Electronic Stability Program (ESP)

ELECTRONIC STABILITY PROGRAM

To determine whether the car is responding properly to cornering commands, ESP(R) uses steering wheel angle, yaw (turning) rate and lateral acceleration sensors (combined into Dynamics Sensor). Using signals from these sensors, in addition to individual wheel speed sensor signals, the system determines appropriate brake and throttle actions. Once initiated, ESP(R) operates much like All-Speed Traction Control, except that the goal is directional stability. If the vehicle yaw response, or rate of turning, is inconsistent with the steering angle and vehicle speed indications, the ESP(R) system applies the brakes and, if necessary closes the throttle, to restore control. This occurs whether the vehicle is turning too rapidly (oversteering) or not rapidly enough (understeering).

HYDRAULIC BRAKE ASSIST

Brake Assist is programmed into the ESP(R) system. During a panic stop, a pressure sensor determines when the driver is doing so by measuring the brake pedal pressure application rate. A high rate of pedal pressure application causes the ESP(R) system to apply maximum available pressure to the brakes and the vehicle stops as quickly as available traction will allow.

TRACTION CONTROL

For information on the All-Speed Traction Control, Brakes - Operation.

ELECTRONIC ROLL MITIGATION

Typically when a vehicle makes a sudden turn, the outside wheel takes the majority of the cornering loads. In order for the vehicle to make the turn, a significant amount of grip must exist at the tire contact patch. The additional body roll that occurs during this event places additional weight on this outside tire. These conditions, plus the lateral acceleration of the vehicle, combined with the center of gravity position in the vehicle can cause the vehicle to lift the two inside wheels in the turn and the vehicle rolls over the outside tire. Electronic Roll Mitigation (ERM) takes advantage of the principle that a tire in slip cannot handle cornering loads by building and applying enough brake pressure to intentionally drive the outside wheel into slip, not to the point of total lockup but close. As a result, the outside tire cannot support the cornering loads and the vehicle cannot maintain its original path. The new path is straighter, reducing the amount of lateral acceleration and transferring some of the weight back over to the inside tires, thus preventing a rollover.

BRAKE LOCK DIFFERENTIALS

Split-friction surfaces (where one wheel has grip and the other side of the vehicle is on a low-friction surface) or where a wheel is suspended off the ground and spins in the air are common off-road situations. An open differential by nature directs torque to the wheel that has the least amount of grip, and in these cases, the wheel spinning on the low-friction surface or the wheel spinning in the air, which is undesirable. While some vehicles use locking differentials to help overcome this situation and provide a solid connection between two wheels on a given axle, Brake Lock Differentials (BLDs) are used on this vehicle as an effective solution.

Vehicles equipped with the Off-Road Group (sales code AWL) include a separate set of BLD calibrations that are designed for Off-Road mode use while crawling over obstacles, during heavy articulation activity, split-friction surfaces and other conditions. These BLDs provide an alternative solution to locking differentials found on other vehicles by replacing the locking differential with the brake control hardware that is already in place on the vehicle. The same hardware used for Electronic Stability Control is used for Brake Lock Differentials.

Under normal conditions with the vehicle in Off-Road mode, for example, assume that the Electronically Controlled Coupling directs 148 ft. lbs. (200 Nm) of torque to the rear differential and each wheel receives an equal share, 74 ft. lbs. (100 Nm) of torque.

When wheel speeds are unequal across a given axle, the BLDs respond. In this example, the vehicle is crawling over rocky terrain and the vehicle experiences heavy axle articulation that suspends the right rear wheel in the air and the wheel spins. The open differential ports all 148 ft. lbs. (200 Nm) of torque to the wheel in the air.

The Antilock Brake Module (ABM) senses the right wheel is spinning faster than its mate on the opposite side of the rear axle and clamps the brake caliper on the spinning wheel to stop it from spinning and form a solid connection across the axle. If 74 ft. lbs. (100 Nm) of brake force is applied to stop the spinning wheel, an equal amount of torque is ported to the wheel with grip. This allows the vehicle to continue moving forward. The amount of torque the BLDs can send to the wheel with traction is dependent on the amount of torque the brake caliper can hold. The brakes are capable of holding the amount of torque used in this example.

HILL DESCENT CONTROL

Hill Descent Control (HDC) included on vehicles equipped with the Off-Road Group (sales code AWL) works automatically once the vehicle is in the Off-Road Mode and the transaxle selector is in Low or Reverse. A green icon will illuminate on the instrument cluster indicating that the HDC is operating. Should a driver decide to back down a hill, HDC will back the vehicle down at a controlled rate of speed. The system determines when the vehicle is on a grade or level terrain by input from the acceleration sensor located in the Dynamics Module. Grade Sensing logic built into it activates the brakes only when descending a hill; the driver does not need to turn it on or off.

As the vehicle crests a hill, the brakes will automatically begin to pulsate and adjust the brake pressure to maintain the vehicle at a set speed of 3 mph (5 km/h.) The vehicle speed can be lowered by applying the brake or raised by applying the accelerator pedal to provide a safe descent down a hill. Once the driver releases the brake or the throttle, vehicle speed will ramp up or down to the Hill Descent Control limit speed.