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Antilock brakes (ABS) on trailers and combinations

Combination Vehicles3 minManual section 6.3

Which trailers must have ABS, how to tell if a trailer has it, and how to brake when it does. Short answer: ABS helps you keep control; it does not let you stop shorter or drive faster.

Which trailers have ABS (6.3.1)

Every trailer and converter dolly made on or after March 1, 1998, is required to have ABS. Many older units also had ABS added voluntarily.

Trailers have a yellow ABS malfunction lamp on the left side, on either the front or rear corner. Dollies built on or after March 1, 1998 must have a lamp on the left side.

For units built before that date it can be hard to tell. Look under the vehicle for the ECU (electronic control unit) and for wheel speed sensor wires coming from the back of the brakes.

Braking with ABS (6.3.2)

ABS is an add-on to your normal brakes. It neither adds to nor takes away from your normal braking power. It only works when a wheel is about to lock.

ABS does not necessarily shorten your stopping distance, but it helps you keep control during hard braking. A computer senses that a wheel is about to lock, reduces the brake pressure to a safe level, and you keep control.

Even trailer-only ABS, or ABS on just one axle, gives you more control while braking. With trailer-only ABS, the trailer is less likely to swing out. But if steering control is lost or the tractor starts to jackknife, ease off the brakes (when safe) until you have control again.

With a tractor-trailer that has ABS, brake the way you always have:

  • Use only the braking force you need to stop safely and stay in control.
  • Brake the same way whether ABS is on the tractor, the trailer, or both.
  • As you slow down, watch the tractor and trailer and back off the brakes (if safe) to stay in control.

If the ABS fails, you still have regular brakes. Drive normally, but get it serviced soon.

ABS does not allow you to drive faster, follow closer or drive less carefully.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 6.3

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

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Review questions with answers

  1. Trailers and converter dollies built on or after what date must have ABS?
    1. January 1, 1975
    2. March 1, 1998
    3. March 1, 1997
    4. July 1, 2001
    Show answer
    March 1, 1998

    Every trailer and converter dolly made on or after March 1, 1998 is required to have ABS.

  2. Where is the ABS malfunction lamp on a trailer?
    1. On the right rear door
    2. Inside the tractor cab only
    3. On top of the trailer roof
    4. On the left side, at the front or rear corner
    Show answer
    On the left side, at the front or rear corner

    Trailers have yellow ABS lamps on the left side, on the front or rear corner.

  3. What color is the trailer ABS malfunction lamp?
    1. Red
    2. Blue
    3. Yellow
    4. Green
    Show answer
    Yellow

    Trailer ABS malfunction lamps are yellow.

  4. On an older trailer with no ABS lamp, how can you tell if it has ABS?
    1. Check underneath for an ECU and for wheel speed sensor wires running from the back of the brakes
    2. Check the color of the glad hands
    3. Pull the trailer hand valve and listen
    4. Look for a red octagon knob
    Show answer
    Check underneath for an ECU and for wheel speed sensor wires running from the back of the brakes

    Check under the unit for an ECU and wheel speed sensor wiring running from the brakes.

  5. Which statement about ABS is correct?
    1. It increases your normal braking power
    2. It always shortens stopping distance
    3. It helps you keep control during hard braking
    4. It works all the time, even in gentle stops
    Show answer
    It helps you keep control during hard braking

    ABS doesn't necessarily shorten stopping distance, but it helps you keep control in hard braking.

  6. When does ABS activate?
    1. Every time you press the brake
    2. Only when wheels are about to lock up
    3. Only above 40 mph
    4. Only when the trailer is empty
    Show answer
    Only when wheels are about to lock up

    ABS only comes into play when wheels are about to lock.

  7. True or False: ABS on just the trailer, or on a single axle, still improves your control while braking.
    1. True
    2. False
    Show answer
    True

    True. Even partial ABS gives more control; with trailer ABS, the trailer is less likely to swing out.

  8. Your tractor-trailer has ABS. How should you brake?
    1. Pump the brakes rapidly
    2. Press as hard as you can every time
    3. Use only the trailer hand valve
    4. The same way you always have, using only the force needed
    Show answer
    The same way you always have, using only the force needed

    Brake as you always have: use only the force needed, the same way whether ABS is on tractor, trailer or both.

  9. If your ABS malfunctions, what should you do?
    1. Stop immediately and wait for a tow
    2. You still have regular brakes; drive normally and get it serviced soon
    3. Use only the trailer hand valve
    4. Drive faster to reach a shop quickly
    Show answer
    You still have regular brakes; drive normally and get it serviced soon

    You still have normal brakes. Drive normally, but get the system serviced soon.

  10. Only your trailer has ABS. You start a tractor jackknife while braking. What should you do?
    1. Brake harder
    2. Pull the trailer hand valve
    3. Let up on the brakes (if safe) until you regain control
    4. Turn off the engine
    Show answer
    Let up on the brakes (if safe) until you regain control

    If you lose steering control or start a tractor jackknife, let up on the brakes until you regain control.

  11. Your tractor-trailer has working ABS on all units. Which is true?
    1. You can safely follow more closely
    2. ABS does not let you drive faster, follow closer, or drive less carefully
    3. You can take curves faster
    4. You no longer need to inspect the brakes
    Show answer
    ABS does not let you drive faster, follow closer, or drive less carefully

    ABS helps you keep control in hard braking, but it is no reason to drive faster, tailgate, or be less careful.