Antilock Braking System (ABS) is a system that monitors and controls wheel speed during braking so as to minimize wheel lockup while maximizing vehicle lateral stability. ABS allows the driver to fully hit the brake with out fear of skidding or loss of control. The following terms are used by one or more manufacturers to describe different aspects of 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 vehicle are commonplace: 4S/4M, 6S/4M, and 6S/6M.

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.

ABS Modulator Valve – An electro-pneumatic control valve that contains the solenoids used to precisely modulate brake air pressure during an ABS event.

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.

Automatic Traction Control (ATC) – 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.

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.

ABS 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.

ABS Chamber Pressure – The air pressure in the brake chambers during a brake application.

ABS 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.

ABS 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).

ABS Control Pressure – The air pressure applied from the foot/hand valve which controls the brake application pressure either directly or through a relay valve. The ABS interrupts this pressure by adding a modulator 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.

ABS Failure Lamp – An indicator lamp that becomes active whenever an ABS is not fully functional. Also called as Malfunction Indicator Lamp.

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.

ABS 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).

Six-Channel ABS – A system that has six sensors and six modulators (6S/6M).

Tandem Control – An ABS design in which the four wheels of the tandem axle are controlled by only one modulator.

ABS 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, expressed as a percentage. The formula is: Wheel Slip = (100)(Vehicle Speed-Wheel Speed)/(Vehicle Speed).