410 Auto Electricity and Electronics
Copyright by Goodheart-Willcox Co., Inc.
Figure 21-25. Stability control system operation. A—With an understeer problem, the stability control system might apply the left rear
brake to keep the front of the vehicle from sliding or drifting to the right in a left-hand turn. B—With an oversteer problem, a stability
control system can apply the right front brake to keep the rear of the vehicle from sliding around in a high-speed turn.
With Stability Control Without Stability Control
Understeer
With Stability Control Without Stability Control
Oversteer
A B
Other sensors—gives the control module information
about the driver’s actions and the resulting vehicle
motion.
These sensors help the control module determine if the
vehicle is skidding out of control so corrective action can be
taken. The control module is programmed to know when the
vehicle is near its maximum rate of directional change. If this
limit is reached, the system activates, Figure 21-25.
The control module is preprogrammed to apply one or
more of the wheel brakes. It can also decrease or increase
engine power to maintain vehicle control. If the driver rotates
the steering wheel hard right for a sharp turn, the steering
angle sensor signals the stability control module. The control
module knows how fast each wheel should be turning. The
inside wheels should be spinning more slowly than the outside
wheels, because they are traveling less distance in the turn.
For example, the control module can use sensor sig-
nals to determine if the vehicle is following its intended
direction of travel. The module knows what frequency
each speed sensor should produce as the steering wheel is
turned. If one or more of the wheels are not spinning at the
right speed (skidding on road surface), the control module
can react within 40 milliseconds to take corrective action.
It might cut engine power slightly and apply one of the
wheel brake calipers to reduce skidding.
If understeer is detected (front tires lose adhesion with
the road in a turn), the control module will apply braking
force to the rear wheel on the opposite side of the vehicle.
This will help bring the front of the vehicle back under con-
trol for making the turn. See Figure 21-25A.
If the stability control system detects oversteer (rear
tires lose adhesion with the road surface and slide side-
ways), additional braking force is automatically applied to
the outside front wheel. This helps correct the oversteer so
the rear of the vehicle does not spin out or slide sideways
in a sharp turn. See Figure 21-25B.
When driving a vehicle equipped with a stability control
system, a common mistake is to try to counter-steer the vehicle
out of a skid when turning. This often causes overcorrection.
It is best to continue steering in the desired direction of travel,
allowing the system to correct the skid. This is similar to get-
ting used to driving a vehicle with anti-lock brakes.
Electronic Climate Control
An electronic climate control system uses an ECM or
processor, sensors, and actuators to maintain a preset passen-
ger compartment temperature. See Figure 21-26. Although
Figure 21-24. Major control of the stability control system is the
inertia or yaw sensor that detects when a car is moving side-
way or skidding when the steering wheel is facing in another
direction. The yaw sensor, wheel speed sensors, steering angle
sensor, and other sensors work together to stiffen the shock
absorbers and apply the brake to improve driver control in an
accident situation. (TRW)
Lateral
Acceleration
Sensor
Steering Angle
Sensor
Wheel Speed
Sensor
Tone Ring
Electronic
Hydraulic
Control Unit
Yaw (Inertial)
Sensor