398 Auto Electricity and Electronics
Copyright by Goodheart-Willcox Co., Inc.
This chapter discusses anti-lock brakes, electronically-
controlled suspension systems, electronic climate control,
and other systems not covered in previous chapters. As
you will learn, electronics is used almost everywhere on
the modern motor vehicle. Electronics are used to avoid
collisions, help you stop your car, and prevent skidding in
slippery pavement. Electronics systems will even drive your
car into a parking spot for you!
Anti-lock Brake System (ABS)
An anti-lock brake system, abbreviated ABS, uses
wheel speed sensors, an electronic control module, and an
electro hydraulic control unit to prevent tire skid during
hard braking. If a tire locks up and skids on the road sur-
face, braking distance can increase and steering control
can be lost.
The anti-lock brake system improves driver and pas-
senger safety by reducing stopping distances and increasing
directional stability under panic stop conditions.
Figure 21-1 shows the advantages of ABS. With this
example, one side of the road is very slippery and the other
side is dry. This poses a problem if a panic stop is required.
Look at Figure 21-1A. Without ABS, the car would tend
to skid to the right because of higher tire adhesion on the
right. With ABS, the car would still travel straight ahead with
hard braking. The brake units would be cycled to prevent
tire skid and a loss of control. Note in Figure 21-1B how the
car can be steered while braking with ABS.
Figure 21-1. Without ABS, tires can lock up and lose adhesion with the road. This can cause a loss of control during hard braking or
when braking and trying to steer. Even if one side of the road is slick, brakes will be cycled so that none of the tires skid. Maximum
braking results when tires are just about ready to skid; traction is reduced if tires lock up.
Without ABS With ABS
With ABS
Dry
Pavement
Wet or Slick
Pavement
Straight-Ahead
Low Friction
Coefficient
Without ABS
Loss of Control
While Turning
and Braking
High Friction
Coefficient
Improved Control
While Braking
and Turning
A
B
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