Antilock Brakes / Traction Control Systems: Description and Operation
Anti-Lock Control
ABS With AdvanceTrac(R)
The ABS with AdvanceTrac(R) consists of the following components:
- ABS module - attached to the Hydraulic Control Unit (HCU), but can be serviced separately.
- Brake booster vacuum sensor - located in the brake booster and can be serviced separately from the booster. Refer to Power Brake Actuation Service and Repair.
- 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 in each wheel bearing and wheel hub assembly and is serviced with the wheel bearing and wheel hub. Refer to Front Suspension - Rear Wheel Drive (RWD) Front Suspension or Front Suspension - Four Wheel Drive (4WD) Front Suspension.
- HCU - contains several valves (inlet, outlet, isolation and dump) and the hydraulic pump motor. It is mounted behind the battery and battery tray.
- 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 in each wheel bearing and wheel hub assembly and is serviced with the wheel bearing and wheel hub. Refer to Rear Suspension Rear Suspension.
- Restraints Control Module (RCM) - located under the front floor console and contains several sensors (yaw rate, roll rate, lateral and longitudinal accelerometers).
- Steering wheel rotation sensor - located at the end of the steering column on the LH side of the column, the slotted wheel is serviced with the steering column.
- Stability/traction control switch - part of the power liftgate/traction control/hazard center stack switch assembly located in the instrument panel finish panel. The switch assembly 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. A slight bump may be felt in the brake pedal when EBD is active.
Standard ABS
The ABS 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 anti-lock control system activates, any 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.
When the traction control system activates, any of the following can occur:
- A rumble or grinding sound may be heard much like ABS
- A small deceleration or a reduction in the acceleration of the vehicle
- The stability/traction control indicator (sliding car icon) flashes
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal much like ABS
Electronic Stability Control (ESC) System
The Electronic Stability Control (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 ESC system activates, any of the following can occur:
- A rumble or grinding sound may be heard much like ABS
- A small deceleration or a reduction in the acceleration of the vehicle
- The stability/traction control indicator (sliding car icon) flashes
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal much like ABS
Roll Stability Control (RSC(R)) System
The Roll Stability Control (RSC(R)) system constantly monitors vehicle motion relative to the driver's intended course, 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, any of the following can occur:
- A rumble or grinding sound may be heard much like ABS
- A small deceleration or a reduction in the acceleration of the vehicle
- The stability/traction control indicator (sliding car icon) flashes
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal much like ABS
Stability/Traction Control Switch
The stability/traction control switch can be used by the driver to disable and enable the traction control, ESC and 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. When the switch is pressed, the systems are disabled. When the systems are disabled, the sliding-car icon in the IC (or message center if equipped) illuminates. The standard ABS cannot be disabled through the use of the stability/traction control switch.
The AdvanceTrac(R) system cannot be disabled when a MyKey(TM)-restricted key is in use that has the AdvanceTrac(R) always-on feature configured "on". For information about MyKey(TM), refer to Anti-Theft - Passive Anti-Theft System (PATS) MyKey(R) Restrictions On Vehicle Operation.
Trailer Sway Control
Trailer sway control is a unique function in the AdvanceTrac(R) with a 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 modulate brake fluid pressure to specific 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 AdvanceTrac(R) system activates. 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.
Optimized Hydraulic Braking
Optimized hydraulic braking is a system built into the ABS module to aid in bringing the vehicle to a safe, controlled stop in the event of severe vacuum loss at the brake booster. The ABS module continually monitors the vacuum in the brake booster through the use of a sensor. If a loss of vacuum is detected, the ABS module activates the pump motor in the HCU to assist with vehicle braking.