400 Auto Electricity and Electronics
Copyright by Goodheart-Willcox Co., Inc.
This induces a weak AC or alternating current signal in the
wheel sensor coil. The frequency of this AC signal is depen-
dent on tire and wheel speed. The AC signal is used by the
anti-lock brake system ECM to check for tire skid. A rapid
decrease in sensor AC signal frequency would indicate that
a tire is starting to lock up and skid.
Note: The operation of a wheel speed sensor
is similar to a crankshaft sensor and a
magnetic pickup coil in an ignition system.
Refer to the discussion of these sensors for more
information on coil and trigger wheel operation.
ABS ECM
The ABS ECM uses wheel speed sensor inputs to con-
trol the operation of the hydraulic actuator. It is constructed
and operates like the other ECMs, or electronic control
modules, discussed in this book. A photo of an ABS control
module is shown in Figure 21-5.
ABS Hydraulic Actuator
The ABS hydraulic actuator regulates the amount of
fl uid pressure applied to each wheel brake assembly during
hard braking. It is controlled by the ABS ECM. In an inte-
grated system, the master cylinder-booster assembly is an
integral part of the hydraulic actuator. In a non-integrated
system, the hydraulic actuator does not contain the master
cylinder-booster assembly. The hydraulic actuator normally
mounts in the engine compartment, Figure 21-6.
An exploded view of an ABS hydraulic actuator is
given in Figure 21-7. Its major parts include:
Fluid reservoir—container for holding extra supply of
brake fl uid.
Solenoid valve block—coil-operated valves that con-
trol brake fl uid fl ow to the wheel brake cylinders; they
are contained in the valve block assembly.
Figure 21-4. Wheel speed sensor is simply a coil mounted around a core. When the teeth of the sensor rotor pass the sensor coil,
an alternating voltage is induced into the sensor coil and the output
Leads to
Computer
Permanent
Magnet
Pick-Up or
Sensor Tip
Sensor Rotor
Coil
Windings
High Speed
Waveform
Low Speed
Waveform
Pick-Up
Clearance
Sensor Rotor
Voltage
Figure 21-5. Study the major parts of an anti-lock brake system.
Hydraulic actuator has solenoid-operated valves that can reduce
pressure to each wheel brake assembly. It is frequently mounted
on the brake master cylinder assembly. (Bosch)
Master
Cylinder
ABS
ECM
Wheel Speed
Sensor
Electric
Motor Pump
Pressure
Accumulator
ABS Hydraulic Valve
Assembly or Actuator
Electrical
Connector