Antilock Brakes / Traction Control Systems: Description and Operation

Anti-Lock Control

AdvanceTrac(R) With Roll Stability Control (RSC(R))

The AdvanceTrac(R) system is made up of the following systems: conventional ABS, traction control and Electronic Stability Control (ESC). The Roll Stability Control (RSC(R)) system is an optional feature that works in conjunction with AdvanceTrac(R) to aid in maintaining vehicle stability. The AdvanceTrac(R) with RSC(R) system consists of the following components:

- ABS module - attached to the Hydraulic Control Unit (HCU), but can be serviced separately from the HCU
- Front wheel speed sensors - one sensor is located in each front wheel knuckle. The sensors are serviced separately from the knuckle
- Front wheel speed sensor tone rings - one tone ring is located on each front axle halfshaft and can be serviced separately from the front axle halfshaft
- HCU - which contains several valves (inlet, outlet, isolation and dump), and the hydraulic pump motor. It is mounted in the RR corner of the engine compartment on hybrid vehicles and in the LR corner of the engine compartment on non-hybrid vehicles. The HCU is serviced as a unit, but does not include the ABS module
- Rear wheel speed sensors - one sensor is located in each rear wheel knuckle, the sensors are serviced separately from the knuckle
- Rear wheel speed sensor tone rings - one tone ring is located on each axle halfshaft in AWD applications and is not separately serviceable. In FWD applications, the tone ring is part of the wheel hub and can be serviced separately
- Stability control sensors - located inside the Restraints Control Module (RCM) and consist of the following sensors: yaw rate, roll rate, lateral accelerometer and longitudinal accelerometer. The sensors are integral to the RCM and cannot be serviced separately
- Steering wheel rotation sensor - located in the Electronic Power Assist Steering (EPAS) steering column assembly, this component cannot be serviced separately
- Stability/traction control switch - located in the lower instrument cluster center finish panel and is serviced separately from the finish panel

Electronic Brake Distribution (EBD)

The ABS module and HCU incorporate a strategy called Electronic Brake Distribution (EBD). The EBD strategy uses the HCU as an electronic proportioning valve to create a balanced braking condition between the front and rear wheels and minimize the chance of rear wheel lockup during hard braking.

Some drivers may feel a slight bump sensation in the brake pedal when the EBD is active.

Anti-Lock Control

The anti-lock control system aids in the prevention of wheel lock-up during braking events which allows the driver to maintain steering control and stop in the shortest distance possible under most conditions. This is accomplished by the ABS module and HCU modulating brake fluid pressure to the affected brake caliper(s).

When the ABS activates, both of the following can occur:

- A rumble or grinding sound may be heard
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal

AdvanceTrac(R) System

Traction Control System

The traction control system aids in the prevention of excessive wheel spin which allows the vehicle to maintain traction during acceleration. This is accomplished by the ABS module and the HCU modulating brake fluid pressure to the affected brake caliper(s) and by the PCM modulating engine torque.

ESC System

The ESC system constantly monitors the vehicle's direction of travel relative to the driver's intended course, except when the vehicle is traveling in reverse, and aids in keeping the vehicle traveling along that course. This is accomplished by the ABS module and the HCU modulating brake fluid pressure to the affected brake caliper(s) and by the PCM modulating engine torque.

When the traction control or ESC system activates, any of the following can occur:

- A rumble or grinding sound may be heard, much like ABS
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal, much like ABS
- A small deceleration or a reduction in the acceleration of the vehicle
- The stability/traction control indicator (sliding-car icon) flashes

Roll Stability Control (RSC(R)) System

The RSC(R) system constantly monitors vehicle roll motion, except when the vehicle is traveling in reverse, and aids in keeping the vehicle stable during sudden or abrupt maneuvers. This is accomplished by the ABS module and the HCU modulating brake fluid pressure to the affected brake caliper(s) and by the PCM modulating engine torque. The combination of reduced engine torque and modulated brake fluid pressure aids in reducing the cornering forces and, therefore, the total roll momentum acting on the vehicle.

When the RSC(R) system activates, the vehicle produces the same noticeable symptoms as it does when the traction control and ESC systems activate.

Stability/Traction Control Switch

The stability/traction control switch can be used by the driver to disable and enable the traction control, ESC and the RSC(R) systems. When the ignition key is turned to the RUN position, all 3 systems are enabled and the sliding-car icon in the Instrument Cluster (IC) illuminates momentarily as a test. Momentarily pressing the switch only disables the traction control system. Pressing and holding the switch for more than 5 seconds with a vehicle speed less than 56 km/h (35 mph) disables trailer sway control. When trailer sway control is disabled through the use of the switch, the stability/traction control OFF indicator (sliding-car OFF icon) flashes for a brief period and then illuminates continuously. When the traction control system is disabled through the use of the switch, the stability/traction control OFF indicator (sliding-car OFF icon) illuminates continuously.

The conventional ABS system cannot be disabled through the use of the stability/traction control switch.

Trailer Sway Control

Trailer sway control is a unique function in the AdvanceTrac(R) with RSC(R) system that recognizes and aids in reducing trailer sway. The trailer sway control system continually monitors the steering wheel rotation sensor and the yaw rate sensor to identify specific, increasing yaw oscillations that occur during a trailer sway event. If the trailer sway event exceeds certain, programmed parameters, the ABS module and the HCU modulates brake fluid pressure to specific brake caliper(s) and the PCM modulates engine torque until the trailer sway is reduced below the programmed parameters.

When the trailer sway control system activates, the vehicle produces the same noticeable symptoms as it does when the traction control and ESC systems activate. In addition, the message center displays TRAILER SWAY, REDUCE SPEED.

The trailer sway control system only activates when the vehicle speed exceeds 64 km/h (40 mph). The system can be disabled during any key cycle. Refer to the Owner's Literature for information on disabling the system. The system is re-enabled when the ignition key is turned to the OFF position and back to the RUN position.

Trailer sway control is not available on hybrid vehicles.