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:
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 antilock braking system components,
The Electronic Control Unit
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
- 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 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
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
- 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,
- 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
- 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
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
ABS Malfunction Indicator
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
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
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
top of page , Driver
Knowledge Page ,
Truck Brakes page , HomePage.