Which combination vehicles are required to have ABS?

Federal regulations require ABS on air-braked tractors built on or after March 1, 1997, and on air-braked trailers built on or after March 1, 1998. Hydraulic-braked vehicles have similar requirements. Vehicles built before these dates are not required to have ABS, but many have been retrofitted. An ABS-equipped vehicle has a yellow ABS warning light that comes on briefly at startup and goes out once the system has self-tested; a steady or recurring yellow ABS light indicates a malfunction. ABS prevents wheel lockup during heavy braking, preserving steering control and often reducing stopping distance on slick surfaces. Drivers should brake firmly with ABS, not pump the brakes.