Technician Guidelines for Antilock Braking Systems
Air-Braked Trucks, Tractors and Trailers

This is the first section of the reprint in HTML format of the government document FHWA-MC-98-008. The original document can be found at the following listed website:


This section reviews several basic antilock braking system (ABS) concepts. When you complete this section, you should be able to answer the following questions:

What is an ABS?
Why are antilock braking systems (ABSs) standard on most new commercial vehicles?
How does an ABS work?
What are the major features and benefits of ABSs?
How should I drive an ABS-equipped vehicle?

What is an ABS?

Antilock braking systems (ABSs) are electronic systems that monitor and control wheel slip during vehicle braking. ABSs can improve vehicle control during braking, and reduce stopping distances on slippery (split or low coefficient of friction) road surfaces by limiting wheel slip and minimizing lockup. Rolling wheels have much more traction than locked wheels. Reducing wheel slip improves vehicle stability and control during braking, since stability increases as wheel slip decreases.

ABSs can be applied to nearly all types of vehicles and can be successfully integrated into hydraulic and air brake systems (including air over hydraulic). This document applies to the ABSs used with air brake systems on commercial vehicles. The National Highway Traffic Safety Administration (NHTSA) requires—through FMVSS 121, “Air Brake Systems” and FMVSS 105, “Hydraulic Brake Systems”—that ABSs be installed on commercial vehicles built (built meaning the official date of manufacture) on or after:

• March 1, 1997, for air-braked truck-tractors.
• March 1, 1998, for other air-braked vehicles (trucks, buses, trailers and converter dollies).
• March 1, 1999, for hydraulically braked trucks and buses with gross vehicle weight ratings of more than 10,000 lbs.

Antilock braking systems (ABSs) are electronic systems that monitor and control wheel slip during vehicle braking. Reducing wheel slip improves vehicle stability and control during braking, since stability increases as wheel slip decreases.

The equipment requirements of FMVSS 121 specify that ABSs on truck-tractors and full trailers must control the brakepressures to at least one front axle and one rear axle. The ABSs on semi-trailers and dollies must control at least one axle of the vehicle. Additionally, the ABSs on tractors must control one of the rear axles with two modulator valves so that the brake pressure on one end of the axle is independent of the brake pressure on the other end. The performance requirements of FMVSS 121 can require an ABS on additional axles.

NHTSA defines an ABS as a portion of a service brake system that automatically controls the degree of rotational wheel slip during braking by:

  • Sensing the rate of angular wheel rotation.
  • Transmitting signals regarding the rate of wheel rotation to one or more devices, which interpret these signals and generate responsive controlling output signals.
  • Transmitting those signals to one or more devices which adjust braking forces in response to the signals.

Other aspects of NHTSA’s rule stipulate that:

  • ABSs on trailers be capable of being powered by the trailer’s stop lamp circuit.
  • New tractors—built on or after March 1, 1997— provide constant electrical power to a tractor-to-trailer electrical connector for powering trailer ABSs.
  • Vehicles required to have an ABS also have a yellow ABS malfunction indicator lamp which lights up to indicate most malfunctions.
  • The power unit’s ABS malfunction lamp be “in front of and in clear view” of the driver. It lights when the ignition key is first switched “on” for a bulb check.
  • The ABS malfunction lamp on trailers be mounted on the left side of the trailer, near the rear side marker lamp. On dollies, the lamp is located on the left side where it can be seen by someone standing about 10 feet from the lamp. The lamp lights for a short bulb check when the vehicle is stopped and the ABS starts receiving electrical power. This lamp will no longer be required after February 2009.
  • Air-braked tractors and trucks which tow other air-braked vehicles—built on or after March 1, 2001— have an in-cab warning lamp which indicates malfunctions in any towed trailer’s or dolly’s ABS. Its location and function are the same as for the powered unit’s ABS malfunction lamp.
  • Trailer and dolly ABSs—built on or after March 1, 2001—have the equipment needed to send an ABS malfunction signal to the towing vehicle. A towing trailer must also be able to relay an ABS malfunction signal from the vehicle it is towing to the vehicle towing it.

How Do ABSs Work?

An ABS consists of several key components:

  • electronic control unit (ECU)
  • wheel speed sensors
  • modulator valves
  • exciter rings.

Here’s how these components work together:

  • Wheel speed sensors constantly monitor and send electrical pulses to the ECU at a rate proportional to the wheel speed.
  • When the pulse rates indicate impending wheel lockup, the ECU signals the modulator valve(s) to reduce and/or hold the brake application pressure to the wheel(s) in question.
  • The ECU then adjusts pressure, seeking one which gives maximum braking without risking wheel lockup.
  • When the ECU acts to modulate the brake pressure, it will also (on most vehicles) turn off the retarder (if so equipped) until the risk of lockup is over.

The ECU continually checks itself for proper operation. If it detects a malfunction/failure in the electrical/electronic system, it will shut down that part of the ABS affected by the problem—or the entire ABS—depending upon the system and the problem. When this happens, the ABS malfunction lamp lights.

An ABS adjusts brake pressure much faster and more accurately than can drivers. It’s faster because:

  • electronic controls are very fast and
  • ABS modulator valves are physically closer to the brakes than is the driver’s foot brake valve.

It is more effective, too, because an ABS can tailor the brake pressure to each wheel or set of wheels to provide maximumbraking/stability. Some vehicles also use a traction control system in conjunction with the ABS.

Electronic controls allow an ABS to adjust brake pressure faster and more accurately than can drivers. An ABS is more effective on slippery roads because it tailors the brake pressure at the wheel to maximize vehicle braking and stability.

Traction control helps the ABS improve vehicle traction by minimizing wheel slip on the drive axle during acceleration. If a wheel on the drive axle starts to slip, the traction control system automatically brakes the wheel slightly, transferring engine torque to the wheels with better traction. If all the drive wheels start to slip, the traction control system may also reduce engine power. Traction control systems are referred to by several different names, depending on the manufacturer. These include:

  • Automatic Traction Control (ATC)
  • Traction Control (TC) • Automatic Slip Regulation/Anti-Spin Regulation (ASR)

How Should I Drive an ABS-equipped Vehicle During?

It is the consensus of brake experts that drivers should brake an ABS-equipped vehicle just as they would brake a non-ABS equipped vehicle. The proper braking technique is to maintain a steady, modulated brake application. Modulated, in this case, means applying only the pressure required to achieve the desired deceleration. Do not slam on the brakes to make speed corrections or routine stops.

When operating on slippery surfaces, with or without an ABS, it is strongly recommended that drivers depress the clutch when braking. Engine braking itself can cause drive wheels to slip. Usually, any retarder will automatically be disabled when the ABS is in use.

Brake an ABS-equipped vehicle just as you would brake a non-ABS equipped vehicle. Only apply the pressure required to achieve the desired deceleration.

Much of what is taught about hydraulic ABSs doesn’t apply to air ABSs. Thus, it’s important to remember the following:

  • Brake as if no ABS is present, with a modulated application as described previously.
  • Unless certain that the entire combination vehicle has a working ABS, don’t stomp on the brakes in a panic situation—one or more wheels could lock and cause the vehicle to jackknife. Even then, be careful because you can still jackknife or lose control if the vehicle is travelling too fast.
  • Do not expect to feel the brake pedal pulsing or hear strange sounds when the ABS activates on air-braked vehicles. These vehicles do not transmit pulsing pressure to the driver’s foot and the driver probably will not hear the system cycling.
  • Operate mixed combination vehicles (with and without an ABS) the same way one would operate totally non-ABS combination vehicles. Apply only the brake pressure needed to achieve the desired deceleration while ensuring vehicle stability. Monitor the combination vehicle behavior and back off the brake pedal, if possible, to keep the units under control.

ABS Features and Benefits
Control of steering, drive and trailer wheels Increases steering ability and vehicle stability during braking.
Reduces possibility of jackknifing and trailer swing.
Reduces tire flatspotting.
Fail-safe electrical/electronic system If the electrical/electronic system fails, the ABS is shut off, returning the vehicle to normal braking. On some systems, the ABS is only shut off at the affected wheels.
Traction Control An optional feature that controls excessive wheel spin during acceleration, reducing the possibility of power skids, spins or jackknifes.
Self-diagnosing system Built-in system makes maintenance checks quick and easy.
Diagnostic tool compatibility ABSs are compatible with industry standard hand-held and computer-based diagnostic tools. Blink codes and other diagnostic schemes can also be used for troubleshooting, if other tools are not available.
ABS Malfunction Indicator Lamp Informs the driver or technician that an ABS fault has occured. The warning lamp may also transmit blink information. It does not signal all possible faults.

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