Chapter 30
ABS and EBS
Objectives (1 of 3)
• Describe how an antilock brake system
(ABS) works to prevent wheel lockup during
braking.
• List the major components of a truck ABS.
• Describe the operation of ABS input circuit
components.
• Outline the role of the ABS module when
managing antiskid mode.
Objectives (2 of 3)
• Explain how the ABS module controls the
service modulator valves.
• Explain what is meant by the number of
channels of an ABS system.
• Describe how trailer ABS is managed.
• Outline the procedure for diagnosing ABS
faults.
Objectives (3 of 3)
• Describe the procedure required to set up
and adjust a wheel speed sensor.
• Explain how an electronic brake system
(EBS) manages service brake applications.
• Outline the reasons why an EBS has to meet
current FMVSS No. 121 requirements.
ABS Schematic
Shop Talk
• An ABS system is a simple computercontrolled system.
• As such, it requires system inputs,
processing capability, and outputs.
• The steps required to produce an outcome
are less complex than in an electronically
managed transmission, but electronically the
process is similar
Caution
• When driving an ABS-equipped truck, drivers
have a tendency to expect too much.
• Always exercise care when braking,
especially under bobtail conditions, even
when the vehicle ABS system is fully
functional.
ABS Component Locations
ABS Components (1 of 2)
• A typical truck ABS system requires the following
components:
– Wheel speed sensor (system input)
– An ABS module or ECU (system processing and
switching)
– Brake modulators or ABS valves (system output)
– Interconnecting wiring and connectors (electrically
connects the first three sets of components)
– Networking capability
ABS Components (2 of 2)
Shop Talk
• Systems that use a 100-tooth wheel sensor
do so for a reason.
– When divided into 360 degrees of one full tire
revolution, the 100 teeth on the pulse wheel
equal 3.6 degrees each.
– When a radial tire on a loaded-to-specification
truck is run down the highway at 60 mph, its
dynamic footprint is about 3.6 degrees.
– One- hundred-tooth ABS reluctor wheels have
become the industry standard.
Wheel Speed Signals
ABS Modulator (1 of 4)
ABS Modulator (2 of 4)
ABS Modulator (3 of 4)
ABS Modulator (4 of 4)
Shop Talk
• Always check the vehicle service literature for
wire and connector identification.
• Individual wire identification will differ
depending on the type of connectors in use,
the vehicle manufacturer, and the system
features in use.
ABS Electrical Schematic
ABS Configurations
•
•
•
•
Six-channel system
Six-/four-channel system
Four-channel system
Two-channel system
Trailer ABS
• 2S/1M = two wheel speed sensors and a single
modulator valve
• 2S/2M = two wheel speed sensors and two
modulator valves
• 4S/2M = four wheel speed sensors and two
modulator valves
• 4S/3M = four wheel speed sensors and three
modulator valves
• 6S/3M = six wheel speed sensors and three
modulator valves
ABS Trailer Cord Schematic
Caution
• ABS effectiveness is fully dependent on the
foundation brake adjustment.
• Use stroke indicators to check brake
adjustment status.
Shop Talk
• Bulb test
– Depress ABS switch for 16 seconds.
– The ABS light should illuminate even when no
codes are present.
ABS Operation
• If an ABS relay valve malfunctions, the wheels
controlled by the valve return to standard service
braking.
• If a sensor malfunctions in a two-sensor system,
control of that affected side of the trailer returns to
standard service braking.
• If one sensor malfunctions in a system that has four
sensors on a tandem axle, the ABS will continue to
function. In this event, the ECU manages the
system using the wheel speed data on the
functioning sensor on the same side of the tandem.
• If a problem completely disables the ABS, control
returns to standard braking.
ABS Tractor Power Supply Test
Warning
• The ABS is an electrical system.
• When working on ABS systems, the same
precautions that must be taken with any other
electrical system should be taken.
• There is a danger of electrical shock or sparks that
can ignite flammable substances.
• Always disconnect the battery ground cable before
working on the electrical system.
– This is of special concern when working with
flammable or explosive truck or trailer cargos.
ABS Diagnostics
Shop Talk
• Blink codes in most systems do not display
faults in the order in which they were
recorded in the ECU memory.
• Also, if codes are blinked that do not appear
in the OEM code chart, the ECU may be
defective.
Shop Talk
• Never use a battery charger to perform a
dynamic check of a trailer ABS system, as
ECU damage can result.
ABS Diagnostics
Stability Control/Rollover
Stability Control/Jackknife
Summary (1 of 4)
• Antilock brake systems (ABS) are designed
to help prevent wheel lockup during severe
braking.
• An ABS system uses microcomputer
technology to sense and reduce braking
force on wheels that are beginning to lockup
under braking.
• The components of a typical ABS system are
wheel speed sensors, control modules, and
brake modulators.
Summary (2 of 4)
• Several different ABS configurations are used on
tractors and trailers, depending on the number of
axles, axle configuration, axle load, brake circuit
distribution, and brake force distribution.
• Truck and trailer ABS are computer-controlled
system with networking capability.
• LED indicators and blink codes are used to
diagnose first generation ABS failures PCs and
proprietary software are used to read, diagnose,
and analyze system performance on current ABS
systems.
Summary (3 of 4)
• Electronic brake systems (EBS), also known
as electro-pneumatic braking, are capable of
replacing the current FMVSS 121-compliant
pneumatic control circuit with an
electronically managed control circuit.
• EBS provides much faster service braking
response and release times because its
electrical signals travel at much higher
speeds than pneumatic control signals.
Summary (4 of 4)
• An EBS system controls pressure in the
brake service chambers under all braking
conditions, unlike ABS, which tends to be a
factor only under severe braking conditions.
• Until FMVSS 121 is revised, EBS can only be
used on North American highways as an
option to a pneumatic control circuit.