Chapter 21 Chassis System Electronics 399
Copyright by Goodheart-Willcox Co., Inc.
ABS Components
Shown in Figure 21-2, the major parts of an anti-lock
braking system include:
Wheel speed sensors—magnetic pickups for detecting
the rolling or rotating speed of each tire and wheel
assembly.
Sensor rotor—toothed wheel that rotates at the same
rpm as the wheel and tire.
Electronic control module—small ECM or processor that
uses sensor inputs to control the hydraulic actuator.
Hydraulic actuator—solenoid-operated valve and
electric pump mechanism for controlling how much
hydraulic pressure is applied to each wheel brake
cylinder.
Indicator light—dash light that informs the driver of a
problem in the ABS system.
Wheel Speed Sensors
The wheel speed sensors produce an AC signal that
corresponds to wheel and tire speed. These signals are fed
into the anti-lock brake system ECM.
Look at Figure 21-3. It shows how one wheel speed
sensor mounts in the car’s steering knuckle assembly. The
sensor tip is located next to the sensor rotor. Only a small
air gap separates the sensor tip from the teeth on the sensor
rotor. In this example, the sensor rotor is mounted on the
back of the brake disc. A hex bolt and O-rings secure the
sensor in the steering knuckle.
Wheel speed sensors can also mount on the axle shaft
and drive shaft. However, their function and operation are
very similar. Refer to Figure 21-4.
Figure 21-2. ABS or anti-skid braking systems use these basic components. Wheel speed sensors send electrical signals to the
ECM. ECM can then use a hydraulic actuator to reduce brake fl uid pressure to any wheel that slows down too much and is ready to
skid. (Honda Motor Co.)
ABS Motor
Relay
Fail-Safe
Relays
ABS Control
Module
Brake Booster
Rear Wheel
Speed Sensor
Master Cylinder
Front Wheel
Speed Sensor
Modulator
Unit
Figure 21-3. Cutaway shows how this wheel speed sensor mounts
in the steering knuckle. Sensor tip is next to the spinning sensor
rotor. Teeth pass next to the sensor tip. (Mercedes Benz)
Brake Disc
or Rotor
Steering
Knuckle
Wires to
Computer
Sensor Rotor
Teeth
Speed Sensor
Sensor Coil
Sensor
Hex Bolt
Sensor
Tip
Sensor
Rotor
Front
Wheel
Hub
As the tire and wheel rotates, the sensor rotor spins
next to the wheel speed sensor. As each tooth passes the
sensor, the magnetic fi eld around the sensor is affected.