Download Textbook–Module 3 - Northwest Technology Center

MODULE 4–SKID CONTROL SYSTEMS
Topic A. System Overview And
Operation
Vehicle
In Control
Begin
Skid
Skid Control
System
Engages
Back Under
Control
Back Tires
Front Tires
A-2 This vehicle has exceeded the limits of traction available to one side.
A skid happens when a vehicle exceeds the traction
limits of the vehicle’s tires to the road surface.
To determine what is happening during a skid,
sensors provide input to the ECM. The steering angle
sensor provides the ECM with the drivers intended
path, or direction. The wheel speed sensors provide
information on individual wheel speed which the
ECM is able to use for many different calculations.
The yaw rate sensor tells the ECM the vehicles’ actual
rate of turn. Lateral accelerometers measure the
cornering force. A brake pressure sensor measures
the amount of pressure the driver is applying to the
brakes.
The ECM is programmed to look for a vehicle’s
correct path when steering through a corner. If the
information being sent to the ECM does not match
this programmed data, the computer determines
that the vehicle is in a skid.
A-3 The front tires have lost traction.
Skids can be divided into two categories, understeer,
and oversteer.
Understeer occurs when a driver attempts to turn
and the front tires begin to slip. The front wheels are
turned but the vehicle does not respond and continues on its original course.
On front-wheel drive vehicles this may be caused by
the drive wheels spinning too fast for the corner (too
much throttle). Front wheel drive vehicles can also
understeer if a corner is entered too fast and the
throttle opened abruptly.
When a front-wheel drive vehicle understeers, the
skid control system instructs the ECM to open or
close the throttle, depending on the torque applied
to the drive wheels.
When a rear-wheel drive vehicle understeers, the
skid control system instructs the ECM to reduce the
throttle before engaging the appropriate brake circuit to correct the slide.
Brake pressure is then applied by the computer to
one or more of the wheel circuits to correct the
vehicle’s skid.
Anti-Lock Brake And Traction Control Systems Program 1 v.2.2–Module 4
© 2000 Inter-Industry Conference On Auto Collision Repair
36
UNDERSTEER AND OVERSTEER
Refer to screen A-5v of your Student
CD-ROM for a video explaining understeer
and oversteer.
A-4 The rear tires have lost traction.
Oversteer occurs when a vehicle is turning and the
rear tires begin to slide away from the direction of the
turn. This occurs most often on rear-wheel drive
vehicles with slipping drive wheels.
When a rear-wheel drive vehicle oversteers, the skid
control system instructs the ECM to open or close
the throttle depending on if the drive wheels have
too much or too little torque applied for the situation.
Skid control systems consider a number of variables
to determine if understeer is occurring. For example,
when a vehicle begins to slide, three sensors provide
input to the ECM, steering angle sensor, yaw rate
sensor, and lateral accelerometer. The steering angle
sensor measures how far the front wheels have been
turned. The yaw rate sensor measures the rate of the
vehicle’s change in direction. The lateral accelerometer measures cornering force. For example, this
force may be described as the force passengers feel
when making a turn.
When the rear tires are losing traction in a corner, the
steering wheel is turned less than the vehicle is
actually turning. Again the lateral accelerometer
senses a reduction in cornering force. The ECM
applies the outside front brake to prevent the rear of
the vehicle from skidding farther out of line.
Different vehicle makers use different methods of
correcting a vehicles’ skid. For example, in a situation
where one vehicle maker would apply the right front
brake (left hand corner oversteer) another vehicle
maker applies front and rear right side brakes, and
still another vehicle maker applies front and rear right
side brakes and left rear brake.
Anti-Lock Brake And Traction Control Systems Program 1 v.2.2–Module 4
© 2000 Inter-Industry Conference On Auto Collision Repair
37
Topic B. Parts Identification And
Function
B-2 The steering angle sensor measures the steering angle, and on some
models, the rate of turn as well.
B-1 Yaw rate sensors are able to measure the effects of momentum on
a vehicle.
Yaw rate sensors measure a vehicle’s actual turn
rate. These sensors may be located under the rear
package shelf, in the center console, or under the
driver’s seat.
One yaw rate sensor is a tuning fork type. Voltage
forces the tuning fork to vibrate. Then, any change in
direction of travel of the vehicle distorts the legs of
the tuning fork. The sensor creates a linear output
voltage signal that tells the ECM the rate of change
in direction, or yaw.
YAW RATE SENSORS
Select the Demonstration Icon found on screen
B-1 of your Student CD-ROM for a demonstration on examples of yaw rate sensors.
The steering angle sensor measures the steering
wheel position relative to straight ahead. It may be
self-centering or have to be calibrated after any
steering (or straightening) work is performed.
Some models also measure the rate (speed) that the
steering wheel turns. This is accomplished using a
potentiometer that measures the turning rate.
Depending on the vehicle, the signal may be either
unmodified, or analyzed through a microprocessor
before being sent to the ECM.
On BMW vehicles equipped with DSC 2 or 3 the
steering sensor must be calibrated after work has
been performed to the steering or suspension systems.
On some GM vehicles (Corvette and Seville) the
ECM runs a centering routine on the steering wheel
position sensor every time the vehicle is started and
driven to compensate for tire and suspension wear.
STEERING ANGLE SENSORS
Select the Demonstration Icon found on screen
B-2 of your Student CD-ROM for a demonstration on examples of steering angle sensors.
Anti-Lock Brake And Traction Control Systems Program 1 v.2.2–Module 4
© 2000 Inter-Industry Conference On Auto Collision Repair
38
LATERAL ACCELEROMETERS
Select the Demonstration Icon found on screen
B-3 of your Student CD-ROM for a demonstration on examples of lateral accelerometers.
B-3 The lateral accelerometer sends a signal to the ECM that represents
the cornering force.
A lateral accelerometer is very similar to an airbag
system crash sensor (decelerometer). The lateral
accelerometer sends a precise reading to the ECM
that represents the cornering force that the vehicle is
experiencing.
The ECM uses the lateral acceleration information
along with the steering wheel position and wheel
speed information to calculate the desired yaw (turn)
rate.
Lateral acceleration sensors may be located ahead
of the B-pillar, on the vertical surface of the inner
rocker panel (one on each side of the vehicle). The
sensors can be located in the C-pillar behind the rear
seat cushion, or in the engine compartment, near the
master cylinder.
Accelerometers are used on technologically sophisticated vehicles for measuring acceleration forces in
almost every direction. These sensors are used for
every system that might possibly need information
on vehicle movement from airbag to suspension
systems. A service manual is essential to identify the
sensors.
B-4 The number location and function of these warning lights will vary
greatly between vehicles.
There are numerous indicator and warning lamps
used in skid control systems. The following is an
example of one vehicle makers set of warning lamps:
■
■
■
■
■
■
SCS ON. Displayed for a few seconds when the
system is turned on.
SCS. Displayed when the skid control system is
activated.
SCS WARMING UP. Displayed if the steering
angle sensor is not centered after 30 seconds.
SERVICE SCS. Displayed continuously when an
SCS fault exists.
SCS OFF. Displayed continuously when SCS is
turned off.
SCS WARM UP COMPLETE. Displayed for a few
seconds after the steering angle sensor is centered.
These lamps may be located in the instrument panel
or the center console.
Most vehicles use two lateral acceleration sensors.
Anti-Lock Brake And Traction Control Systems Program 1 v.2.2–Module 4
© 2000 Inter-Industry Conference On Auto Collision Repair
39