All trailers with air brakes, manufactured after March 1, 1998, must have ABS in order to comply with FMVSS 571.121. This standard
also applies to air-over-hydraulic brake systems.
An ABS system consists of wheel sensors that send electric impulses to an Electronic Control Unit (ECU) which pulses the service brakes
in the event of wheel lockup. The minimum ABS con figuration for a tandem axle trailer is a 2S/1M. This indicates two wheel sensors will
be connected to and monitored by the ECU, and one modulator valve will control the service brakes. 4S/2M ABS con figuration is also