Outsmarting Today’s Anti-Lock Brake Systems
MAY 2014
Smart braking is the next
big thing in ABS. Active
safety systems warn the
driver when braking may be
needed, and can even slow or
stop the vehicle if the driver
fails to react in time.
In This Issue
u
How ABS Works
u
ABS System
Components
u
Antilock Problems
u
ABS Codes
u
Wheel Speed Sensors
u
WSS Sensor
Diagnosis
u
Hydraulic and
Electronic Faults
u
Replacing ABS Parts
Antilock brakes were
introduced back in
the 1980s as a safety
upgrade to reduce the
risk of skidding and losing
control when braking
hard, especially on wet or
slick surfaces. Over the
years, antilock brakes have
progressed from being an
extra cost factory option
to standard equipment.
Antilock brake technology
has also evolved, becoming
smaller, lighter, faster
and smarter. It has been
integrated with traction
control and stability control
to improve driving safety
and handling under all
driving conditions.
Some late model vehicles
with active safety systems
can even identify the need
for emergency braking
and slow or stop the
vehicle if the driver fails
to react in time. These
are very sophisticated
systems that rely on inputs
from cameras and radar
sensors to estimate closing
distances to other vehicles
or objects in the road
ahead (or when backing
up). These systems
typically flash a visual
and/or audible warning to
the driver if the vehicle is
closing too fast and braking
is needed to avoid a
collision. The system may
also “pre-charge” (apply
slight pressure to bring the
pads up against the rotors
but not actually apply the
brakes) in anticipation of
braking. If the driver fails
to react, the system then
intervenes and applies the
brakes as needed to slow
or stop the vehicle.
How ABS Works
The basic idea behind ABS
is to minimize tire slip
and loss of traction 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, it’s
essentially a hockey puck.
The vehicle takes further
to stop, and with reduced
directional stability the
vehicle may fishtail, spin or
slide one way or another.
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 the brakes are
applied. On older trucks
with rear-wheel only ABS
(RABS or RWAL), only the
rear wheels are monitored.
But on all passenger cars
and most newer trucks
and SUVs, all four wheels
are monitored (though
a common wheel speed
sensor may be used in the
rear axle on some to monitor
the average speed of both
rear wheels).
If one wheel starts to slow
at a faster rate than the
others, it tells the ABS
control module the wheel
is starting to slip and is
in danger of locking up
and skidding. The ABS
system responds by cycling
hydraulic pressure in that
wheel’s brake circuit. A
© 2014 National Automotive Parts Association
solenoid valve in the ABS hydraulic
modulator 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 the brake.
By rapidly cycling the brakes on and
off from 3 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 is normal and
provides audible and tactile feedback
to the driver that the ABS system is
responding to a loss of traction.
ABS System Components
The major components in an ABS
system include the electronic control
module (which may be a separate
module or integrated into the body
control module, but is usually attached
to the hydraulic unit on most late
model applications), the hydraulic
modulator unit that contains the brake
circuit solenoid valves, wheel speed
sensors (WSS), and a high pressure
electric pump and accumulator (not
used on some applications).
Pressure generated by the pump and
stored in the accumulator is used to
pump the brakes during an ABS stop,
when the stability control system
intervenes to maintain directional
stability, to limit wheel spin when
traction control is needed, and for powerassisted braking on some applications.
On late model vehicles with stability
control, additional system components
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 unit.
The vacuum booster is not part of the ABS
system, nor is the master cylinder except
on older vehicles with “integral” ABS
systems where the master cylinder and
ABS unit are combined into one assembly.
Antilock Problems
All ABS systems have extensive selfdiagnostic capabilities and will run
various self-tests when a vehicle is
driven to check for possible faults. If
a fault is found, it should set a trouble
code and turn on the ABS warning light
(and/or traction control light or stability
control light and/or brake warning light
depending on the fault).
Most antilock brake faults that
are serious enough to turn on the
ABS warning light will temporarily
deactivate the ABS system. This will
not affect normal braking or driving, but
it will prevent the ABS
system from kicking
in should ABS, traction
control or stability
control become
necessary.
False ABS activation
is another common
problem. This
may occur during
normal braking on
dry pavement. The
underlying cause is
usually a weak or erratic
signal from one of the
In addition to the basic ABS components shown, a
wheel speed sensors.
steering angle sensor provides additional inputs
Diagnosing ABS
for stability control.
problems requires a
scan tool with ABS
The ABS control module checks the
health of the ABS system every time
the vehicle is driven. Codes are accessed via the OBD II connector.
capability. Most basic DIY scan tools
can only read powertrain codes and
data, not ABS codes or system data,
though some offer this capability.
What’s usually needed is a professional
level scan tool with bidirectional
communications that can not only
read and display ABS codes but also
access various system self-tests such
as running the ABS pump and actuating
the ABS solenoids.
A scan tool is required not only to read
and clear ABS codes, but also to look
at system data such as wheel speed
sensor readings, pump operating,
system pressure, switch status and
so on. This kind of information is
essential for accurate diagnose of ABSrelated problems.
ABS Codes
Diagnosis starts with a code, but
doesn’t end there because a code only
tells you the circuit or component
where the fault has occurred, not
what part you need to replace. So
no conclusions should be made until
further testing and inspection has been
done. Common ABS fault codes include
those for the wheel speed sensors
circuits, ABS control solenoids, high
pressure pump and accumulator, and
the control unit and other sensors.
Pressure leaks or failure to develop
normal pressure can set a pump or
accumulator code. The accumulator
is a small metal ball or canister with
a nitrogen-filled bladder inside. Over
time, the rubber bladder can crack and
leak, preventing the accumulator from
code may be set if there is a loss of a
signal in a WSS circuit (shorted or open),
or the reading from one WSS sensor
differs significantly from the rest. On
some newer vehicles, “plausibility” codes
may also be set if the speed reading of
any WSS sensor differs from the vehicle
speed sensor by more than 3 to 5 mph.
A scan tool like this can read ABS
codes and data, but lacks bidirectional
communication for activating the ABS
pump and solenoids.
holding pressure. The o-ring at the base
of the accumulator may also be leaking.
CAUTION: Accumulators can hold 1500
to 2600 PSI of pressure! Even a leaky
accumulator may contain some residual
pressure so it must be fully discharged
before it or any other hydraulic
component in the brake system can be
safely replaced. The accumulator can be
discharged by pumping the brake pedal
30 to 40 times with the ignition key OFF.
CAUTION: On vehicles that have active
brake pre-charging, the system may
remain active even when the key is
off. To prevent possible personal injury
when serving the brakes, the brake
system must first be deactivated by
removing the brake system fuse.
Wheel speed sensor faults can be caused
by broken wires, loose or corroded wiring
connectors, damage or corrosion on the
sensor itself or its notched tone ring. A
Wheel speed sensors may be
magnetic (“passive” or magnetoresistive (“active”). Both
types monitor wheel speed but
function differently.
Wheel Speed Sensors
Most wheel speed sensors are magnetic
and produce an alternating current
(AC) signal that increases in frequency
and amplitude with speed. These are
sometimes called “variable reluctance”
(VR) or “passive” WSS sensors because
they generate their own voltage signal
when the vehicle is in motion. They
have two wires: signal and ground.
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. Air gaps typically
range from .016 in. to as much as .050
in. (0.40 to 1.3 mm) depending on the
application. As a rule, there should be
no more than .006 inches of runout in
the tone ring because too much runout
may cause an erratic signal.
The teeth on the sensor tone ring
must also be rust-free and undamaged
to produce a good signal. Common
problems here include rusted, cracked or
nicked sensor tone rings on exposed CV
joints or corroded tone rings inside sealed
wheel bearings with integral wheel speed
sensors. If the tone ring is corroded,
cracked 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. Most sensors
should read 450 to 2200 ohms (always
look up the exact specifications because
they vary depending on the application).
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. But 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.
Another type of wheel speed sensor is
a “magneto-resistive” or “active” WSS
sensor. These work like a Hall-effect
sensor and generate a digital signal.
You’ll find them on certain Chrysler,
Jeep and Mercedes models, Ford Focus,
Toyota Tundra and others. Most active
WSS sensors have three wires: a voltage
reference input (5 to 12 volts), a signal
output and ground, although some only
have two wires (reference voltage and
signal return).
WSS Sensor Diagnosis
Most magnetic wheel speed sensors
(except active WSS sensors) don’t
generate a readable signal until a vehicle
is traveling 3 to 5 mph or faster. You
can spin each wheel by hand to see if
the WSS is generating a signal, but that
won’t tell you if the signal is accurate
or how one wheel compares to another.
To check and compare WSS inputs, the
vehicle needs to be driven in a straight
line at 12 to 25 mph while observing the
WSS readings on your scan tool.
If all of the sensors are generating a
speed signal, and they all agree, the
fault is not a bad WSS but something
else (possibly an intermittent wiring
fault in one of the WSS circuits). Since
most WSS problems are due to broken
wires or loose or corroded wiring
connectors, visually inspect the wires,
and use an ohmmeter or continuity
tester to check the wiring while wiggling
the wires back and forth. If the circuit
suddenly goes open or shows a jump in
resistance, you’ve found the fault.
If one of the WSS PIDs reads zero mph
and the others are reading vehicle
speed, the fault is a bad sensor, tone
ring or wiring problem.
If all four WSS are indicating a speed
reading, but one is reading differs by more
than a couple of mph from the others,
stop the vehicle and check the wheel
and tire size on the one that is reading
different than the others. Someone may
have mounted a tire that has a larger or
smaller diameter, or the brake rotor, drum, axle or CV joint may
have the wrong tone ring.
If the front WSS sensors are both showing a different speed
than the rear WSS sensors, this too may be the result of
someone having replaced the original front or rear tires/
wheels with ones that are different sizes.
Changing tire sizes will change the WSS inputs, which may
upset the operation of the ABS, traction control or stability
control systems. On some vehicles, the control module
can be reflashed to accommodate different tire/wheel
sizes. But if such a reflash is not offered by the vehicle
manufacturer, and customer wants to install different sized
tires or wheels, they should go with “plus wheel sizing”
and lower profile tires to maintain the same overall tire
diameter as before.
Hydraulic and Electronic Faults
Problems 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 when the brakes are relined, corrosion inhibitors in
the fluid can become depleted and allow rust to attack the
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.
ABS 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 has failed.
Other problems that can
affect the operation of the
Problems in the ABS unit may
ABS system include a bad
power relay, or blown fuses require replacing individual
(which indicates a short or components (where possible)
system overload).
or the entire unit.
Replacing ABS Parts
Replacing a bad wheel speed sensor is fairly simple, but
replacing the hydraulic modulator, pump or accumulator requires
bleeding the system to remove air. If all of the air is not removed,
you can end up with a soft pedal or poor ABS performance.
The hydraulic units on many ABS systems are quite complex
internally with a lot of ports and passageways, making
bleeding a time-consuming and difficult process. Refer to
the vehicle service literature for specific procedures and
requirements. On many applications, a bidirectional scan
tool must be used to cycle the ABS unit during the bleeding
procedure. Some ABS modulators also have one or more
bleeder screws that may need to be opened in a specified
sequence to remove trapped air.
Always 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.
Tech Tips
u You can test a WSS by spinning the tire by hand at a rate
of about one revolution per second. With a voltmeter
attached to the sensor’s terminals, a good WSS sensor
should generate about 50 to 700 millivolts AC.
u For advanced diagnostics, use a Digital Storage Oscilloscope
(DSO) to observe WSS signal waveforms. A scope will show
the voltage and frequency of the signal, as well as any
missing or distorted humps in the signal pattern. If one or
more humps in the wave pattern are significantly shorter
than the others, the sensor tone ring has damaged teeth.
u When replacing brake rotors, drums, axles or outboard
CV joints, compare the old and new parts to make sure
the WSS tone rings are the same. Mismatched parts with
different teeth counts will set WSS fault codes.
u Use a digital voltmeter to check active wheel speed
sensors. This type of sensor must be connected to its
wiring harness in order to test it, and the ignition must be
ON so the control module can provide reference voltage
to the sensor. With the wheel stationary, the sensor
should generate a static return signal of 0.9 volts. When
the wheel is spun, it should produce a digital signal that
changes back and forth from 0.9 volts and 1.65 volts.
Review Questions
1. All of the following are part
of the ABS system EXCEPT:
a. Vacuum booster
b. Hydraulic modulator assembly
c. Wheel speed sensors
d. Pump and high pressure
accumulator
2. A faulty wheel speed sensor
may affect the operation of
which of the following?
a. ABS system
b. Stability control system
c. Traction control system
d. All of the above
3. What should be done when
replacing an ABS hydraulic
unit?
a. De-pressurize the accumulator
before removing it
b. Bleed the brakes
c. Change the brake fluid
d. All of the above
© 2014 National Automotive Parts Association
Answers: 1. a, 2. d, 3. d