The vehicle stability enhancement system
(VSES) provides added vehicle stability during
aggressive driving maneuvers. Yaw rate is the
rate of rotation around the vehicle's vertical axis.
The system is activated when the ABS brake
module determines that the desired yaw rate
does not match the actual yaw rate as measured
by the sensor.

The desired rate is calculated by the ABS
module  the following inputs...

•  The position of the steering wheel

•  The speed of the vehicle

•  The lateral, or sideways acceleration of the
vehicle

The difference between the desired yaw rate and
the actual rate is called a rate error, which is a
measurement of over steer or under steer.
When an error is detected, the computer
attempts to correct the vehicle's motion by
applying brake pressure to one or more of the
wheels. The amount of brake pressure which is
applied varies, depending on the correction
required. The engine torque may be reduced
also, if it is necessary to slow the vehicle while
maintaining stability.

VSES activations generally occur in turns during
aggressive driving. When braking during system
activation, the pedal may pulsate. The brake
pedal pulsates at a higher frequency during
VSES activation than during ABS activation.
The vehicle stability enhancement system sensors values may vary
slightly due to differences in temperature, sensor mounting,
connector resistances, manufacturing, etc. Since the system is a
very sensitive and precise control system, it is imperative that the
electronic brake control module (EBCM) be able to accurately
equate a given sensor voltage with an actual unit of measurement.
For example, the yaw rate signal of one vehicle may be 2.64 volts
at +18.0 deg/sec yaw rate while the yaw rate signal of another
vehicle may be 2.64 volts at +17.5 deg/sec yaw rate. Therefore, at
the beginning of each ignition cycle, the EBCM must perform an
initialization procedure to observe how the sensors are correlated
with each other and also to determine what each sensor value is
when the applicable unit of measurement equals 0. This voltage is
referred to as the sensor bias voltage. Although some activation of
the system may occur if required to prior to full initialization, the
system does not give optimum performance until the sensors are
fully initialized.

The following VSES sensors require initialization.

•  The yaw rate sensor

•  The lateral accelerometer

•  The longitudinal accelerometer

•  The master cylinder pressure sensor

•  The steering wheel position sensor

When the vehicle speed is greater than 25 km/h (15 mph), full
sensor initialization must occur during 3 km (1.8 mi) of driving or 1
km (0.6 mi) of straight and stable driving, whichever occurs first.
Although an attempt at initialization may fail due to driving
conditions, such as driving on a very winding road, failed
initialization is usually caused by a sensor bias voltage which is not
within an acceptable range. Often, a DTC sets soon after a failed
initialization attempt. The stability system not ready indicator
illuminates when sensor initialization fails.
Vehicle Steering Stability System Explained

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