Technician Guidelines for Antilock Braking Systems
Air-Braked Trucks, Tractors and Trailers
This is the ABS Troubleshooting section of the reprint
in HTML format of the government document FHWA-MC-98-008. The
original document can be found at the following listed website:
This section describes general principles
of electrical, electronic, and air system diagnostics to provide
technicians with a plan of action for ABS troubleshooting. 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:
General 8-step Diagnostic
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.
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:
- Service newsletters
- 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.
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
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. (Truck
Troubles Editor Note: although fancy diagnostic tools are nice,
utilization of the LED diagnostic codes normally proves quite
adequate for solving ABS brake problems. Don't believe that unavailabilty
of the fancy tools will prevent typical repair actions. Those
LED diagnostic codes can serve repair actions quite nicely!)
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.
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.
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.
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.
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
Notes on Electrical/Electronic
The following section contains general service information that
should be considered if electrical/electronic connections need
repair during ABS servicing.
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
- 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
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
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
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.
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.
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
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.
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.
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.
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
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
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.
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
blowdown 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
Top 10 Most Commonly Encountered Problems
That Trigger ABS Malfunction Indicator Lamps
- Abraded or cut wires in convuluted tubing near frame
- Cut or corroded wires near sharp frame members and
- Wire jacket worn through from overlapping sensor
and modulator wires near frame members and frame-mounted
- Corroded connectors and connections not properly
sealed or damaged seals.
- Damaged connector latches or connectors not completely
sealed to mating assemblies.
- Terminals not completely latched or seated into connectors
- Excessive sensor air gap, sensor clip tension or
excessive wheel bearing endplay.
- Damage to exposed wires exiting or entering wire
- Worn, chipped or damaged sensor or modulator.
- Non-functioning controller (ECU).
Sample ABS Malfunction Indicator Lamp Flowchart
Common ABS Errors and Responses
|Component or wiring failure.
||ABS malfunction indicator lamp informs driver of fault.
Affected wheel(s) is removed from ABS control and switched
to normal braking. The remaining ABS valves may continue
providing braking controls at the wheels. The error is recorded
as a fault code and stored in the ECU. The information can
be recalled by the technician through the blink code lamp
or an electronic diagnostic tool.
|Power supply to ECU is interrupted or ABS connector not
||All antilock and traction control systems would be inoperative.
Normal, non-ABS controlled braking would be available at
all wheels. ABS malfunction indicator lamp indicates system
|Individual electronic component fails internally in ECU.
||All or part of the ABS is shut off and that part of the
vehicle reverts to normal non-ABS braking. ABS malfunction
indicator lamp indicates system fault.
|Error not detected. ABS remains inoperative but warning
light remains off.
||Certain failures, mainly mechanical, can occur and cause
the ABS to malfunction but not illuminate the indicator
lamp. Depending on the fault, the ABS will be inoperable
on one or more axles. In the rare event a valve hangs open,
system air could be lost, impairing all braking.
Causes of Common ABS
The following Table 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
Common ABS Sensor Problems and Causes
|Sensor signal is erratic.
||Damaged tooth wheel, excessive hub runout, and/or sensor
gap is to wide. Check sensor adjustment, resistance. Check
wheel bearing adjustment.
|Open sensor circuit.
||Damaged sensor, cable or loose cable connections. Replace
|Shorted sensor circuit.
||Damaged sensor, cable or cable connections, Replace
|One sensor is not producing a signal when other sensors
are producing signals, and sensor resistance is within
||Improper air-gap at non-producing sensor. Gaps should
not exceed 2 mm (0.080 in.). Check sensor adjustment.
Check wheel bearing alignment.
|ABS malfunction indicator lamp does not go out when
vehicle reaches speed needed for minimum "threshold"
voltage (such as five mph.)
||All of the items listed above could be causes. Also,
no tooth wheel or sensor installed. Install tooth wheel
|Sensor cap and cables lose elasticity, swell, or become
||Corrosion of bushing, sensor, and/or sensor clip. Make
any necessary replacements. Check manufacturer's recommendations
for proper lubricant.
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
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.
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