Antilock Brakes / Traction Control Systems: Description and Operation
Anti-Lock Control
Anti-Lock Control with AdvanceTrac(R) and Roll Stability Control (RSC(R))
The ABS with AdvanceTrac(R) and Roll Stability Control (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 - the tone ring is part of the front wheel bearing and is serviced with the bearing on Four-Wheel Drive (4WD) vehicles. On Rear Wheel Drive (RWD) vehicles, the tone ring is part of the front brake rotor and is serviced with the brake rotor.
- HCU - which contains several valves (inlet, outlet, isolation and dump) and the hydraulic pump motor. The HCU is mounted on the LH side of the engine compartment.
- Rear wheel speed sensors - one sensor is located at each end of the rear axle housing. It is mounted to the brake backing plate and can be serviced separately.
- Rear wheel speed sensor tone rings - one tone ring is located on each rear axle shaft and is available for service separate from the axle.
- Stability control sensors - located inside of 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 at the end of the steering column on the LH side of the column, the slotted wheel is serviced with the steering column but the sensor can be serviced separately.
- Stability/traction control switch - located in the Instrument Cluster (IC) center finish panel and is serviced separately from the finish panel.
ABS aids in the prevention of wheel lock-up during braking events which will allow 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) and wheel cylinder(s).
When the ABS activates, the driver may notice the following operational symptoms:
- A rumble or grinding sound may be heard (HCU pump motor)
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal (HCU pump motor and valves)
Stability/Traction Control Indicator and Stability/Traction Control OFF Indicator
Status of the traction control and AdvanceTrac(R) systems is indicated by a stability/traction control indicator (also known as the sliding-car icon) and a stability/traction control OFF indicator (sliding-car OFF icon) located in the Instrument Cluster (IC). When the driver disables the systems through the use of the stability/traction control switch, the sliding-car OFF icon will illuminate solidly. When one or more of the systems is currently active, the sliding-car icon will flash, and when the system is disabled due to DTCs being present in the ABS module, the sliding-car icon will illuminate in conjunction with the yellow ABS warning indicator.
Traction Control System
The traction control system aids in the prevention of excessive drive-wheel spin which will allow the vehicle to maintain traction during acceleration. This is accomplished in the same manner as anti-lock control but also includes the use of the PCM for modulating engine torque.
ESC and RSC(R) Systems
Both systems constantly monitor vehicle direction of travel relative to the driver's intended course and aid in keeping the vehicle traveling along that course. The RSC(R) system also monitors the vehicle roll motion and aids in keeping the vehicle stable during sudden or abrupt maneuvers. This is accomplished in the same manner as anti-lock braking and traction control.
When any of the above 3 systems activate, the driver may notice the same operational symptoms as ABS along with the following additional symptoms:
- A small deceleration or a reduction in the acceleration of the vehicle due to the PCM modulating engine torque and the HCU modulating brake pressure
- The sliding-car icon flashes (indicating a traction or stability event is taking place)
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 sensation may be felt in the brake pedal when EBD is active.
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 IC will illuminate momentarily as a test. Momentarily pressing the switch will only disable 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]) will disable all 3 systems. When all 3 systems are disabled, the sliding car OFF icon in the IC will extinguish for 3 seconds before illuminating solidly.
There are 2 separate sliding car icons; the sliding car OFF icon and the sliding car icon. The sliding car OFF icon is used to alert the driver that the traction control portion of the AdvanceTrac(R) system has been disabled by the driver. This icon will only illuminate solidly and does not flash. The sliding car icon is used to alert the driver that a stability/traction event is taking place (flashes twice per second) and to alert the driver of potential concerns in the AdvanceTrac(R) system (illuminates solidly).
On vehicles equipped with Four-Wheel Drive (4WD), the sliding car OFF icon will turn on and all 3 systems will be disabled when the 4WD Low Locked transfer case mode is selected. Additionally, if one or more of the vehicle dynamic systems has been disabled through the use of the switch and the driver shifts the transfer case into 4WD HI or 2-Wheel Drive (2WD), all disabled systems will be enabled.
If all 3 systems have been disabled by the driver through the use of the switch as described above and the vehicle exceeds 56 km/h (35 mph), the AdvanceTrac(R) system will enable the ESC and RSC(R) systems, but the traction control system will remain disabled and the sliding car icon will remain illuminated until the switch is pressed again, or the ignition key is cycled to the OFF position and back to the RUN position.
The conventional ABS system cannot be disabled through the use of the stability/traction control switch.