Traction Control System (TCS)
Traction is maintained by limiting the amount of torque produced by the drivetrain and also by applying brake pressure to slipping wheels during acceleration. This causes power to transfer through the driveline to wheels which are not slipping. The transfer case used on 4 wheel drive vehicles equipped with VSES does not contain a viscous coupling and therefore allows the front and rear driveshafts to turn at substantially different speeds. This front to rear differential must be kept within acceptable parameters by the VSES. The two methods of traction control are performed as follows.
Engine torque reduction
The EBCM uses a 5-volt Pulse Width Modulated (PWM) signal to request that the PCM reduce the amount of torque to the drive wheels. The PCM reduces torque to the drive wheels by retarding spark timing and commanding the throttle actuator control.
The PCM uses a 12-volt PWM signal to report to the EBCM the amount of torque that is being delivered to the drive wheels. Engine torque reduction is mostly used to reduce vehicle speed during VSES events and during TCS events when the brakes are in danger of being overheated or all of the driven wheels are slipping at the same rate. Engine torque reduction can be disabled by pressing the traction control switch.
Brake pressure application
The EBCM uses brake pressure application to control traction by transferring torque through the driveline to wheels which are not slipping. The precharge pump motor, ABS pump motor and appropriate valve solenoids are commanded ON and OFF to apply brake pressure to the slipping wheels. Brake pressure application is used in an attempt to maintain equal WSS signals at the driven wheels.
The EBCM does not allow excessive brake pressure application due to the fact that the solenoid coils or the brakes may become overheated, damaging the EBCM or reducing the driver's ability to stop the vehicle. Estimated coil and brake temperatures are determined by a calculation in the EBCM software. Overheated solenoid coils cause all brake pressure application to become disabled and the stability system disabled message to be displayed. Overheated brakes cause brake pressure application during TCS events to disable, yet the VSES remains functional and as long as the engine torque reduction is enabled, there is no indication to the driver when this occurs and no DTC sets. In either case, engine torque reduction is the only function governing front to rear differential speeds within the transfer case on 4 wheel drive vehicles. Also on 4 wheel drive vehicles, if the engine torque reduction is disabled for any reason when brake application disables, the traction off indicator flashes if wheel slip is detected. Any time the traction off indicator is flashing, there is danger of transfer case damage.
Vehicle Stability Enhancement System (VSES)
VSES provides added stability during aggressive maneuvers. Yaw rate is the rate of rotation about the vehicle's vertical axis. The VSES is activated when the EBCM determines that the desired yaw rate does not match the actual yaw rate as measured by the yaw rate sensor.
The desired yaw rate is calculated by the EBCM using, primarily, 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 yaw rate is the yaw rate error, which is a measurement of oversteer or understeer. When a yaw rate error is detected, the EBCM attempts to correct the vehicle's yaw 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 VSES activation, the pedal may pulsate. The brake pedal pulsates at a higher frequency during VSES activation than during ABS activation.
