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2.18 – Antilock Braking Systems (ABS)

  • ABS is a computerized system that keeps your wheels from locking up during hard brake applications.
  • ABS is an addition to your normal brakes. It does not decrease or increase your normal braking capability. ABS only activates when wheels are about to lock up.
  • ABS does not necessarily shorten your stopping distance, but it does help you keep the vehicle under control during hard braking.

2.18.1 – How Antilock Braking Systems Work

  • Sensors detect potential wheel lock up. An electronic control unit (ECU) will then decrease brake pressure to avoid wheel lockup.
  • Brake pressure is adjusted to provide the maximum braking without danger of lockup.
  • ABS works far faster than the driver can respond to potential wheel lockup. At all other times the brake system will operate normally.

2.18.2 – Vehicles Required to Have Antilock Braking Systems

The Department of Transportation requires that ABS be on:

  • Truck tractors with air brakes built on or after March 1, 1997.
  • Other air brake vehicles, (trucks, buses, trailers and converter dollies) built on or after March 1, 1998.
  • Hydraulically braked trucks and buses with a gross vehicle weight rating of 10,000 lbs. or more built on or after March 1, 1999.

Many commercial vehicles built before these dates have been voluntarily equipped with ABS.

2.18.3 – How to Know If Your Vehicle Is Equipped with ABS

  • Tractors, trucks, and buses will have yellow ABS malfunction lamps on the instrument panel.
  • Trailers will have yellow ABS malfunction lamps on the left side, either on the front or rear corner.
  • Dollies manufactured on or after March 1, 1998, are required to have a lamp on the left side.
  • As a system check on newer vehicles, the malfunction lamp comes on at start-up for a bulb check and then goes out quickly. On older systems, the lamp could stay on until you are driving over 5 mph.
  • If the lamp stays on after the bulb check or goes on once you are underway, you may have lost ABS control.
  • In the case of towed units manufactured before it was required by the Department of Transportation, it may be difficult to tell if the unit is equipped with ABS. Look under the vehicle for the ECU and wheel speed sensor wires coming from the back of the brakes.

Multiple-Choice Questions:

Question #156 (1 of 4)

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Which of the following are not true about ABS?

  • ABS is a computerized system that keeps your wheels from locking up during hard brake applications.
  • ABS does not necessarily shorten your stopping distance, but it does help you keep the vehicle under control during hard braking.
  • ABS creates more friction on the brake shoes and drums, which in turn creates more heat. Therefore, you must manage the heat in ABS systems more stringently than on non-ABS systems
  • ABS is an addition to your normal brakes. It does not decrease or increase your normal braking capability. ABS only activates when wheels are about to lock up.
  • ABS is a computerized system that keeps your wheels from locking up during hard brake applications.
  • ABS is an addition to your normal brakes. It does not decrease or increase your normal braking capability. ABS only activates when wheels are about to lock up.
  • ABS does not necessarily shorten your stopping distance, but it does help you keep the vehicle under control during hard braking.
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Question #157 (2 of 4)

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Which of the following statements are not true about the way ABS works?

  • ABS works far faster than the driver can respond to potential wheel lockup. At all other times, the brake system will operate normally.
  • Sensors detect potential wheel lock-up. An electronic control unit (ECU) will then decrease brake pressure to avoid wheel lockup.
  • ABS will minimize wear on the brake shoes and drums by applying the minimum necessary stopping force based on sensor input
  • Brake pressure is adjusted to provide maximum braking without the danger of lockup.
  • Sensors detect potential wheel lock up. An electronic control unit (ECU) will then decrease brake pressure to avoid wheel lockup.
  • Brake pressure is adjusted to provide the maximum braking without danger of lockup.
  • ABS works far faster than the driver can respond to potential wheel lockup. At all other times the brake system will operate normally.
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Question #158 (3 of 4)

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Which of the following is not a way to determine if your vehicle has anti-lock brakes?

  • As a system check on newer vehicles, the malfunction lamp comes on at start-up for a bulb check and then goes out quickly.
  • Trailers will have yellow ABS malfunction lamps on the left side, either on the front or rear corner.
  • There will be an ABS inspection plate on the right-front corner of the tractor, normally near the headlight
  • Tractors, trucks, and buses will have yellow ABS malfunction lamps on the instrument panel.

2.18.3 – How to Know If Your Vehicle Is Equipped with ABS

  • Tractors, trucks and buses will have yellow ABS malfunction lamps on the instrument panel.
  • Trailers will have yellow ABS malfunction lamps on the left side, either on the front or rear corner.
  • Dollies manufactured on or after March 1, 1998, are required to have a lamp on the left side.
  • As a system check on newer vehicles, the malfunction lamp comes on at start-up for a bulb check and then goes out quickly. On older systems, the lamp could stay on until you are driving over 5 mph.
  • If the lamp stays on after the bulb check or goes on once you are under way, you may have lost ABS control.
  • In the case of towed units manufactured before it was required by the Department of Transportation, it may be difficult to tell if the unit is equipped with ABS. Look under the vehicle for the ECU and wheel speed sensor wires coming from the back of the brakes.
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Question #159 (4 of 4)

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How can you tell if your ABS system has malfunctioned?

  • The ABS light on the rear of the trailer goes out
  • The ABS light on the dash goes out while you are driving, you may have lost ABS
  • The tires refuse to lock up under hard braking
  • The lamp stays on after the bulb check or goes on once you are underway, you may have lost ABS
If the lamp stays on after the bulb check or goes on once you are underway, you may have lost ABS control.
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