Anti-lock brakes (ABS) were first sold commercially in in 1987 when Freightliner offered a system. The systems were so much better than what was previously available that in 1997 the government required ABS in tractor trailers. Today all air-braked tractors manufactured after March 1, 1997 are required to have ABS, and all trailers and single unit trucks were required to have ABS in 1998.
Generally anti-lock braking systems sense a wheel or axle’s rate of deceleration. The wheel speed is sent to the ABS computer which knows how fast a wheel should decelerate without spinning or locking. If the unit determines that the wheel is about to lock up, it can pump the brake. This regains the wheels ability to brake.
This same technology serves as the basis for stability, traction control and anti-roll systems.