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://


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 Relay Valve.

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 tractor operation.

Braked Wheel Behavior - The study of wheel reactions during braking, particularly be-tween the road surface and the tire.

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 “Mu.”

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 corrected.

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 Fault.

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) - Individual.

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 travel).

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 “computer chips.”

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 speed.

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 lock.

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 also cycle.

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 modulators (6S/6M).

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 Fault.

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 electrical connector.

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 proper operation.

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. These include:

• 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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