Truck Driver's Guide to Antilock Braking Systems
This is a reprint in HTML format of the government
document Truck Driver's Guide to Antilock Braking Systems
. The original document can be found at the following listed website:
This publication was produced through a cooperative
agreement between the Trucking Research Institute and the Federal
Highway Administration, Office of Motor Carrier and Highway Safety.
It is disseminated under the sponsorship of the Federal Highway
Administration in the interest of information exchange. The United
States Government assumes no liability for its contents or use
The contents do not necessarily reflect the official
policy of the Federal Highway Administration. This pamphlet does
not constitute a standard, specification, or regulation. The Trucking
Research Institute has made a reasonable effort to ensure the
accuracy of the information contained in this booklet. However,
every carrier should satisfy itself that the procedures outlined
herein are appropriate for its own use. Office of Motor Carrier
and Highway Safety Federal Highway Administration
Why this HTML Document Reprint?
The original Truck Driver's Guide to Antilock
Braking Systems document, is only available in Adobe Acrobat
Reader (PDF) file format. We here at Truck Troubles dot Com find
this document to have immense value for truck drivers.
Because many truck drivers do not have access to
their own personal computer, they can not install the required
PDF file viewer on the system they use to access the internet.
Also, when using slow modem connections to the internet, this
HTML format also enables much quicker access to the information,
because these HTML files are significantly smaller to retrieve
for viewing than the original PDF file.
By converting this document into HTML, all persons
who browse the internet, now have quicker access to this very
valuable document. In the conversion process, some very minor
wording changes have been implemented to support the HTML requirements.
Overall, this HTML reprint of the document is extremely close
to the original document. Any of these minor differences will
not materially affect the user of this reprint document. If you
require access to the original document, then go to http://mchs.fhwa.dot.gov/safetyprogs/brakesaft.htm
What Is An Antilock Braking
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
How Do Antilock Braking Systems
Sensors and computer logic detect potential wheel
lock up, by sensing excessive deceleration rates and noting substantial
differences in wheel speeds. An electronic control unit (ECU)
then tells the appropriate modulator valve(s) to decrease brake
pressure to avoid wheel lock up. Brake pressure is adjusted to
a level providing the maximum braking without danger of lockup.
ABS works far faster than the driver can respond to potential
wheel lock up. And it only adjusts the brake pressure to the wheels
that are in danger of locking up. At all other times the brake
system will operate normally.
How Is ABS Going To Help
First, it is important to understand what can happen
to you without ABS: When you brake hard on slippery surfaces in
a vehicle without ABS, your wheels may lock up. When your steering
wheels lock up, you lose steering control. When your other wheels
lock up, you may skid, jackknife or even spin the vehicle.
ABS helps you avoid wheel lockup. The computer
senses impending lockup, reduces the braking pressure to a safe
level, and you maintain control. You may or may not be able to
stop faster with ABS, but you should be able to steer around an
obstacle while braking, and avoid skids caused by overbraking.
But What If ABS Is Only On
The Tractor, or Only On The Trailer?
Having ABS on only the tractor, only the trailer,
or even on only one axle, still gives you more control over the
vehicle during braking. BRAKE NORMALLY When only the tractor has
you should be able to maintain steering control and there is less
chance of jackknifing. BUT keep your eye on the trailer and let
up on the brakes (if you can safely do so) if it begins to swing
out. When only the trailer has ABS, the trailer is less likely
to swing out, but if you lose steering control or start a tractor
jackknife, let up on the brakes (if you can safely do so) until
you gain control.
How Should I Brake With
When you drive a tractor-trailer combination with
ABS, you should brake as you always have. In other words:
- Use only the braking force necessary to stop safely and
stay in control.
- Brake the same way, regardless of whether you have ABS on
the tractor, the trailer, or both.
As you slow down, monitor your tractor and trailer
and back off the brakes (if it is safe to do so) to stay in control.
There is only one exception to this procedure. If you always drive
a straight truck or combination with working ABS on all axles,
in an emergency stop, you can fully apply the brakes.
Which Vehicles Have Antilock
Braking Systems, and Which Ones Don't?
The Department of Transportation requires that
ABS be on:
- Air-braked truck tractors built on or after March 1, 1997.
- Other air-braked 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
Many commercial vehicles built before these dates
have been voluntarily equipped with ABS.
How Do I Know If My Vehicle
Is Equipped With ABS?
- Check the certification label for the date of manufacture,
and compare it with the ABS schedule.
- 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
after February 1998 are required to have a lamp on the left
- Some antilock braking systems have diagnostic lamps incorporated
into their electronic control unit.
- 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.
What Happens If The Antilock
Braking System Isn't Working?
Without ABS, you still have normal brake functions.
Drive and brake as you always have. Vehicles with ABS have yellow
malfunction lamps to tell you if something isnít working. The
location of the
lamps are described above.
As a system check, on newer vehicles, the malfunction
lamp comes on at start-up for a bulb check, then goes out quickly.
On older systems the lamp could stay on until you are driving
over five miles per hour. If the lamp stays on after the bulb
check, or goes on once you are underway, you may have lost ABS
control at one or more wheels.
Remember, if your ABS malfunctions, you still have
regular brakes. Drive normally, but get the system serviced soon.
What ABS Won't Do!
- IT WONíT allow you to drive faster, follow more closely,
or drive less carefully.
- IT WONíT prevent power or turning skids; ABS should prevent
brake-induced skids or jackknifes, but not those caused by spinning
the drive wheels (traction-control might help there) or going
too fast in a turn.
- IT WONíT necessarily shorten stopping distance; ABS will
help maintain vehicle control, but not always shorten stopping
- IT WONíT increase or decrease ultimate stopping power;
ABS is an ďadd-onĒ to your normal
brakes, not a replacement of them.
- IT WONíT change the way you normally brake. Under normal
brake conditions, your truck will stop as it always stopped.
ABS only comes into play when a wheel would normally have
locked up because of overbraking.
- IT WONíT compensate for bad brakes or poor brake maintenance.
What's The Most Important
Thing To Remember About ABS?
- The best vehicle safety feature is still a safe driver.
- Drive so you never need to use your ABS.
- If you need it, ABS could help to prevent a serious accident.
to top of page , Driver
Knowledge Page ,
Truck Brakes page , HomePage.