CDL · Antilock Brakes

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

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

Correct answer

Trailers and converter dollies built after the required date have ABS, indicated by a yellow ABS malfunction lamp on the trailer (and on the dolly) and often an ABS label

A ABS is shown by a red light
B Only the tractor can have ABS
C ABS is never used on trailers
D Trailers and converter dollies built after the required date have ABS, indicated by a yellow ABS malfunction lamp on the trailer (and on the dolly) and often an ABS label

Why this is the answer

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.

Source: FMCSA CDL Manual 7.4.1 Antilock Brakes on Trailers and Dollies

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About this topic: Antilock Brakes

ABS prevents wheel lockup during heavy braking, preserving steering and reducing stopping distance on slick surfaces. Drivers must brake firmly, not pump, on ABS-equipped vehicles. This is one of 5 Doubles and Triples questions on antilock brakes, each with a full explanation on a single study page.

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

Why this is the answer

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About this topic: Antilock Brakes

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