• Each truck and bus manufactured on or after March 1, 1999, with a hydraulic brake system, must have both an ABS and ABS malfunction indicator system that meets federal standards.
• Each air braked truck tractor manufactured on or after March 1, 1997, and each CMV other than a truck tractor (i.e. trucks, buses, semitrailers, converter dollies, and full trailers), manufactured on or after March 1, 1998, must have an ABS that meets federal requirements.
• Certain vehicles with air brake systems must be equipped with an electrical circuit that sends a signal warning in the event of an ABS malfunction.

In March 1995, the National Highway Traffic Safety Administration (NHTSA) issued rules requiring antilock brake systems (ABS) for heavy trucks, tractors, trailers, and buses. Since the Federal Motor Carrier Safety Administration (FMCSA) has adopted these regulations in 393.55, all commercial motor vehicles (CMVs) built with ABS must have working ABS. These ABS rules provide specific requirements for:

• Hydraulic brake systems
  • ABS malfunction indicators for hydraulic braked vehicles
• Air brake systems
  • ABS malfunction circuits and signals for air braked vehicles
• Exterior ABS malfunction indicator lamps for trailers

The ABS rules in 393.55 do not apply to vehicles engaged in driveaway-towaway operations.

Hydraulic brake systems

Each truck and bus manufactured on or after March 1, 1999, with a hydraulic brake system, must have an ABS meeting the requirements of Federal Motor Vehicle Safety Standard (FMVSS) No. 105 (571.105, S5.5).

ABS malfunction indicators — Each hydraulic braked vehicle manufactured on or after March 1, 1999, must have an ABS malfunction indicator system that meets the requirements of FMVSS No. 105 ( 571.105, S5.3).

Air brake systems

Truck tractors — Each air braked truck tractor manufactured on or after March 1, 1997, must have an ABS meeting the requirements of FMVSS No. 121 (571.121, S5.1.6.1(b)).

CMVs (other than truck tractors) — Each air braked CMV other than a truck tractor, manufactured on or after March 1, 1998, must have an ABS that meets the requirements of FMVSS No. 121 (571.121, S5.1.6.1(a) for trucks and buses, and S5.2.3 for semitrailers, converter dollies and full trailers).

ABS malfunction circuits and signals — Certain vehicles with air brake systems must be equipped with an electrical circuit that sends a signal warning in the event of an ABS malfunction:

• Each truck tractor manufactured on or after March 1, 1997, and each single-unit air braked vehicle manufactured on or after March 1, 1998, must have an electrical circuit that will signal a malfunction that affects the generation or transmission of response or control signals to the vehicle’s ABS. (571.121, S5.1.6.2(a))
• Each truck tractor manufactured on or after March 1, 2001, and each single-unit vehicle that is equipped to tow another air-braked vehicle, must have an electrical circuit that will transmit a malfunction signal from the ABS on the towed vehicle(s) to the trailer ABS malfunction lamp in the cab of the towing vehicle, and must connect the electrical circuit to the towed vehicle. (FMVSS No. 121, 571.121, S5.1.6.2(b))
• Each semitrailer, trailer converter dolly, and full trailer manufactured on or after March 1, 2001, must have an electrical circuit that will signal a malfunction in the trailer’s ABS and must have the means for connection of the ABS malfunction circuit to the towing vehicle. In addition, each trailer manufactured on or after March 1, 2001, that is designed to tow another air-brake equipped trailer must be able to transmits a malfunction signal from the ABS of the trailer(s) it tows to the vehicle in front of the trailer. (FMVSS No. 121, 571.121, S5.2.3.2)

Exterior ABS malfunction indicator lamps for trailers — Each air braked trailer (including a trailer converter dolly) manufactured on or after March 1, 1998, must have an ABS malfunction indicator lamp which meets the requirements of FMVSS No. 121 (571.121, S5.2.3.3).