Where are antilock braking systems (ABS) required on trailers and dollies, and how can you tell a unit has it?

Trailers and converter dollies manufactured on or after the federally required date are equipped with ABS, just as tractors are. You can identify ABS-equipped units by the yellow ABS malfunction lamp, usually on the left side of the trailer, and frequently by an ABS label or the presence of wheel-speed sensor wiring at the wheels. When the ignition is turned on, the lamp should light briefly and then go out; if it stays on, the ABS has a fault and should be repaired, though the normal brakes still function. Knowing which trailers and dollies have ABS tells the driver how the combination will behave during hard braking.