vehicle enters a 
corner traveling faster 
than physically possible to 
negotiate the turn. 

In this situation, antilock 
brakes will not prevent 
the vehicle from leaving 
the road. 
They will allow the 
vehicle to be slowed and 
steered in the process, 

thus lessening the 
severity of the eventual 
impact. 

Continued

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ABS OPERATION
All ABS control tire slip by monitoring relative deceleration rates 
of the wheels during braking, by one or more wheel speed sensors.
If one wheel starts to slow at a faster rate than others, or at a faster 
rate than programmed in the control module, it indicates a wheel 

is starting to slip and is in danger of losing traction and locking. 

Continued
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Figure 81–6 A typical stop on a slippery road surface without antilock brakes. Notice that the
wheels stopped rotating and skidded until the vehicle finally came to a stop.

Continued
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The ABS responds by momentarily reducing hydraulic pressure to 
the brake on the affected wheel or wheels. This allows the wheel 
to speed up momentarily so it can regain traction. As traction is 
regained, brake pressure is reapplied to again slow the wheel.
Electrically operated solenoid valves (or motor­driven valves in 
the case of Delphi ABS­VI applications) are used to hold, release, 
and reapply hydraulic pressure to the brakes. This produces a 
pulsating effect, which can be felt in the brake pedal. 
The effect is much the same as pumping the brakes, except that 
the ABS does it automatically for each brake circuit, and at speeds 
that would be humanly impossible—up to dozens of times per 
second depending on the system (some cycle faster than others).

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SYSTEM CONFIGURATIONS
All ABS keep track of wheel deceleration rates with wheel speed 
sensors. The various ABS use a different number of sensors, 
depending on how the system is configured. 
Figure 81–7 ABS configuration includes four-channel, three-channel, and single-channel.

Continued
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Four­Channel ABS  On some applications, each wheel is 
equipped with its own speed sensor.
This type of arrangement is called a “four­wheel, four­channel” 
system since each wheel speed sensor provides input for a separate 
hydraulic control circuit or “channel.”
The term channel always refers to the number of separate or 
individually controlled ABS hydraulic circuits in an ABS,
not the number of wheel speed sensor electrical circuits.
NOTE: For vehicle stability systems to function, there has to be four 

wheel speed sensors and four channels so the hydraulic control unit can 
pulse individual wheel brakes to help achieve vehicle stability.
Continued
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Three­Channel ABS  Some four­wheel ABS have a separate 
wheel speed sensor for each front wheel but use a common speed 
sensor for both rear wheels.
These are called “three­channel” systems. The rear wheel speed 
sensor is mounted in either the differential or the transmission.
The sensor reads the combined or average speed of both rear 
wheels. This type of setup saves the cost for an additional sensor 
and reduces the complexity of the system by allowing both rear 
wheels to be controlled simultaneously.
This is known as the select low principle. Three­channel systems 
are the most common type of ABS setup used on rear­wheel­drive 
applications.
Continued
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Single­Channel ABS  The single­channel rear­wheel­only ABS is 
used on many rear­wheel­drive pickups and vans. Ford’s version is 
Rear Antilock Braking System (RABS), while GM and Chrysler 
call theirs Rear Wheel Anti­Lock (RWAL).
The front wheels have no speed sensors, and only a single speed 
sensor mounted in the differential or transmission is used for both 
rear wheels.
Rear­wheel antilock systems are typically used on applications 
where vehicle loading can affect rear wheel traction, which is
why it is used on pickup trucks and vans.
Because the rear­wheel antilock systems have only a single 
channel, they are much less complex and costly than their 
multichannel, four­wheel counterparts.

Continued
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Integral and Nonintegral  Another distinction between ABS is 
whether they are integral or nonintegral ABS.
Integral systems combine the brake master cylinder and ABS 
hydraulic modulator, pump, and accumulator into one assembly.
Integral systems do not have a vacuum booster for power  assist 
and rely instead on pressure generated by the electric pump for 
this purpose.
Most of the older ABS applications are  integral systems. Integral 
ABS include the Bendix 10 and Bendix 9 (Jeep) ABS, Bosch 3, 
Delco Moraine Powermaster III, and Teves Mark 2.

See Figure 81­8. 
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Figure 81–8 A typical integral ABS unit that combines the function of the master cylinder,
brake booster, and antilock braking system in one assembly.

Continued
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Nonintegral ABS, sometimes referred to as “add­on” systems, are 
the dominant type of ABS because of lower cost and simplicity. 
They have a conventional brake master cylinder and vacuum power 
booster with a separate hydraulic modulator unit.
Some have an electric pump for ABS braking (to reapply pressure 
during the ABS hold­release­reapply cycle), but do not use the 
pumps for normal power assist.
Nonintegral (add­on) systems include Bendix 3, Bendix 6, Bendix 
ABX­4, Bendix Mecatronic, Bosch 2, Bosch 2S Micro, Bosch 2U, 
Bosch 2E, Bosch 5, Delco Moraine ABS­VI, Kelsey­Hayes 
RABS/RWAL, 4WAL, EBC­5 and EBC­10, Sumitomo ABS, Teves 
Mark 4 ABS and MK20, and Toyota rear­wheel ABS.
See Figure 81–9.
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Figure 81–9 A typical nonintegral-type (remote) ABS.

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