Chapter 30 ABS and EBS

Objectives (1 of 3) Describe how an antilock brake system (ABS) works to prevent wheel lockup during braking. List the major components of a truck ABS. Describe the operation of ABS input circuit components. Outline the role of the ABS module when managing antiskid mode.

Objectives (2 of 3) Explain how the ABS module controls the service modulator valves. Explain what is meant by the number of channels of an ABS system. Describe how trailer ABS is managed. Outline the procedure for diagnosing ABS faults.

Objectives (3 of 3) Describe the procedure required to set up and adjust a wheel speed sensor. Explain how an electronic brake system (EBS) manages service brake applications. Outline the reasons why an EBS has to meet current FMVSS No. 121 requirements.

ABS Schematic

Shop Talk An ABS system is a simple computer-controlled system. As such, it requires system inputs, processing capability, and outputs. The steps required to produce an outcome are less complex than in an electronically managed transmission, but electronically the process is similar

Caution When driving an ABS-equipped truck, drivers have a tendency to expect too much. Always exercise care when braking, especially under bobtail conditions, even when the vehicle ABS system is fully functional.

ABS Component Locations

ABS Components (1 of 2) A typical truck ABS system requires the following components: Wheel speed sensor (system input) An ABS module or ECU (system processing and switching) Brake modulators or ABS valves (system output) Interconnecting wiring and connectors (electrically connects the first three sets of components) Networking capability

ABS Components (2 of 2)

Shop Talk Systems that use a 100-tooth wheel sensor do so for a reason. When divided into 360 degrees of one full tire revolution, the 100 teeth on the pulse wheel equal 3.6 degrees each. When a radial tire on a loaded-to-specification truck is run down the highway at 60 mph, its dynamic footprint is about 3.6 degrees. One- hundred-tooth ABS reluctor wheels have become the industry standard.

Wheel Speed Signals

ABS Modulator (1 of 4)

ABS Modulator (2 of 4)

ABS Modulator (3 of 4)

ABS Modulator (4 of 4)

Shop Talk Always check the vehicle service literature for wire and connector identification. Individual wire identification will differ depending on the type of connectors in use, the vehicle manufacturer, and the system features in use.

ABS Electrical Schematic

ABS Configurations Six-channel system Six-/four-channel system Two-channel system

Trailer ABS 2S/1M = two wheel speed sensors and a single modulator valve 2S/2M = two wheel speed sensors and two modulator valves 4S/2M = four wheel speed sensors and two modulator valves 4S/3M = four wheel speed sensors and three modulator valves 6S/3M = six wheel speed sensors and three modulator valves

ABS Trailer Cord Schematic

Caution ABS effectiveness is fully dependent on the foundation brake adjustment. Use stroke indicators to check brake adjustment status.

Shop Talk Bulb test Depress ABS switch for 16 seconds. The ABS light should illuminate even when no codes are present.

ABS Operation If an ABS relay valve malfunctions, the wheels controlled by the valve return to standard service braking. If a sensor malfunctions in a two-sensor system, control of that affected side of the trailer returns to standard service braking. If one sensor malfunctions in a system that has four sensors on a tandem axle, the ABS will continue to function. In this event, the ECU manages the system using the wheel speed data on the functioning sensor on the same side of the tandem. If a problem completely disables the ABS, control returns to standard braking.

ABS Tractor Power Supply Test

Warning The ABS is an electrical system. When working on ABS systems, the same precautions that must be taken with any other electrical system should be taken. There is a danger of electrical shock or sparks that can ignite flammable substances. Always disconnect the battery ground cable before working on the electrical system. This is of special concern when working with flammable or explosive truck or trailer cargos.

ABS Diagnostics

Shop Talk Blink codes in most systems do not display faults in the order in which they were recorded in the ECU memory. Also, if codes are blinked that do not appear in the OEM code chart, the ECU may be defective.

Shop Talk Never use a battery charger to perform a dynamic check of a trailer ABS system, as ECU damage can result.

ABS Diagnostics

Stability Control/Rollover

Stability Control/Jackknife

Summary (1 of 4) Antilock brake systems (ABS) are designed to help prevent wheel lockup during severe braking. An ABS system uses microcomputer technology to sense and reduce braking force on wheels that are beginning to lockup under braking. The components of a typical ABS system are wheel speed sensors, control modules, and brake modulators.

Summary (2 of 4) Several different ABS configurations are used on tractors and trailers, depending on the number of axles, axle configuration, axle load, brake circuit distribution, and brake force distribution. Truck and trailer ABS are computer-controlled system with networking capability. LED indicators and blink codes are used to diagnose first generation ABS failures PCs and proprietary software are used to read, diagnose, and analyze system performance on current ABS systems.

Summary (3 of 4) Electronic brake systems (EBS), also known as electro-pneumatic braking, are capable of replacing the current FMVSS 121-compliant pneumatic control circuit with an electronically managed control circuit. EBS provides much faster service braking response and release times because its electrical signals travel at much higher speeds than pneumatic control signals.

Summary (4 of 4) An EBS system controls pressure in the brake service chambers under all braking conditions, unlike ABS, which tends to be a factor only under severe braking conditions. Until FMVSS 121 is revised, EBS can only be used on North American highways as an option to a pneumatic control circuit.