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Technician Guidelines for Antilock Braking Systems

Air-Braked Trucks, Tractors and Trailers

FHWA-MC-98-008

Prepared for the by
The Maintenance Council
American Trucking Associations
2200 Mill Road
Alexandria, Virginia 22314

U.S. Department of Transportation
Federal Highway Administration

Technical Report Documentation Page

PURPOSE

The purpose of this document is to provide truck technicians
with general guidelines for ABS operation, maintenance,
inspection and troubleshooting. Technicians should always
consult the appropriate vehicle or component manufacturer’s
information for specific ABS procedures.

DISCLAIMER

This document is disseminated under the sponsorship of
the Department of Transportation in the interest of information
exchange. The United States Government assumes no liability
for its contents or use thereof. The contents of this document
do not necessarily reflect the official policy of the Department
of Transportation. This publication does not constitute a
standard, specification or regulation.

The Maintenance Council and the Trucking Research
Institute have made a reasonable effort to ensure the accuracy
of information contained in this publication. However, all
equipment users should satisfy themselves that the procedures
outlined herein are appropriate for their own use.

The United States Government does not endorse products
or manufacturers. Trade or manufacturers’ names appear
herein only because they are considered essential to the object
of this document.

ACKNOWLEDGMENT

The authors extend their thanks to the following organizations
which contributed to the development of this document.
• American Trucking Associations’ Engineering Dept.
• The ATA Foundation
• Bendix/AlliedSignal Corporation
• Eaton-Bosch
• Federal Highway Administration
• Midland-Grau
• Rockwell WABCO
• The Maintenance Council’s ABS/EBS Task Force and
S.6 Chassis Study Group.

TABLE OF CONTENTS

I. AN INTRODUCTION TO ANTILOCK BRAKING............ 3
A. What is an ABS? ............................................... 3
B. How Do ABSs Work?......................................... 5
C. How Should I Drive an ABS-equipped Vehicle
During Road Tests? ........................................... 6
D. What Are the Features and Benefits of ABSs? ..... 7
II. ABS COMPONENT DESCRIPTIONS AND
OPERATION ......................................................... 8
A. Electronic Control Unit (ECU) ............................. 8
B. Modulator Valves ............................................ 10
C. Wheel Speed Sensors ...................................... 11
D. ABS Malfunction Indicator Lamps..................... 12
E. ABS Diagnostics .............................................. 12
F. Traction Control Systems ................................. 13
III. ABS TROUBLESHOOTING, MAINTENANCE AND
INSPECTION ....................................................... 14
A. ABS Troubleshooting ...................................... 14
1. General Diagnostic Principles ..................... 14
2. Notes on Electrical/Electronic Connections .. 19
3. Error Detection Methods ............................ 23
4. Causes of Common ABS Sensor Problems .... 25
B. ABS Maintenance and Inspection ..................... 27
1. ABS Sensor Pickup Adjustment ................... 27
2. ABS Sensor Pickup Removal and Installation 27
3. Sensor Pickup Removal—Front Axle ............ 27
4. Sensor Pickup Installation—Front Axle ........ 27
5. Sensor Pickup Removal—Rear Axle ............. 28
6. Sensor Pickup Installation—Rear Axle ......... 28
7. Proper ABS Sensor Resistance ..................... 29
8. Modulator Valves/Routine Inspection .......... 29
9. Modulator Valve Removal and Installation .. 30
10. Proper ABS Modulator Valve Resistance .... 30
IV. ABS SPEC’ING CONSIDERATIONS ......................... 31
V. GLOSSARY OF ABS TERMS ................................... 33
VI. INDEX................................................................ 42

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
pressures 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 airbraked
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.

B. How Do ABSs Work?

An ABS consists of several key components: electronic
control unit (ECU), wheel speed sensors, modulator valves, and
exciter rings. Here’s how these components work together:
1. Wheel speed sensors constantly monitor and send
electrical pulses to the ECU at a rate proportional to
the wheel speed.
2. 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.
3. The ECU then adjusts pressure, seeking one which
gives maximum braking without risking wheel lockup.
4. 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.
5. 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 maximum
braking/stability. Some vehicles also use a traction control
system in conjunction with the ABS. 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)

C. How Should I Drive an ABS-equipped Vehicle During Road Tests?

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.

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.

D. What Are the Features and Benefits of ABSs?

Table 1 lists the major features and benefits offered by ABSs:

II. ABS COMPONENT DESCRIPTIONS & OPERATION

This section describes the design and operation of ABS
components.
When you complete this section, you should understand the
purpose and function of all major ABS parts including: the ECU,
the modulator valve, the wheel speed sensor, ABS malfunction/
indicator lamp, ABS diagnostic components, and traction
control.
Modern antilock braking systems all feature the following
major components (See Fig. 1 on page 9 for typical system):
• Electronic Control Unit (ECU)
• Modulator Valves
• Wheel Speed Sensors (pickup and exciter)
• ABS Malfunction

A. Electronic Control Unit (ECU)

The ECU processes all ABS information and signal functions.
It receives and interprets voltage pulses generated by the
sensor pickup as the exciter teeth pass by, and uses this
information to determine:

• impending wheel lock-up and
• when/how to activate the ABS modulator valves.

The ECU connects to the following ABS components: wheel
speed sensors, ABS modulator valves, power source, ground,
warning lamps, blink code switch, J1587* diagnostic connector,
and retarder control device (usually by relay or the J1922**/
J1939*** datalink.) The ECU also makes self-diagnostic checks
during normal operation.

During braking, the ECU uses voltage pulses from each
wheel speed sensor to determine wheel speed changes. If the
ECU determines that the pulse rate of the sensed wheels
indicates imminent lock-up, it cycles the ABS modulator valves
to modify brake air pressure as needed to provide the best
braking possible.

The ECU sends signals to the ABS malfunction indicator
lamp or blink code lamp to communicate ABS faults. It also
sends signals to the retarder control to disengage the retarder
when the ABS is working. When the ABS stops modulating the
brake pressure, the ECU permits retarder use once again.

* SAE J1587, Joint SAE/TMC Recommended
Practice for Electronic Data Interchange Between
Microcomputer Systems in Heavy-duty Vehicle
Applications. (See Glossary of ABS Terms for definition
of SAE.)

**SAE J1922, Powertrain Control Interface for
Electronic Controls Used in Medium- and Heavy-duty
Diesel On-highway Applications.

***SAE J1939, A series of SAE Recommended
Practices that define architecture and protocol
for a serial control and communications network
for various equipment types.

FIGURE 1: TYPICAL TRACTOR ABS SCHEMATIC

Technicians can communicate with the ECU through a
standard SAE J1587 diagnostic connector (See Fig. 1).
Technicians can read and clear fault codes stored in the ECU
and run various diagnostic tests with this connector.
The type of ECU used and its location (in-cab or frame) vary
by manufacturer and application. A detailed description of all
the different ECU types used today is beyond the scope of this
manual. Consult either the vehicle or component
manufacturer’s service information for specifics.

B. Modulator Valves

ABS modulator valves regulate the air pressure to the brakes
during ABS action. When not receiving commands from the
ECU, the modulator valve allows air to flow freely and has no
effect on the brake pressure. The ECU commands the
modulator valve to either:

• change the air pressure to the brake chamber, or
• hold the existing pressure.

However, it cannot automatically apply the brakes, or
increase the brake application pressure above the level applied
by the driver.

The modulator valve typically contains two solenoids. The
modulator valve and relay valve may be incorporated into a
single unit. The modulator valve may also be separate, inserted
into the service line to the brake chamber(s) after any relay
valve, located as close as practicable to the chamber(s) itself.
When the modulator valve is separate, it has to control
more air flow and, therefore, includes two larger diaphragm
valves which are controlled by the solenoids. It usually has
three ports: the supply port, the delivery port and the exhaust
port.

• The supply port receives air from a quick release or
relay valve.
• The delivery port sends air to the brake chambers.
• The exhaust port vents air from the brake chamber(s).

Typically, when an ECU controlling a separate modulator
valve detects impending wheel lockup, it activates the solenoids
to close the supply port and open the exhaust port. When
enough air is vented to prevent wheel lockup, the exhaust valve
will close and the ECU will—depending on the situation—either:

• keep the supply port closed to maintain existing
pressure, or

• open the supply port to allow brake application
pressure to increase and repeat the cycle.

C. Wheel Speed Sensors

The wheel speed sensor has two main components: the
exciter and the pickup. Other components include associated
wiring and mounting equipment.

Exciter—The exciter is a ring with notched teeth. The most
commonly used exciter has 100 evenly spaced teeth, but the
number of teeth can vary depending on the system design. The
component is known by several names: sensor ring, tooth
wheel, tone ring, and exciter.

Pickup—The pickup is commonly called “the sensor.” It
contains a wire coil/magnet assembly, which generates pulses
of electricity as the teeth of the exciter pass in front of it. The
ECU uses the pulses to determine wheel speeds and rates of
acceleration/deceleration. The strength of these electrical pulses
decreases rapidly with slight increases in the gap between the
pickup and the exciter.

Wheel speed sensor location varies. It can be located
anywhere on the axle to sense wheel speed. The sensor can be
an assembly containing both the exciter and the pickup with a
fixed gap. Or, the pickup and the exciter can be mounted
separately on different parts of the axle assembly. The sensor
pickup is a sealed unit and typically of elbow or straight design.

On most ABS air-braked vehicles, the pickup is located in
the mounting flange on the wheel end. The exciter usually is
either mounted on—or integrated with—the wheel hub.
Since the output of the pickup decreases so rapidly with
slight increases in exciter-pickup gap, it is imperative that the
wheel end and sensor gap be maintained within the
manufacturer’s specification.

When the wheels of only one tandem axle have wheel
speed sensors, they are usually placed on the axle whose
wheels are most likely to lock-up first during braking. On a
tandem with a four-spring suspension, the sensors are generally
on the lead axle. On a tandem with air suspension, the sensors
are generally located on the trailing axle.

ABS configuration is defined by the arrangement and
number of sensors and modulator valves used. The most
common configurations for power units are:

• four sensors/four modulators (4S/4M),
• six sensors/four modulators (6S/4M), and
• six sensors/six modulators (6S/6M).

Common configurations for trailers are 2S/1M, 2S/2M, 4S/
2M and 4S/3M.

D. ABS Malfunction Indicator Lamps

Vehicles required to have an ABS must have ABS
malfunction indicator lamps. These lamps must be yellow and
light up when the ABS has a “malfunction that affects the
generation or transmission of response or control signals” in
the ABS.

ABS malfunction indicator lamps are not required to light up
for every type of malfunction. However, they are required to
light up for short periods of time for a bulb check whenever the
ABS starts to receive electrical power. The warning lamps for
trailers and dollies are not required to light up for a bulb check
unless the vehicle is stopped.

All trailers/dollies built on or after March 1, 1998 must
feature an external ABS malfunction indicator lamp as part of
the ABS. All new trailers must be capable of activating an in-cab
trailer warning lamp beginning in March 2001. The
requirement for an external trailer/dolly indicator lamp expires
in March 2009.

In-cab ABS indicator lamps are typically located on the
instrument panel. The exact location and appearance vary by
vehicle/component manufacturer. Consult the manufacturer’s
service information for specifics.

E. ABS Diagnostics
Although not required by law, all air brake ABSs have selfdiagnostic
capability. On truck-tractors and single-unit or
straight trucks, an ABS provides this information to technicians
through the malfunction indicator lamp and/or an electronic
diagnostic tool, which plugs into an on-board diagnostic
connector. The connector is typically located inside the tractor
cab just underneath the left end of the instrument panel. It is
usually the same connector that’s used to troubleshoot
electronic engines.

Truck-tractors and trucks may also use the ABS malfunction
indicator lamp to signal stored fault information through a blink
code. Vehicles using this system have a switch to activate the
blink code system. Other ABSs may also have light-emitting
diode (LED) lamps on the ECU to indicate problems.
ABSs used on trailers sometimes have a place to connect an
electronic diagnostic tool. The connector is either on a pigtail to
the ECU, on the outside of the ECU, or inside the ECU box.
Others have either LED lamps on the ECU box or number
codes displayed inside the ECU which give diagnostic
information.

F. Traction Control Systems

Traction control systems are designed to prevent wheel spin
in the power mode. Traction control attempts to regain traction
by braking the spinning wheels, and sometimes throttling back
engine power. Unlike an ABS, traction control can automatically
apply the brakes. The driver does not need to depress the brake
pedal for traction control to engage.

Traction control is not
required by law, but it is
a common ABS option.

Traction control electronics are integrated into the ABS ECU.
The system applies the brakes on the spinning wheel(s) when
the wheel speed sensors tell the ECU that a wheel is
accelerating at a much faster speed than the wheel on the other
end of the axle. It does this by energizing a solenoid valve,
which directs reservoir pressure to the relay valve and
simultaneously activates the modulator valves to keep air
pressure from the brake chambers. The ECU then directs the
modulator valve to open, and pulse air into the brake chamber
on the spinning wheel until wheel speed balance is regained.

On some systems, the ECU will throttle back engine power
if both wheels are spinning too fast. If all the drive wheels on a
tractor are spinning too fast, the tractor can become unstable,
spin or jackknife. Traction control is especially valuable when a
light drive wheel load might allow the wheels to spin under
power, or when a tractor is pulling multiple trailers.

III. ABS TROUBLESHOOTING, MAINTENANCE &
INSPECTION

Although an ABS generally requires no routine
maintenance, it should be checked periodically like other
components of the air brake system.
In this section, we review various aspects of ABS
troubleshooting, maintenance and inspection. When you
complete this section, you should understand:

• General ABS troubleshooting principles
• Special concerns about connector repairs
• ABS error detection methods
• Common ABS errors and causes
• General ABS component adjustment, installation and
removal procedures

A. ABS Troubleshooting

1. General Diagnostic Principles

This section describes general principles of electrical,
electronic, and air system diagnostics to provide technicians
with a plan of action for ABS troubleshooting. Chart 1 on page
15 illustrates these diagnostic principles in flow chart form. The
following sub-sections—based on The Maintenance Council’s
Recommended Practice TMC RP 1406, “Basic Electrical/
Electronic Diagnostic Procedures”—cover this process in detail.

Step 1: Verify the problem or driver concern.
Establish the connection between the symptom and the
underlying cause of the problem. Use the vehicle
manufacturer’s recommended information collection methods
for verification.

Step 2: Perform preliminary checks.
Operational, visual and audio checks are generally easy to
perform, do not require the use of special tools and may result
in a quick diagnosis. This is a critical step in the diagnostic
process.

CHART 1: GENERAL DIAGNOSTIC PRINCIPLES

Step 3: Refer to service information.

Vehicle manufacturers provide service procedures which
must be followed to ensure proper repair. Training/service
information is readily available from various sources such as:

• Bulletins
• Service newsletters
• Videotapes
• Service manuals
• Manufacturers’ and dealers’ “Help Line Phone
Numbers”
• Troubleshooting guides

Be sure to confirm that the reference material is applicable
to the specific problem or vehicle being diagnosed. Also, ensure
information is current. Vehicle and supplier manufacturers’
service information—specifically bulletins and newsletters—is
very effective and may help shorten diagnosis.

Hands-on training may also be available from the vehicle/
ABS manufacturer at dealer locations or on site at the fleet. The
Brake Training Resource Directory contains a list of brake
training resources in North America. It is available from the
Office of Motor Carriers, Federal Highway Administration, 400
7th St., S.W., Washington, DC 20590, (202) 366-4009 or from
The Maintenance Council by calling (800) ATA-LINE or (703)
838-1763.

Step 4: Perform electrical, electronic and air system checks.
Systems checks found in service manuals provide a systematic
approach to identifying the probable cause of a system fault.
This step is important to properly define the correct approach
for the repair and to avoid unnecessary time-consuming
repairs. Additionally, systems checks will help to define what
the problem is not. Systems checks may require the use of original
equipment manufacturer (OEM) service tools and should isolate
a particular component in the system as a probable cause.

i. Electrical diagnostic procedures
Electrical problems are a common cause of ABS faults. It is
beyond the scope of this document to explain electrical
diagnostic procedures for all ABSs and vehicle manufacturers in
great detail. References for diagnosing electrical systems can be
readily obtained from component, vehicle, and test equipment
manufacturers. (TMC Recommended Practice 129, “Heavy-
Duty Vehicle Systems Wiring Checks,” is a good source of
general information on electrical diagnostic procedures.)

ii. Electronic diagnostic procedures
To diagnose an electronic system properly, specialized test
equipment approved by the electronic system manufacturer
may be required. Failure to use the correct diagnostic tool may
result in inaccurate or incomplete diagnosis or cause ECU damage.

iii. Air system diagnostics
It is beyond the scope of this document to explain air
system diagnostic procedures in great detail. However, several
TMC Recommended Practices—such as RP 619, “Air System
Inspection Procedure”—are a good source of general
information on this topic. Other references for diagnosing air
brake systems can be readily obtained from component,
vehicle, and test equipment manufacturers.
Chart 2 on page 18 is an example of a troubleshooting flow
chart for a common modulator valve problem.

Technician Tip—
If a suspect part can be easily installed and
removed, remove and temporarily replace it
with a known good part to see if the problem
remains.

If the problem disappears, reinstall the
suspect component to see if the problem
returns. If so, replace the suspect component.

Step 5: Find and isolate problem
For an active problem, the diagnosis should narrow and/or
eliminate possible causes. Find and isolate the faulty part of the
system or circuit by breaking the problem into smaller pieces.
For an intermittent problem, attempt to simulate/recreate the
conditions where the fault would exist. Monitor suspect circuits
and components to pinpoint the probable cause while the
problem is occurring.

Step 5a: Reexamine complaint
Review all information describing the complaint. When did
the problem occur? What conditions are present when the
symptom occurs (weather conditions, driving conditions, etc.)?
Contact the driver, if necessary, to gather more information or
to arrange a “show me” or test drive interview.

Step 6: Repair and verify
Once the suspect component is found, carefully disconnect
the old component and inspect its connections to the harness.
If the component connections are OK, temporarily connect a
known good component (without installing) to ensure the
problem is corrected.

After the problem is corrected with the known good
component, reconnect the suspect component to make sure
the problem returns. Temporarily connecting a known good
component, and then reconnecting the suspect component,
will help reduce replacement of incorrect components. If
reconnecting the suspect component does not cause the
problem to recur, thoroughly inspect the connectors and
harnessing for the cause of the problem. Reconnect the suspect
component and move (jiggle) the harness while monitoring for
the problem to return. If the problem returns with the
connection of the suspect component, permanently install the
new component.

CHART 2:
SAMPLE ABS MODULATOR VALVE PROBLEM FLOW CHART
Chart 2 represents a typical troubleshooting flow chart for a
common ABS modulator valve problem.


Step 7: Clear fault codes.
Clear any codes stored in the ECU identifying the problem.

Step 8: Implement any possible preventive measures.
Review the vehicle maintenance schedule for required
service intervals and perform necessary maintenance. Check
for other areas of apparent concern and notify the fleet
manager—or fix—prior to release of vehicle.

2. Notes on Electrical/Electronic Connections

The following section contains general service information
that should be considered if electrical/electronic connections
need repair during ABS servicing.

a. Wiring Termination Techniques
Termination is the process of either ending a wire or
attaching a device to be used at the end of a wire. Wiring
terminations are made in a variety of ways. Wires can be
terminated with butt splices, the application of a terminal, and
by simply “tinning” or sealing the wire’s end.

The primary considerations during a termination are
mechanical strength, vibration resistance, electrical integrity,
and environmental protection.

• Mechanical Strength—Whenever a wire is terminated,
the mechanical strength of the termination should
meet or exceed the mechanical strength of the
conductor without the termination.

• Vibration Protection—Always place conductors back in
any holding device that they were in prior to the
modification/repair or attach the conductors to the
vehicle in a manner which will prevent the conductor
from vibrating during operation.

• Electrical Integrity—The termination must be able to
fulfill the electrical needs of the circuit (for example,
current-carrying capability, minimal voltage drop).
Whenever a termination or splice is made in a
conductor, an inherent voltage drop will be present.
Special connectors are available to minimize the
voltage drop, but these connectors normally are cost
prohibitive. Terminations made carefully normally
provide an acceptable voltage drop.

• Environmental Protection—Whenever a termination is
made in a conductor which disturbs the integrity of the
insulation on the conductor, measures must be taken
to ensure that the termination is not susceptible to
moisture damage or other damage which may result
from the conductor or termination being exposed to its
normal operating environment. Additionally,
consideration must be given to the type of insulating
material being used to ensure that it has an acceptable
heat range and is compatible with the intended
environment.

• Electromagnetic/Radio Frequency Interference
Protection—The ECU contains components that can
detect radio waves and other electromagnetic “noise”
and unintenionally send false signals because of them.
To prevent radio frequency interference (RFI) and
electromagnetic interference (EMI), ABS cables contain
special shielding. When making repairs, take care to
ensure the integrity of the shielding is not
compromised.

For terminations that are made to a threaded stud which is
exposed to salt spray or other corrosive environments, a
suitable coating material should be applied to the connection to
ensure adequate service life.

Conventional Terminations—Conventional terminations are
terminations made using commercially available terminals such
as ring terminals, spade terminals, etc. Terminals of this type
are available through many different outlets.

Selection of good quality terminals is crucial to making a
dependable connection. The selection should include the
considerations mentioned in “Wiring Termination Techniques,”
as well as specific considerations about the location of the
termination on the vehicle (for example, heat exposure). Some
fleets have established specific methods for making
terminations. These methods were developed to ensure
consistent terminations which will yield an acceptable service
life. These recommendations should be followed when
applicable.

Proprietary Terminations—Proprietary terminations are
terminations made using proprietary terminals and connector
bodies. These terminations are very common on commercial
vehicles and come in a variety of configurations. Multiple
connections in one connector body are typical. Also, various
types of proprietary terminations on the same vehicle are
common. When repairing or replacing these terminations,
special techniques are needed. These techniques include tools,
special assembly methods and, many times, special training.

When servicing special connectors, use of OEM
recommended tools is critical to making a good termination.
Repair or replacement of these special terminations should not
be attempted without the specific tools recommended.
Manufacturers’ service manuals and bulletins typically detail the
techniques to be used for proper repair.

Butt Splices—A butt splice is any splice where wires are joined
together “end-to-end.” In this case, the wires may be either
twisted together and soldered, or crimped together using a
commercially available terminal. Butt splices should always be
covered with insulation and heat shrink tubing which has a
meltable inner liner or another suitable protective insulation.
The use of pressure sensitive tape is not recommended as the
tape will likely deteriorate with time.

Conductor Terminations—Terminations of conductors are
made to attach the conductor to another conductor or to a
device on the vehicle. These terminations must be carefully
made in order to provide acceptable serviceability. Attaching a
wire to another wire (not using a butt splice) is an example of a
conductor termination.

Technician Tip—
Whenever an additional grounding point is to be
established on the vehicle, consult the
vehicle manufacturer to ensure that the planned
alteration does not result in an inadequate
ground path for other components on the
vehicle.

Terminations Without Terminals—Occasionally a wire is
terminated without a terminal to facilitate the attachment of the
wire to an accessory. If this situation is unavoidable, the wire
should be “tinned” to prevent fraying and breakage at the point
of connection. Using a heat shrink process at the end of the
wire is also acceptable.

b. Grounding Recommendations
Grounding problems occur in a variety of ways (such as
corrosion or inadequate current-carrying capacity). As a result,
grounding terminations should be coated with a suitable
material to prevent corrosion as a result of exposure to salt
spray or other corrosive environments.

Whenever an additional grounding point is to be established
on the vehicle, consult the vehicle manufacturer to ensure that
the planned alteration does not result in an inadequate ground
path for other components on the vehicle. This is especially
important when establishing a grounding point between chassis
and body.

c. Wiring Damage Caused During Repair
Mechanical damage to wiring must be avoided during
vehicle repair. Insulation cuts and “pinch points” are common
problems which may cause failure.
Conductor insulation should not be pierced while
troubleshooting electrical problems. Piercing of the protective
covering results in corrosion which can cause circuit failure. If
piercing of the insulation is unavoidable, suitable insulation to
avoid water entry must be used at the point where the
conductor was pierced.

d. Vehicle Repairs—Special Care
Many times vehicle repairs include welding operations. All
welding on a vehicle should be done using methods and
techniques which are acceptable to the OEM in order to avoid
damage to the electrical and electronic system of the vehicle.
This damage normally occurs due to unwanted circuit paths or
to voltage spikes created in the electrical and electronic systems
which cause component failure.

CAUTION: When welding on an ABS-equipped vehicle,
disconnect power and ground leads from the ECU to avoid
unintended grounding through the ECU which will damage
electronic components. Other damage may occur to vehicle
systems as a result of heat generated during the welding
process. Special care must be taken to ensure that heat buildup
does not melt conductors and other susceptible electrical
components.

e. Miscellaneous
The use of “star washers” in the electrical path is
discouraged. Often, an open circuit or high resistance results
when the “points” of the washer are exposed to salt spray and
other corrosive materials. If the use of star washers cannot be
avoided, a suitable material should be applied to the
connections to ensure as much protection from corrosion as
possible.

3. Error Detection Methods

One ABS benefit is the ability to electronically detect
component or system failures. This electronic detection occurs
either during self-test checks at start-up, or during continuous
passive monitoring.

At start-up, the ECU will activate the ABS malfunction
indicator lamp and briefly energize the ABS modulator valves
(ignition-on blow down or "chuff test"). At the same time, the
ECU checks the wheel speed sensors and other essential
components for proper operation. If no problems are found,
and the ECU detects that wheel speed sensors were functioning
properly just prior to the last vehicle shutdown, the ABS
malfunction indicator lamp will go out. On earlier systems,
the lamp would not go out until the vehicle reached about 5 mph.

During vehicle operation, the various ABS components also
continually monitor each other for failures and "out-of-range"
operating parameters. Through this process, the ECU detects
abnormalities during operation and activates the ABS malfunction
indicator lamp as appropriate.

The ECU will generally detect two types of faults: active and
stored. An active fault is a current and continuous failure in
need of repair (such as a broken connector). A stored fault is
a failure that affects ABS operation intermittently (such as a
loose connector). Technicians typically can retrieve failure
information either through blink codes or an electronic diagnostic
tool. For explanations of manufacturer-specific diagnostic and
troubleshooting tools and procedures, consult the appropriate
manufacturer's service information.

Top 10 Most Commonly Encountered Problems
That Trigger ABS Malfunction Indicator Lamps

1. Abraded or cut wires in convuluted tubing near frame clamps.
2. Cut or corroded wires near sharp frame members and frame-mounted
modulators.
3. Wire jacket worn through from overlapping sensor and modulator wires near
frame members and frame-mounted modulators
4. Corroded connectors and connections not properly sealed or damaged seals.
5. Damaged connector latches or connectors not completely sealed to mating
assemblies.
6. Terminals not completely latched or seated into connectors
7. Excessive sensor air gap, sensor clip tension or excessive wheel bearing endplay.
8. Damage to exposed wires exiting or entering wire tubing.
9. Worn, chipped or damaged sensor or modulator.
10. Non-functioning controller (ECU).

CHART 3: SAMPLE ABS MALFUNCTION INDICATOR LAMP
(WARNING LIGHT) DIAGNOSTIC FLOW CHART
NOTE: The notations to various sections that appear in Chart 3 refer to the source document from which this chart originally
appeared—not this manual. Chart 3 represents a typical ABS warning light diagnostic flow chart.

Chart 3 on page 24 shows an ABS indicator lamp diagnostic
flow chart. Table 2 on page 25 offers a list of common ABS
errors and responses.

4. Causes of Common ABS Sensor Problems
Table 3 on page 26 summarizes the causes of several
common ABS sensor problems.

One benefit of monitoring wheel speed with a exciter ring/
sensor system is that dirt or dust does not affect operation.

Even if the space between the teeth of the exciter ring is full of
dust and particles from the brake linings, the monitoring
operation is not affected. In fact, the magnetic property of the
dirt in the gaps is similar to that of air. The change in the
magnetic field is determined by the spacing of the teeth of the
exciter ring. The output voltage is unaffected by dirt.
Therefore, an ABS fault cannot be remedied by cleaning the
tooth wheel with compressed air. Dirt in the gaps doesn’t affect
voltage output, so removing it will not remedy an ABS fault.

B. ABS Maintenance and Inspection

1. ABS Sensor Pickup Adjustment

CAUTION: Follow all recommended safety warnings and
cautions. To prevent eye injury, always wear safe eye
protection when performing maintenance or service. Do not
work under a vehicle supported only by jacks. Jacks can slip or
fall over and cause serious personal injury.

To adjust the ABS sensor pickup, gently push the sensor
pickup in until it contacts the tooth wheel:

• On the steering axle, the sensor pickup may be accessible
on the in-board side of the steering knuckle.
• On the drive axle, the wheel and drum assembly must
be pulled to gain access to the pickup. Prior to pulling
the wheel and drum assembly, observe the output
voltage of the pickup while rotating the wheel by hand.
The amount of output voltage is dependent upon the
sensor pickup gap and wheel speed. Refer to the
manufacturer’s recommendations for proper voltage
levels.

2. ABS Sensor Pickup Removal & Installation
The following installation and removal procedure is a
guideline only. When removing or installing a sensor pickup on
your system, always follow the procedures detailed in the
manufacturer’s maintenance manual.

3. Sensor Pickup Removal—Front Axle
To remove the sensor pickup from the front axle:

1. Put wheel chocks under the rear tires to keep the
vehicle from moving. Apply the parking brake.

2. Remove the pickup and spring clip from the steering
knuckle. Use a twisting motion if necessary. Never
pull or tug on the cable.

3. Disconnect the pickup cable from the chassis harness.
Be careful not to criss-cross wiring.

4. Sensor Pickup Installation—Front Axle
To replace the sensor pickup in the front axle:

1. Connect the sensor cable to the chassis harness. Be
careful not to criss-cross wiring.

2. Install the fasteners used to hold the sensor pickup
cable in place.

3. Apply lubricant to the sensor spring clip and to the
body of the pickup. NOTE: Use a mineral oil-based
lubricant that contains molybdenum disulfide. The
lubricant should have excellent anti corrosion and
adhesion characteristics, and be capable of continuously
functioning in a temperature range of -40° to
300°F (-40°- 150° C).

4. Clean and inspect the hole in the steering knuckle.
Install the sensor pickup spring clip. Make sure the
flange stops are on the inboard side of the vehicle.

5. Push the sensor spring clip into the bushing in the
steering knuckle until the clip stops.

6. Push the sensor pickup completely into the sensor
spring clip until it contacts the tooth wheel/exciter.

7. Install fasteners and straps to retain the pickup wiring.

8. Remove the wheel chocks.

5. Sensor Pickup Removal—Rear Axle

1. Put chocks under the front tires to keep the vehicle
from moving.

2. Raise the rear tire off the ground. Put safety stands
under the axle.

3. Release the parking brake and back off the slack
adjuster to release the brake shoes.

4. Remove the wheel and tire assembly from the axle.

5. Remove the brake drum.

6. Remove the pickup from the mounting block in the
axle housing. Use a twisting motion if necessary.
Never pull or tug on the cable.

7. Remove the sensor spring clip from the mounting block.

8. Disconnect the fasteners that hold the sensor cable
and the hose clamp to the other components.

9. Disconnect the pickup cable from the chassis harness.

6. Sensor Pickup Installation—Rear Axle

To reinstall the sensor pickup in the rear axle:

1. Apply lubricant to the sensor spring clip and to the body
of the pickup. Follow manufacturer’s recommended lube
specification (See lubricant recommendation in
previous section “ Sensor Pickup Installation—Front
Axle”).

2. Clean and inspect the hole in the mounting block.
Install the sensor spring clip. Make sure the flange
stops on the inboard side of the vehicle.

3. Push the sensor spring clip into the mounting block
until it stops.

4. Push the pickup completely into the sensor spring clip
until it contacts the tooth wheel. See figure at left.

5. Insert the pickup cable through the hole in the spider
and axle housing flange. Route the cable to the frame
rail. Be sure to route the cable in a way that will
prevent pinching or chafing, and will allow sufficient
movement for suspension travel.

6. Connect the pickup cable to the chassis harness.

7. Install the fasteners that hold the pickup cable in place.

8. Install the brake drum on the wheel hub.

7. Proper ABS Sensor Resistance

For most common types of ABS sensors, the sensor circuit
resistance is between 700-3000 ohms. Resistance can be
measured at the sensor connection when it is removed from
the ECU, or right at the sensor when the extension cable is
removed. Follow the manufacturer’s specifications to
determine the correct sensor resistance.

8. Modulator Valve/Routine Inspection

As part of a routine vehicle preventive maintenance
program, ABS modulator valves should be checked for proper
operation and condition. This inspection generally should
include:
1. Removal of contaminates and a visual inspection for
excessive corrosion and physical damage.
2. Inspection of all air lines and wiring harnesses for
signs of wear or physical damage.
3. Testing for leakage and proper operation.
For specific modulator valve inspection and testing
procedures, consult the manufacturer’s service information.

9. Modulator Valve Removal and Installation
The following removal and installation information is
offered as a guideline only. Always refer to the manufacturer’s
specific instructions when removing or installing ABS
modulator valves.
Removal
1. Disconnect the harness connector from the modulator
valve. Be careful not to criss-cross wiring.
2. Disconnect the air supply and air delivery lines from
their respective ports.
3. Remove modulator valve mounting fasteners.
4. Remove the modulator valve.

Installation
1. Install the modulator valve with appropriate mounting
fasteners. Tighten to specified torque.
2. Connect the air supply and air delivery lines at their
respective ports.
3. Connect the harness connector to the modulator
valve. Be careful not to criss-cross wiring.
4. Check installation by applying the brakes, listening for
leaks at the modulator valve.
5. Turn the ignition on, and listen for the modulator
valve to cycle. If the valve fails to cycle, check the
electrical connection and any stored or active fault
codes. Drive the vehicle to verify that the ABS and its
malfunction lamp operate properly.

10. Proper ABS Modulator Valve Resistance
For most ABS modulator valves, the resistance range
between each valve solenoid coil terminal and the ground on
the ABS valve connector is between 3-10 ohms. To test this
resistance, disconnect the wiring connector from the modulator
and test the resistance between the two pins of each solenoid.
Follow the manufacturer’s instructions for determining valve
resistance.

IV. ABS SPEC’ING CONSIDERATIONS

The Federal Government’s requirement for full-time
electrical power to ABSs has prompted both equipment users
and manufacturers to reconsider the way trailers are supplied
with such power. Since a particular powering configuration is
not required in the ABS rule, manufacturers and equipment
users can decide for themselves how to achieve the full-time
power requirement.

There are several different methods of supplying full time
power to the trailer ABS:

• If the auxiliary circuit of the seven-pin connector is not
in use, it can be used to supply full-time power as long
as the circuit is always “on” or “hot” when the key
switch is “on.” NOTE: Unless otherwise specified,
many manufacturers will supply a “hot” auxiliary
circuit as standard equipment. It is very important that
vehicles use this option if they are commonly coupled
to vehicles in other fleets.

• A second connector can be used specifically to power
the trailer ABS. (for example, the ISO 3731 connector)

• A special connector which is compatible with the
existing seven-pin connector can be used if it can
accommodate additional circuits (for example, a
13-pin connector).

Each of these methods has certain advantages and
drawbacks. However, it is the consensus of the members of
The Maintenance Council that the existing seven-pin connector
design should be preserved if possible for important reasons of
compatibility, safety, and maintainability.

Another important consideration is ensuring that adequate
power is available for proper ABS function. Voltage drops
between the battery and the last unit of a combination vehicle
can impact the amount of power available for the ABS,
especially in doubles and triples combinations.

For these reasons, TMC developed two recommended
practices to promote power supply and connector
standardization—TMC RP 137, “Antilock Electrical Supply From
Tractors Through the SAE J560 Seven-pin Connector,” and TMC
RP 141, “Trailer ABS Power Supply Requirements.”
To ensure adequate power is provided to the trailer from
the tractor, TMC RP 137 recommends that at least 12.5 volts be
available at the J560 connector with a 10-amp load on both the
stop lamp and auxiliary circuit. Industry consensus is that
meeting this minimum recommendation will ensure adequate
power for trailer ABSs.

Additionally, TMC RP 141 recommends that:

• Pin 7 of the J560 connector be reassigned as a continuous
power circuit, activated when the ignition is on.
NOTE: For the purposes of RP 141, Pin 7 of the SAE
J560 seven-pin connector (so designated by SAE as the
auxiliary circuit) is referred to as the continuous power
circuit.
• There be a minimum of 9.5 volts (which includes a 1-
volt safety margin) available at the trailer ABS ECU—
when coupled to a tractor complying with RP 137— to
ensure adequate power for proper ABS operation.
• Trailer manufacturers provide equipment purchasers
with written information regarding the voltage and
current characteristics of the stop lamp and auxiliary
circuits at the SAE J560 seven-pin receptacle. TMC
recommends this information be included in the
owner’s manual.

TMC does not recommend specific wiring gauge sizes,
lighting technology, ABS power consumption, or grounding
methods. It is the consensus of TMC’s membership to leave
those decisions to the manufacturer/equipment user.
All vehicle/component manufacturers have agreed to
incorporate TMC’s recommendations into their vehicle design
as either standard or at the request of the equipment user.

Antilock Braking System (ABS)........................... 3
adequate power ................................................. 31-32
benefits ...................................................................... 7
common problems ................................................... 23
common sensor errors .............................................. 26
common system errors ............................................. 25
component descriptions ........................................ 8-13
connector standardization ........................................ 31
configuration ................................................ 11-12, 33
defined ................................................................... 3-4
driving technique ....................................................... 6
fail-safe ....................................................................... 7
features ...................................................................... 7
faults ........................................................................ 23
inspection ........................................................... 27-30
maintenance ....................................................... 27-30
mandate .................................................................... 3
operation ................................................................... 5
requirements for new vehicles ................................. 3-4
self-diagnostic capability ........................................... 12
spec’ing considerations ............................................ 31
troubleshooting .................................................. 14-19
welding cautions related to ABS ............................... 22

Connectors ........................................... 19, 31-32
ISO Connectors .................................................. 31, 36
Seven-pin Connector (J560)........................... 31-32, 39

Diagnostics ................................ 12, 14-19, 22-23
general principles ................................................ 14-18
tool compatibility ........................................................ 7

Disclaimer.......................................................... 2

Electrical/Electronic Connections
service recommendations .................................... 19-22

Electronic Control Unit (ECU) ............................. 8
defined ................................................................ 8, 35
location.................................................................... 10
operation ................................................................... 8

Error Detection ........................................... 23-25

Exciter .............................................................11

FMVSS 121 .................................................. 3, 36

Glossary of Terms ............................................33

Grounding Recommendations ..........................21

Inspection, ABS .......................................... 27-30
modulator valves ...................................................... 29

J560..................................................... 31-32, 36

J1587 .......................................................8,9, 36

J1708 ..............................................................37

J1922 ................................................................8

J1939 ................................................................8

Maintenance, ABS....................................... 27-30
modulator valves ................................................. 29-30
sensor pickup adjustment .........................................27
sensor pickup rem