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 in each wheel bearing and wheel hub assembly and is serviced with the wheel bearing and wheel hub
- HCU - contains several valves (inlet, outlet, isolation and dump) and a hydraulic pump motor. It is mounted behind the battery and battery tray. The HCU is serviced as a unit, but does not include the ABS module
- Rear wheel speed sensors - one sensor is located in each end of the rear axle housing. The sensors are serviced separately from the knuckle
- Rear wheel speed sensor tone rings - one tone ring is located on each axle shaft
- 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 on the end of the steering column near the intermediate shaft. The sensor is serviced separately from the steering column but the sensor ring is serviced with the column
- Stability/traction control switch - located in the instrument panel 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.
A slight bump sensation may be felt in the brake pedal when EBD is active.
Anti-Lock Control
The anti-lock control system 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).
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 will allow 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.
Electronic Stability Control (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 AdvanceTrac(R) 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 Roll Stability Control (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 will produce the same noticeable symptoms as it does when the AdvanceTrac(R) system activates.
AdvanceTrac(R) With Roll Stability Control (RSC(R))
Stability/Traction Control Switch and Stability/Traction Control Message Center Indicator ("Sliding-Car Icon")
The stability/traction control switch can be used by the driver to disable and enable the traction control, ESC and RSC(R) systems as shown in the table below.
AdvanceTrac(R) with RSC(R) Features
a Traction control is enabled, but with a higher wheel spin tolerance before system intervention occurs
b Lamp lights start blinking 3 seconds after system is disabled
c Traction control switch is not pressed. Sliding car icon turns on when 4WD Low Locked transfer case mode is selected
d If the traction control switch is pressed in 2WD or 4WD High, any change in the transfer case mode will return the AdvanceTrac(R) to fully enabled status
Status of the traction control, ESC and RSC(R) is indicated by a sliding-car icon located in the Instrument Cluster (IC) message center. The IC message center is a reconfigurable telltale screen which displays messages and indicators including door open/closed messages, Four-Wheel Drive (4WD) system status and AdvanceTrac(R) with RSC(R) system status. When multiple messages need to be displayed, the re-configurable telltale will alternate between all messages.
The conventional ABS cannot be disabled through the use of the stability/traction control switch.
The systems cannot be disabled when a MyKey(TM)-restricted key is in use with the Advance Trac(R) always-on feature configured to "on".
Trailer Sway Control
Trailer sway control is a unique function in the AdvanceTrac(R) with RSC(R) system that will recognize and aid 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 will modulate brake fluid pressure to specific brake caliper(s) and the PCM will modulate engine torque until the trailer sway is reduced below the programmed parameters.
For vehicles equipped with an optional Trailer Brake Control (TBC) module, the trailer sway control system will utilize the module to apply the trailer brakes to more quickly and better mitigate the trailer sway.
When the trailer sway control system activates, the vehicle will produce the same noticeable symptoms as it does when the AdvanceTrac(R) system activates. In addition, the message center will display TRAILER SWAY, REDUCE SPEED and if the vehicle is equipped with a TBC module, the module will show output to the trailer brakes.
The trailer sway control system will only activate 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 will be 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 low vacuum is detected during a braking event, the ABS module will activate the pump motor in the HCU to assist with vehicle braking.
Hill Descent Control(TM) (SVT Raptor Only)
Hill Descent Control(TM) can be used to maintain vehicle speed on a downhill grade, operating at speeds between 3-32 km/h (2-20 mph). If the vehicle is on flat or uphill terrain, the system will go into standby mode, remaining activated up to 64 km/h (40 mph). At speeds greater than 64 km/h (40 mph), the Hill Descent Control(TM) system is deactivated.
When Hill Descent Control(TM) is active, a rumble or grinding sound may be heard and/or felt.