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ABS has had three different configurations for distribution of wheel
speed sensors and hydraulic lines from the hydraulic control unit since
The earliest ABS configuration used one wheel speed sensor that
monitor the speed of both rear wheels. If the rear wheels would lock up,
the ABS computer would pulse a solenoid in the hydraulic control unit,
which would then change the brake fluid hydraulic pressure applied to
the rear wheels.
The next evolution in ABS added wheel speed sensors to both front
wheels, while still having a single wheel speed sensor in the rear to
monitor both rear wheels. Under this system, if the left front, right front,
or both rear wheels rotate at a speed different from each other, the ABS
computer could use up to three separate solenoids to control brake fluid
hydraulic pressure. This allowed the ABS system to control the left front,
right front, and both rear wheels together independently.
Today, some vehicles have one wheel speed sensor per wheel for a
total of four. On these vehicles, the ABS computer can independently
control brake fluid hydraulic pressure on every wheel.
ABS Repair Tips
• Before servicing the ABS, refer to the manufacturer's recommendations.
• Torque the wheel lug nuts properly to avoid bending a rotor or drum, which
can cause inaccurate wheel speed sensor readings.
• Bleed the brake system properly.
• When necessary, always add fresh brake fluid to the master cylinder.
• Always replace tires with the manufacturer's recommended size.
• Always rotate tires as recommended by manufacturer.
• When interpreting DTCs retrieved from the vehicle, always follow the man-
ufacturer's recommendations for repair.
Supplemental Restraint System
Supplemental Restraint System (SRS) is a vehicle safety device which
was introduced in the 1970s. Its purpose is to supplement the
over-the-shoulder seatbelt to help prevent vehicle occupants from
hitting interior objects such as steering wheels, dashboards, and the
like. Vehicles can and often do contain multiple airbags in various
locations throughout the vehicle.
How SRS Works
The SRS, or airbags, are activated through a central Airbag Control Unit
(ARU). The ARU monitors sensors such as wheel speed,
accelerometers, brake pressure, seat occupancy, and in some cases
gyroscopes, to provide a basic threshold. When the threshold is