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Antilock braking systems (ABS)

General Knowledge4 minManual section 2.18

ABS is a computer system that keeps your wheels from locking up during hard braking. It is an addition to your normal brakes. It does not increase or decrease your normal braking power, and it only works when wheels are about to lock. ABS does not always shorten your stopping distance, but it helps you keep control during hard braking.

How ABS works

  • Sensors detect when a wheel may lock.
  • An electronic control unit (ECU) then lowers brake pressure to stop the lockup.
  • Pressure is adjusted to give the most braking without lockup.
  • ABS reacts much faster than a driver can. At all other times the brakes work normally.

Which vehicles must have ABS

The Department of Transportation (DOT) requires ABS on:

  • Air-brake truck tractors made on or after March 1, 1997.
  • Other vehicles with air brakes (trucks, buses, trailers and converter dollies) made on or after March 1, 1998.
  • Hydraulic-brake trucks and buses with a GVWR of 10,000 lb or more built on or after March 1, 1999.

Many vehicles built before these dates were voluntarily equipped with ABS.

How to know if your vehicle has ABS

  • Tractors, trucks and buses have yellow ABS malfunction lamps on the instrument panel.
  • Trailers have yellow ABS malfunction lamps on the left side, on the front or rear corner.
  • Dollies built on or after March 1, 1998 must have a lamp on the left side.
  • Bulb check: on newer vehicles the lamp comes on at start-up and then goes out quickly. On older systems it may stay on until you are going over 5 mph.
  • If the lamp stays on after the bulb check, or comes on while driving, you may have lost ABS control on one or more wheels.
  • On towed units built before ABS was required, it can be hard to tell. Check underneath for an ECU and for wheel speed sensor wires running from the rear of the brakes.

How ABS helps you

Without ABS, hard braking on a slippery road can lock the wheels. If the steering wheels lock, you lose steering. If the other wheels lock, you may skid, jackknife or even spin.

ABS helps prevent lockup so you keep control. You may or may not stop faster, but you should still be able to steer around something while braking, and you avoid over-braking skids.

ABS on only part of the rig (only the tractor, only the trailer, or even one axle) still gives you more control. Brake normally.

  • Only the tractor has ABS: you should keep steering control and there is less chance of a jackknife. But watch the trailer and ease off the brakes (if safe) if it starts to swing out.
  • Only the trailer has ABS: trailer swing-out is less likely. If you lose steering or start a tractor jackknife, ease off the brakes (if safe) until you regain control.

Braking with ABS — and without it

With ABS, brake the way you always have:

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

One exception: in a straight truck or combination with working ABS on all axles, you may fully apply the brakes in an emergency stop.

If ABS is not working, you still have normal brakes. Drive and brake as usual, but get the system serviced soon.

Safety reminders: what ABS will not do

  • It will not let you drive faster, follow closer, or drive less carefully.
  • It will not prevent power skids or turning skids. It should prevent brake-caused skids and jackknifes, but not those from spinning drive wheels or going too fast in a turn.
  • It will not necessarily shorten stopping distance. It helps control, not always distance.
  • It will not increase or decrease ultimate stopping power. It is an add-on, not a replacement for normal brakes.
  • It will not change how you normally brake. It only acts when a wheel would lock from over-braking.
  • It will not make up for bad brakes or poor brake maintenance.

Remember: the best safety feature is still a safe driver. Drive so you never need your ABS — but if you need it, it could help prevent a serious crash.

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: 2.18

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

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

Preparing your questions…

Review questions with answers

  1. ABS is designed to:
    1. Increase your normal braking power
    2. Replace the service brakes
    3. Keep the wheels from locking during hard braking
    4. Make the vehicle stop in half the distance
    Show answer
    Keep the wheels from locking during hard braking

    ABS keeps wheels from locking during hard braking. It is an addition to normal brakes and does not increase or decrease braking power.

  2. In an ABS, what lowers brake pressure when a wheel is about to lock?
    1. The electronic control unit (ECU)
    2. The driver's foot
    3. The slack adjuster
    4. The air compressor governor
    Show answer
    The electronic control unit (ECU)

    Sensors detect possible lockup, and the ECU reduces brake pressure to prevent it.

  3. Truck tractors with air brakes must have ABS if they were built on or after:
    1. March 1, 1999
    2. January 1, 2000
    3. March 1, 1995
    4. March 1, 1997
    Show answer
    March 1, 1997

    DOT requires ABS on air-brake truck tractors built on or after March 1, 1997.

  4. Air-brake trailers, buses, trucks and converter dollies must have ABS if built on or after:
    1. March 1, 1997
    2. March 1, 1998
    3. March 1, 1999
    4. March 1, 2001
    Show answer
    March 1, 1998

    Other air-brake vehicles (trucks, buses, trailers, converter dollies) need ABS if built on or after March 1, 1998.

  5. Hydraulically braked trucks and buses need ABS if they have a GVWR of 10,000 lb or more and were built on or after:
    1. March 1, 1997
    2. March 1, 1998
    3. March 1, 1999
    4. March 1, 2005
    Show answer
    March 1, 1999

    Hydraulic-brake trucks and buses of 10,000 lb GVWR or more built on or after March 1, 1999 must have ABS.

  6. Where are the ABS malfunction lamps on a trailer?
    1. On the left side, at the front or rear corner
    2. On the right rear door
    3. Inside the tractor only
    4. On the landing gear
    Show answer
    On the left side, at the front or rear corner

    On trailers, the yellow ABS malfunction lamp is mounted on the left side, near the front or the rear corner.

  7. What color are ABS malfunction lamps?
    1. Red
    2. Green
    3. Blue
    4. Yellow
    Show answer
    Yellow

    Tractors, trucks, buses and trailers use yellow ABS malfunction lamps.

  8. On a newer vehicle, the ABS lamp comes on at start-up and goes out quickly. This means:
    1. The ABS has failed
    2. It is a normal bulb check
    3. The trailer has no ABS
    4. You must not drive
    Show answer
    It is a normal bulb check

    The lamp lights briefly at start-up as a bulb check, then goes out. That is normal.

  9. On older ABS systems, the malfunction lamp may stay on until you are driving over:
    1. 15 mph
    2. 25 mph
    3. 5 mph
    4. 1 mph
    Show answer
    5 mph

    On older systems the lamp could stay on until the vehicle is going faster than 5 mph.

  10. How can you tell if an older trailer, built before ABS was required, has ABS?
    1. Check underneath for an ECU and for sensor wires running from the rear of the brakes
    2. Check the color of the glad hands
    3. Listen for a beep when braking
    4. Read the tire sidewall
    Show answer
    Check underneath for an ECU and for sensor wires running from the rear of the brakes

    On older towed units, check underneath for an ECU and for wheel speed sensor wires.

  11. The main benefit of ABS during hard braking on a slippery road is that:
    1. You will always stop shorter
    2. You can follow more closely
    3. Brakes will never overheat
    4. You can keep steering control and avoid skids from over-braking
    Show answer
    You can keep steering control and avoid skids from over-braking

    ABS helps you avoid lockup so you can steer around obstacles while braking. You may or may not stop faster.

  12. Only your tractor has ABS. During braking, the trailer begins to swing out. You should:
    1. Brake harder
    2. Let up on the brakes, if it is safe
    3. Turn sharply toward the trailer
    4. Use the trailer hand valve
    Show answer
    Let up on the brakes, if it is safe

    With tractor-only ABS, watch the trailer and ease off the brakes (if safe) if it starts to swing out.

  13. Having ABS on only the trailer, or even on just one axle:
    1. Gives no benefit at all
    2. Means you must brake differently
    3. Still gives you more control during braking
    4. Is illegal
    Show answer
    Still gives you more control during braking

    ABS on only the tractor, only the trailer, or one axle still gives more control. Brake normally.

  14. In which case may you fully apply the brakes in an emergency stop?
    1. A straight truck or combination with working ABS on all axles
    2. Any vehicle with ABS on the tractor only
    3. Any vehicle without ABS
    4. A bus with ABS warning lamp on
    Show answer
    A straight truck or combination with working ABS on all axles

    The only exception to normal braking: with working ABS on all axles, you may fully apply the brakes in an emergency stop.

  15. Your ABS warning lamp stays on while driving. What is true?
    1. You have no brakes at all
    2. You must stop and call a tow truck right away
    3. You must use stab braking only
    4. You still have normal brakes; drive normally but get it serviced soon
    Show answer
    You still have normal brakes; drive normally but get it serviced soon

    If ABS malfunctions you still have regular brakes. Drive and brake normally, and get the system fixed soon.

  16. Which skid will ABS NOT prevent?
    1. A skid from over-braking
    2. A skid from spinning the drive wheels or going too fast in a turn
    3. A brake-caused jackknife
    4. Wheel lockup on wet pavement
    Show answer
    A skid from spinning the drive wheels or going too fast in a turn

    ABS prevents brake-induced skids, but not power skids or turning skids.

  17. With ABS, you can safely follow other vehicles more closely.
    1. True
    2. False
    Show answer
    False

    False. ABS will not let you drive faster, follow more closely or drive less carefully.

  18. When driving a vehicle with ABS, you should brake:
    1. The same way you always have, using only the force needed to stop safely and stay in control
    2. By pumping the pedal rapidly
    3. With full force every time
    4. Only with the trailer hand valve
    Show answer
    The same way you always have, using only the force needed to stop safely and stay in control

    Brake as you always have. ABS only acts when a wheel would lock.