you believe antilock brake systems (ABS) have been around for roughly a
quarter-century? The very first electrohydraulic ABS systems made their
debut in Europe back in 1978 on a couple of Mercedes and BMW models.
ABS didn’t arrive in the U.S. until 1985 when Mercedes, BMW and Audi
began offering it as an option on some of their high-end luxury models.
Ten years later, ABS was available as an option or standard
equipment on most passenger cars and light trucks. Like air bags, ABS
quickly became a “must-have” feature because of the safety benefits it
supposedly provided. Nobody would argue the fact that ABS helps
prevents skids when braking hard on slick or wet surfaces. Most
consumers are sold on ABS and want it on the vehicles they buy.
brake systems are made by a number of OEM suppliers including Bosch,
Delphi, Kelsey-Hayes and Continental Teves. Bendix was another OEM
supplier, but was bought by Bosch a number of years ago.
steadily evolved and improved over the years, getting smaller, lighter,
faster and smarter. Many of today’s ABS systems also incorporate
traction control to prevent wheel spin when accelerating on slick
surfaces, and stability control to improve handling and driving safety
under all driving conditions.
The basic idea behind ABS is to
maximize braking traction and steering control by minimizing wheel slip
when braking. A tire that is just on the verge of slipping produces
more friction with respect to the road surface than one which is locked
and skidding. Once a tire loses its grip and skids, friction is reduced
and the vehicle takes longer to stop. Directional stability also is
lost, which may allow the vehicle to fishtail, spin or slide to either
The only exception to this rule is when a tire is braking on
loose snow. A locked tire allows a small wedge of snow to build up
ahead of it. This actually allows the vehicle to stop in a shorter
distance than if ABS were allowing the tires to roll. Because of this,
some cars have an ABS on/off switch so the driver can deactivate the
system when driving on snow.
HOW ABS WORKS
No matter what
type of ABS system is on a vehicle, they all control tire slip by
monitoring the relative deceleration rates of the wheels when braking.
On older trucks with rear-wheel only ABS, only the rear wheels are
monitored. But on all passenger cars and most newer trucks, all four
wheels are monitored (though a common wheel speed sensor may be used in
the rear axle to monitor the average speed of both rear wheels on some
If one wheel starts to slow at a faster rate than
the others, it’s an indication that the wheel is starting to slip and
is in danger of locking up and skidding. The ABS system responds by
cycling hydraulic pressure in the wheel’s brake circuit. A solenoid
valve in the ABS hydraulic control unit closes to isolate the brake
circuit. Then another solenoid opens to momentarily vent pressure. This
releases the brake and allows the wheel to regain momentum and
traction. The solenoid valves move again and pressure is reapplied to
By rapidly cycling the brakes on and off from three to
15 times per second (the speed varies with the capabilities of the
system), traction is maintained, the wheels don’t lock up and the
vehicle stops straight. The driver can usually hear a buzzing or
ratcheting noise from the ABS unit, and feel the rapid pulsations in
the brake pedal. This provides feedback to the driver that the ABS
system is kicking in and doing its thing to help prevent skidding.
ABS SYSTEM COMPONENTS
major components in an ABS system include the electronic control module
(which may be integrated into the Body Control Module or BCM), the
hydraulic modulator unit that contains the brake circuit solenoid
valves, and wheel speed sensors. Many ABS systems also have a pump and
a high-pressure accumulator that stores hydraulic pressure. The
pressure stored in the accumulator may be used to reapply the brakes
during ABS braking, and/or for power-assisted braking during normal
If the application is a late-model vehicle with stability
control, additional sensor inputs include a steering angle sensor, a
lateral acceleration sensor to monitor sideways forces acting on the
body, and a yaw sensor to monitor understeer and oversteer as the
vehicle responds to curves in the road and steering inputs. The lateral
acceleration sensor and yaw sensor are often combined into a single
WHEN THINGS GO WRONG
All ABS systems have
self-diagnostic capability and run various self-tests when a vehicle
starts out on a trip to check for faults. If a fault is found, it will
set a trouble code and turn on the ABS warning light (and/or traction
control light or stability control light is so equipped).
faults that are serious enough to turn on the ABS warning light will
also deactivate the ABS system. Consequently, the ABS system won’t be
able to intervene should it be needed until the fault is diagnosed and
repaired. This requires a scan tool that has software that can access
the ABS systems and codes. An ordinary OBD II scan tool or code reader
can’t do this because it only reads powertrain codes. So step one in
diagnosing any ABS problem is to plug in a scan tool that can read ABS
Once a technician has a code, it will give him a pretty
good idea of what might be wrong. Like powertrain codes, a code by
itself won’t always tell you which part needs to be replaced. But it
will identify the type of fault and/or the part of the system where the
Common ABS faults include codes for bad
accumulators (usually a low-pressure code or a failure to hold pressure
code), codes for wheel speed sensor circuits (which may be the sensor,
sensor ring, sensor wiring connector or wiring harness) and ABS
The accumulator is usually a small metal ball or
canister with a nitrogen-filled bladder inside. Over time, the rubber
bladder can crank and leak, preventing the accumulator from holding
pressure. The o-ring at the base of the accumulator may also leak.
can hold 1,500 to 2,600 PSI of pressure, so the ABS system must be
completely discharged prior to replacing an accumulator or any other
hydraulic component. This can be done by pumping the brake pedal 30 to
40 times while the ignition key is off. Even a leaky accumulator may
contain quite a bit of residual pressure, so warn your customers to
make sure the ABS system has been completely discharged before they
attempt to replace any hydraulic parts.
Most wheel speed sensors
are magnetic and produce an alternating current signal that increases
in frequency and amplitude with speed.
The air gap between the tip
of the sensor and the teeth on the rotating steel tone ring is critical
for an accurate wheel speed signal. The teeth must also be rust-free
and undamaged to produce a good signal. Common problems here include
rusted and cracked or nicked sensor tone rings on exposed CV joints or
inside sealed wheel bearings with integral wheel speed sensors. If the
tone ring is badly corroded or damaged, it must be replaced which
means replacing the CV joint or axle on some applications, the brake
rotor on others, or the wheel bearing and hub assembly if the sensor is
inside the hub.
Magnetic wheel speed sensors can be checked with
an ohmmeter to see if they are within specifications. A short or open,
or excessive resistance calls for replacement. In some cases, just
removing and cleaning the tip of the sensor to remove any debris that‘s
sticking to the tip is all that’s needed to restore normal operation.
if a code indicates a problem in a wheel speed sensor circuit and the
sensor checks out okay, the fault is usually a bad tone ring.
in the ABS hydraulic unit can be caused by internal rust and corrosion
in the brake lines and solenoid valves. Most people totally ignore
their brake fluid. If the fluid is not changed with the brakes are
relined, the corrosion inhibitors can become depleted over time and
allow rust to start eating away at the inside of the brake system.
Little flakes of rust can become lodged in the ABS solenoid valves,
preventing them from fully closing or causing them to leak or stick.
solenoids can also fail electrically by shorting out or going open. If
this happens, the solenoid won’t move when commanded to do so by the
ABS control module.
Bad solenoids on some ABS systems can be
replaced individually, but on many systems the whole ABS modulator unit
has to be replaced if a single solenoid goes bad.
that can affect the operation of the ABS system include a bad power
relay, blown fuse (which indicates a short or system overload), or even
changes in tire/wheel sizes. ABS systems are programmed to work with
the OEM wheels and tires. Replacing the stock wheels with larger
diameter wheels or taller tires may adversely affect the way the system
functions, making it overly sensitive or not sensitive enough.
is especially true on applications that also incorporate stability
control. If a vehicle owner wants larger aftermarket wheels, he’ll have
to go with plus sizing, which requires lower profile tires to maintain
the same tire diameter and circumference as before.
PARTS REPLACEMENT ISSUES
addition to heeding the caution to always make sure the ABS
high-pressure accumulator has been completely discharged prior to
working on the system, a customer may need information on how to bleed
the ABS system once repairs have been made.
Like a conventional
brake system, air in the brake lines, master cylinder, calipers or
wheel cylinders can cause increased pedal travel as well as a soft
The hydraulic units on many ABS systems are quite complex
internally with lots or ports and passageways, so it’s essential that
the system be completely bled to get rid of all the air.
modulators have a number of bleeder valves for this purpose, but the
valves must be opened in a certain sequence to get rid of all the
This information can be found in a service manual or by going to the vehicle manufacturer’s technical Web site.
ABS systems require a scan tool to cycle the ABS solenoids when
bleeding the system. If this is not done, air will remain trapped
inside the hydraulic unit and cause problems.
It’s also important
to use the type of brake fluid (DOT 3 or DOT 4) specified by the
vehicle manufacturer. Silicone-based DOT 5 brake fluid is NOT
recommended for any ABS-equipped vehicles because it tends to aerate
and foam when cycled rapidly inside an ABS modulator. This may cause a
loss of pedal or brake pressure at the worst possible moment.