Chapter 21 Chassis System Electronics 409
Copyright by Goodheart-Willcox Co., Inc.
body against a turn to improve handling. It can also prevent
nose dive on braking and body squat on acceleration.
Basically, pressure sensors on each hydraulic ram
are used as the main control for the system. They react to
suspension system movement and send signals to the ECM.
The ECM can then extend or retract each ram to match the
road surface.
For example, if one side of the car travels over a bump
in the road, the pressure sensors can instantly detect a rise
in pressure inside the ram as the tire and wheel push up
on the suspension and hydraulic ram. Instead of making
the car body rise with spring action, the ECM can release
enough ram pressure to allow the suspension to move up
over the bump without body movement.
Then, as the tire travels back down over the bump, the
sensor detects a pressure drop in the ram and the ECM can
increase ram pressure so the tire follows the road surface.
A hydraulic pump provides pressure to operate the
suspension system rams. Look at Figure 21-22.
Traction and Stability Control Systems
A traction control system is designed to prevent the
vehicle’s wheels from spinning and losing traction under
hard acceleration. Most traction control systems work with
Figure 21-22. Simplifi ed illustration of active suspension system
shows major components. Pressure sensor on hydraulic ram
reacts to up-and-down movement of ram and resulting pressure
changes. If pressure in the ram rises from going over a bump,
the sensor signals the computer. Computer can quickly react to
release ram pressure so suspension moves up with the bump.
As the wheel travels down the other side of the bump, sensors
make the computer increase ram pressure so suspension trav-
els back down to the original road surface.
Computer
Active Suspension
To Other
Sensors
Motor-Pump
Assembly
Pressure
Sensor
Pressure
Control
Valves
Hydraulic
Ram
Irregular
Road
Surface
Causes
Pressure
Drop
Causes
Pressure
Rise
Fluid
Reservoir
the anti-lock brake system to cycle hydraulic pressure to the
wheel spinning the fastest. The control module is capable
of applying one wheel brake at a time. Some systems also
reduce engine power output to reduce skidding.
An indicator light comes on anytime the traction con-
trol system is activated. This warns the driver that the tires
are losing traction and slipping on the road surface. He or
she then knows that the traction control system is trying to
limit or prevent this problem.
A stability control system is an advanced system that
reduces tire spin on acceleration and prevents tire skid
when cornering too quickly. This system uses more input
signals from various sensors to provide greater control over
vehicle handling under severe cornering, braking, and
acceleration conditions. It is designed to make the vehicle
more stable under emergency handling situations, such as
when trying to avoid a collision.
As shown in Figure 21-23, a stability control system
uses the anti-lock brake system, electronically-controlled
shock absorbers, traction control, and electronically-
controlled steering assist system to improve vehicle safety.
The major parts of an electronic stability control system are
shown in Figure 21-24.
In addition to the wheel speed sensors, a stability con-
trol system may use inputs from the following sensors:
Steering angle sensor—measures how sharply the
steering wheel is rotated to the right or left.
Lateral acceleration sensor—measures how much
side force is generated by a turn.
Yaw sensor—also called an inertia sensor, measures the
direction of the thrust generated by vehicle movement.
Throttle position sensor—measures how far the driver
has pressed down on the accelerator to control engine
power output.
Brake pressure sensor—measures the amount of
hydraulic pressure produced by the driver pressing on
the brake pedal.
Figure 21-23. Electronic stability control networks the anti-lock
brake system, electronic shock absorbers, transmission, and
engine control systems to increase vehicle safety while driving.
Anti-Lock Brake System
Electronic Suspension Control
Electronic
Steering Control