Technical information

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Technician Guidelines for Antilock Braking Systems
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I. AN INTRODUCTION TO ANTILOCK BRAKING
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 during a
road test?
A. 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.
The equipment requirements of FMVSS 121 specify that
ABSs on truck-tractors and full trailers must control the brake
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.