Technician Guidelines for Antilock Braking Systems
Air-Braked Trucks, Tractors and Trailers

This is the ABS Components 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 the design and operation of ABS components. When you complete this section, you should understand the purpose and function of all major ABS antilock braking system components, including:

The 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.

  • *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.

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. 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.

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.

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)
  • Ssix sensors/four modulators (6S/4M)
  • Six sensors/six modulators (6S/6M).

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

ABS Malfunction Indicator Lamp

sVehicles 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.

ABS Diagnostics

Although not required by law, all air brake ABSs have self-diagnostic 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 utside 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.

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 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.

  • Traction control systems are designed to prevent wheel spin in the power mode.
  • Unlike an ABS, traction control can apply the brakes automatically. 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.

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