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 Principles

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:

  • 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.

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.)

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 vehicle.

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.

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.

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 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.

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 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.


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 information.

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

  • Abraded or cut wires in convuluted tubing near frame clamps.
  • Cut or corroded wires near sharp frame members and frame-mounted modulators.
  • Wire jacket worn through from overlapping sensor and modulator wires near frame members and frame-mounted modulators
  • 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 tubing.
  • Worn, chipped or damaged sensor or modulator.
  • Non-functioning controller (ECU).


Sample ABS Malfunction Indicator Lamp Flowchart

Common ABS Errors and Responses
Detected Error
System Response
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 plugged in. 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 fault.
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 Sensor Problems

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 affect operation.

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 as needed.
Shorted sensor circuit. Damaged sensor, cable or cable connections, Replace as needed.
One sensor is not producing a signal when other sensors are producing signals, and sensor resistance is within specification. 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 and sensor.
Sensor cap and cables lose elasticity, swell, or become mechanically sensitive. 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 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.

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