Technical information

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8
Technician Guidelines for Antilock Braking Systems
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II. 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 parts including: the ECU,
the modulator valve, the wheel speed sensor, ABS malfunction/
indicator lamp, ABS diagnostic components, and traction
control.
Modern antilock braking systems all feature the following
major components (See Fig. 1 on page 9 for typical system):
• Electronic Control Unit (ECU)
• Modulator Valves
• Wheel Speed Sensors (pickup and exciter)
• ABS Malfunction Indicator Lamps
• Diagnostics
A. 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.
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.
* 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.