Technician Guidelines for Antilock Braking Systems
Air-Braked Trucks, Tractors and Trailers
This is a continuation of the reprint in HTML format
of the government document FHWA-MC-98-008. This is the glossary
section of that document. The original document can be found at
the following listed website: htttp://mchs.fhwa.dot.gov/safetyprogs/brakesaft.htm
GLOSSARY OF ABS TERMS
The following terms are used by one or more manufacturers to
describe different aspects of ABSs:
Antilock Braking System (ABS) - A system that monitors
and controls wheel speed during brak-ing so as to minimize wheel
lockup while maximizing vehicle lateral stability. Plural form—ABSs.
ABS Configuration - The arrangement of antilock braking
system components, which varies by the number of sensors and modulator
valves used. The following configurations for tractors are common-place:
4S/4M, 6S/4M, and 6S/6M. For trailers, 2S/1M., 2S/2M, 4S/2M and
4S/3M. (S=sensor. M=modulator.)
ABS Inline Valve - A modulator valve located in the service
brake delivery line near the wheel’s brake chamber which modifies
brake pressure during an ABS event. Also see ABS Modulator Valve
or ABS Relay Valve.
ABS Modulator Valve - An electro-pneumatic control valve
that contains the solenoids used to precisely modulate brake air
pressure during an ABS event. Also see ABS Inline Valve or ABS
ABS Relay Valve - A valve that performs the service relay
function as well as the ABS modulator valve function to modify
brake air pressure during an ABS event. Also see ABS Modulator
Valve or ABS Inline Valve.
Anti-Spin Regulation (ASR) - See Traction Control.
Automatic Traction Control (ATC) - See Traction Control.
Axle Control - That mode of ABS control whereby one modulator
controls the air pressure to the brake chambers on both ends of
a given axle. Also referred to as axle-by-axle control.
Bracket Mounting - The means of installing the ABS modulator-controller
on the host vehicle by using a supplied, pre-formed bracket.
Brake Proportioning - The limiting of brake air pressure
to a specific axle or tandem to compensate for varying vehicle
loading. Brake proportioning is most beneficial during bobtail
Braked Wheel Behavior - The study of wheel reactions
during braking, particularly be-tween the road surface and the
Category (I, II, & III) - A means of categorizing ABS
performance used in Europe.
Chamber Pressure - The air pressure in the brake chambers
during a brake application.
Channel - The electrical connection between the ECU and
the modulator. The term is also used to describe the number of
individual modulators in a particular antilock system.
Chuff Test - Also called ignition blowdown test. A test—designed
to simplify diagnostics—used to exercise the ABS modulator(s)
upon initial power-up. The “chuff” sound is made by air escaping
from rapid exercising of the exhaust solenoid (and supply solenoid)
on each modulator.
Coefficient of Friction - A measure of the friction (such
as between a tire and the road surface) available to use as surface
retardation. The ratio is defined as “Force Required to Overcome
Friction/Weight” and is denoted by the Greek letter m. See also
Control Algorithm - The specific configuration of logical
decisions implemented to determine the characteristics of an ABS
cycle. Apply, release, hold, etc., determinations are made in
the control algorithm, which is implemented in the ABS software
contained in the electronic control unit (ECU).
Control Pressure - The air pressure applied from the
foot/hand valve which con-trols the brake application pressure
either directly or through a relay valve. The ABS interrupts this
pressure by adding a modu-lator in series such that the air pressure
at the individual brake chambers may vary from the control pressure.
During ABS operation, therefore, chamber pressure may be equal
to or less than the control pressure.
Controller - Another name for the electronic control unit
(ECU). See Electronic Control Unit.
Current - Current represents the flow of electrons through
a conducting medium, such as copper. Current is measured in amperes
or amps and can be derived through the following formula: Amp
= Volt/Ohm or I=V/R.
Cycle - A single sequence of pressure application and
release during ABS operation. This cycle repeats during an ABS
event as long as impending wheel lock-up is identified. Also referred
to as “cycling.”
Data Link - The TMC/SAE J1708/J1587 Serial Data Link
Standard used in most vehicle-mounted ECUs.
Diagnostics - A method of identifying faulty components
or parameters. For example, a series of LED lights may be used
to identify specific ABS components that need to be serviced or
Diagonal Split - The case in which ABS is disabled on
both the specific wheel with an ABS failure and its diagonal counterpart
to maintain vehicle control during emergency stops.
Dynamic Fault - A fault detected with the wheel speed
sensors or modulators when the wheels are rotating. See Static
Electronic Control Unit (ECU) - An on-board vehicle computer
that controls the ABS, traction control and diagnostic functions.
The ECU receives input sig-nals, processes the information, and
sends output signals to the necessary ABS components.
Electromagnetic Interference - Electromagnetic interference
(EMI) disrupts the proper opera-tion of an electronic device or
system. EMI is caused by electro-magnetic field(s).
EPROM - EPROM stands for Erasable Programmable Read Only
Memory. The term refers to an integrated circuit that contains
the ABS control algorithm.
Exciter - A metal ring, normally with 100 evenly spaced
teeth, although sometimes with 80 or 120 teeth, depending on tire
size. It is usually attached to the barrel of the hub on each
ABS-monitored wheel. When the wheel rotates, the teeth move past
the wheel speed sensor pickup to create an electrical signal that
the ECU uses to determine wheel speed. Also called a Tooth Wheel.
Failure Lamp - An indicator lamp that indicates ABS operational
status. See Malfunction Indicator Lamp.
FMVSS - Federal Motor Vehicle Safety Standard. FMVSS
121, “Air Brake Systems,” is the regulation that applies to air
brakes used on commercial vehicles.
Four-Channel ABS - A system that has four sensors and
four modulators (4S/4M) or six sensors and four modulators (6S/4M).
Full-Time Power - This term refers to an ABS design in
which a circuit connects the tractor and trailer to supply constant
electrical power for an ABS. See Stop-lamp Power.
Ghost Sensing - In-axle Speed Sensing Systems where one
wheel/axle is sensed and the differential gear is sensed. The
ECU uses these two inputs to calculate the speed of the unsensed
wheel (i.e., the ghost sensor). Ghost Sensed Speed = (2)(Average)
In-Axle Sensor/Sensing - The practice of locating wheel
speed sensing devices inside the drive axle housing of the ABS-equipped
vehicle. This sensing option offers additional environmental protection
for the wheel speed sensor, but presents special service considerations
for equipment users.
ISO Connector - A multi-pin tractor-trailer electrical
connector used in Europe that meets International Standards Organization
(ISO) require-ments. This connector carries power, failure lamp
status, and serial communications to and from European trailer
ABSs. The ISO 7638 connector, for example, provides a dedicated
ABS power source for European tractor-trailers. The ISO 3731 connector
is used by a North American manufacturer for ABSs as well.
J560 Connector - See Seven-Pin Connector.
J1587 - An SAE Recommended Practice for applications
dealing with the J1708 serial data bus. This standard deals with
the assign-ment of specific parameter codes including diagnostics
and other system attributes. SAE J1587 and J1708 must be used
together to fully implement the noncritical data exchange on heavy
vehicles. See J1708.
J1708 - An SAE Recommended Practice for serial exchange
of vehicle-based, noncritical parametric information. This standard
estab-lishes the hardware and protocol requirements for the serial
data bus. See J1587.
Jackknife - A condition that can occur when either tractor,
trailer, or tractor and trailer wheels lose traction and lateral
vehicle stability cannot be maintained.
Lateral Stability - The resistance of a vehicle to forces
which attempt to change its direction of travel. Maximum lateral
stability is achieved at zero percent wheel slip (free rolling
LED - Light-emitting diode used in some ABS diagnostic
systems to convey diagnostic information.
Malfunction Indicator Lamp - A lamp that becomes active
whenever an ABS is not fully functional. The tractor/truck lamp
is on the instrument panel. A trailer/dolly in-cab indicator is
not required by law until March 2001. However, an external trailer/dolly
indicator lamp is required, effective March 1998. By March 2009,
the external lamp will no longer be required. Also called Warning
Lamp or Failure Lamp.
Manifold - The central device on which the modulators
of a two- or three-channel system may be commonly mounted.
Microcontroller - An application-specific microprocessor
geared around a specific control function. Also referred to as
Modulator - See ABS Modulator Valve.
Mu - Refers to the Greek letter m which represents coefficient
of friction. See Coefficient of Friction.
NHTSA - National Highway Traffic Safety Administration.
This division of the U.S. Department of Transportation regulates
the safety of new vehicles. NHTSA is the federal agency that requires
the installation of ABSs on new commercial vehicles.
Non-Volatile Memory (NOVRAM) - Solid-state electronics
capable of retaining electrical information in the absence of
system power. This is how diagnostics information is saved in
the ABS ECU.
Power Jackknife - A non-braking induced condition whereby
the drive wheels of a tractor will spin under engine power, resulting
in a loss of lateral stability.
Quick-Release Valve - A commonly used valve located close
to a brake chamber that decreases the time required to exhaust
air pressure from it.
Reference Speed - An ideal rate of wheel speed deceleration
(optimum wheel slip) calculated by the ECU and based on actual
wheel speed infor-mation at the moment that the ABS is activated.
The ECU compares actual wheel speed to the reference wheel speed
during an ABS event and adjusts the brake application pressure
in an attempt to match the actual wheel speed with the ideal reference
Relay Valve - See ABS Relay Valve for definition as it
pertains to ABSs.
Retarder Control - A system which prevents the tractor
drive axle(s) from locking on slippery surfaces by disabling the
engine retarder during an ABS event.
RFI - Radio frequency interference. A type of electromagnetic
inter-ference (EMI) that occurs only in the radio frequency band.
See Electromagnetic Interference.
SAE - Society of Automotive Engineers. An organization
that sets voluntary engineering standards for automotive and aerospace
components, systems, and vehicles. See J560, J1587 and J1708.
Select High - A system design in which ABS bases all
control decisions to release or apply brakes to an axle or tandem
on the highest measured wheel speed. Under this design, ABS won’t
start cycling until all sensed wheels experience a tendency to
Select Low - A system design in which the ABS bases all
control decisions to release or apply brakes to an axle or tandem
on the lowest measured wheel speed. If only one wheel locks, the
ABS on all other controlled wheels on that axle or tandem will
Sensor Bushing - The friction spring device that is first
inserted into the sensor block, allowing the sensor pickup to
be adjusted and holding it in position during vehicle operation.
Also called a spring clip.
Seven-Pin Connector - An electrical connector used between
units of combination vehicles in North America to conduct electrical
power for the Stop Lamps, Turn Signals, Running Lamps, Ground,
and Auxil-iary (or ABS) circuits. Also known as the SAE J560 connector
Side-by-Side Control - A control system that uses one
modulator valve on each side of an axle or axle group to control
brake pressures independently, to improve braking performance
on split-co road surfaces.
Six-Channel ABS - A system that has six sensors and six
Skid Number - A term representing the coefficient of friction
( m) of a given surface as a whole number by multiplying the coefficient
of friction by 100: 0.70m x 100 = 70 Skid Number.
Software - As applied to ABSs, the complete package of
programs consist-ing of the ABS algorithm, error checking diagnostics,
engine management interface, and the structure which links every-thing
together. Software, which is contained in the EPROM(s) inside
the ECU, is a specific set of instructions that the ECU will execute
to perform a particular task.
Solenoid - A device that converts an electrical signal
into mechanical movement. It consists of a coil with a moveable
core that changes positions by means of electromagnetism when
current flows through the coil.
Split-Co - Split coefficient of friction. A condition
in which one side of a vehicle is on a high coefficient of friction
while the other side is on a low coefficient of friction (e.g.,
one side of the vehicle on dry pavement and one side on wet or
icy pavement). This condition is most likely to cause the vehicle
to experience yaw or a twisting/turning action during a stop without
an operational ABS.
Spring Clip - See Sensor Bushing.
Static Fault - A fault detected with the wheel speed
sensors or modulators when the wheels are not rotating. See Dynamic
Stop-Lamp Power - A design in which the ABS is powered
only by the stop lamp circuit and requires no additional dedicated
connector. See Full-Time Power.
Stopping Distance The distance required to stop a vehicle.
Stopping distance measurements begin when application (control)
pressure first begins to increase, and end when the vehicle comes
to a com-plete stop.
Tandem Control - An ABS design in which the four wheels
of the tandem axle are controlled by only one modulator.
TMC - The Maintenance Council of the American Trucking
Associa-tions. An industry group which develops voluntary recom-mended
practices on maintenance- and operation-related issues pertaining
to commercial vehicles, based on input from equipment users, vehicle
manufacturers, component suppliers, academia, and government representatives.
TMC RP 137 - TMC Recommended Practice 137 is a voluntary
standard that states tractors should deliver a minimum power level
of 12.5 volts at 10 amps load to the trailer half of the tractor-to-trailer
TMC RP 141 - TMC Recommended Practice 141 is a voluntary
standard that states at least 9.5 volts (which includes a 1.0-volt
safety margin) must be available at the trailer ABS ECU to ensure
Tone Ring - See Exciter.
Traction Control - A system to minimize drive wheel slip
(improve traction) under acceleration. Traction control uses the
ABS to apply braking pressure to a spinning wheel, transferring
engine power to the wheel(s) with better traction. Should all
the drive wheels start to slip, traction control system can improve
vehicle traction by reducing engine torque. Traction control systems
are referred to by several different names, depending on the manufacturer.
• Automatic Traction Control (ATC)
• Traction Control (TC)
• Automatic Slip Regulation or Anti-Spin Regulation (ASR)
Tooth Wheel - See Exciter.
Vehicle Power - The voltage and current delivered to various
electrical and/or electronic devices on a vehicle. Typical vehicle
power in North America is 9.0-16.0 volts direct current. European
vehicles typically operate from 18.0-32.0 VDC. TMC Recommended
Practice 137 establishes a voluntary standard that tractors should
deliver a minimum power level of 12.5 volts at 10 amps load. TMC
Recommended Practice 141 establishes a voluntary standard that
at least 9.5 volts (which includes a 1.0-volt safety margin) must
be available at the trailer ABS ECU to ensure proper operation.
Warning Lamp - See Malfunction Indicator Lamp
Wheel-by-Wheel Control - A type of ABS control in which
each wheel is controlled individually.
Wheel Slip - The difference between vehicle speed and
wheel speed, ex-pressed as a percentage. The formula is: Wheel
Slip = (100)(Vehicle Speed-Wheel Speed)/(Vehicle Speed).
Wheel Speed - The measured velocity of an individual
(sensed) wheel which is derived by the ABS ECU. Wheel speed may
differ from vehicle speed during wheel slip. See Wheel Slip.
Wheel Speed Sensor Pickup - A magnetic pickup-type sensor—coupled
with an exciter or tooth wheel—that produces a signal to indicate
wheel speed to the ECU. A permanent magnet and passing metal teeth
com-bine to produce an electrical signal with a frequency propor-tional
to the wheel speed. The teeth alter the magnetic field produced
by the sensor. The changing magnetic field produces an AC voltage
in pickup coil within the sensor.
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