Principles of Operations

Anti-Lock Control

Principles of Operations

Anti-Lock Braking

The ABS module uses a Hydraulic Control Unit (HCU) to prevent the wheels from locking up during a braking event. The HCU contains several solenoid-controlled valves, a pump motor, an accumulator and several sensors. The fluid pressure is transmitted through 4 normally open inlet valves inside the HCU, through the outlet ports of the HCU and finally to the brake calipers. One circuit of the brake master cylinder feeds the front brakes while the other circuit feeds the rear brakes. If the ABS module senses that a wheel is about to lock, based on wheel speed sensor data, the inlet valve pulses closed, preventing more fluid from entering that circuit. The ABS module then reads the sensor signal from the affected wheel again. If the wheel is still decelerating, the normally closed outlet valve for that circuit is opened. This allows a controlled amount of hydraulic pressure in the brake caliper to be relieved into the HCU accumulator. Once the speed of the affected wheel matches the speeds of the other wheels, the ABS module returns the valves to their normal position, allowing fluid flow to the affected brake caliper.

The ABS module is self-monitoring and carries out self-tests after the ignition is turned to the RUN position, after an ABS stop and after an AdvanceTrac(R) system activation. When the ignition is turned to the RUN position, the ABS module carries out a preliminary electrical check of the wheel speed sensors and their circuits by sending voltage through the sensor and checking for the voltage to return. When the vehicle is traveling at speeds above 21 km/h (13 mph) and the brake pedal is not being pressed, the HCU pump motor is commanded on for approximately 0.5 second to check pump motor operation. Also, during all phases of operation while the vehicle is in motion, the ABS module checks for correct operation of the wheel speed sensors by comparing wheel speed input to other sensor input. Depending on the nature of the malfunction detected, the ABS module deactivates the anti-lock control system and sends a message over the High Speed Controller Area Network (HS-CAN) to the Instrument Panel Cluster (IPC). When the IPC receives this message, it illuminates the yellow ABS warning indicator and the stability/traction control indicator (sliding car icon).

Electronic Brake Distribution (EBD)

On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module then uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a predetermined threshold, the ABS module closes the appropriate isolation valves in the HCU to hold the rear brake pressure constant while allowing the front brake pressure to build. This creates a balanced braking condition between the front and rear wheels and minimizes the chance of rear wheel lockup during hard braking. As the vehicle decelerates, the valves are opened to increase the rear brake pressure in proportion to the front brake pressure.

If ABS is disabled due to DTCs being present in the ABS module, EBD continues to function unless the DTCs are for wheel speed sensors or the HCU. When EBD is disabled, the red brake warning light, the ABS warning indicator and the stability/traction control indicator (sliding car icon) illuminate.

Optimized Hydraulic Braking

When the vacuum sensor in the booster indicates severely low vacuum, a DTC is set in the ABS module. The brake vacuum pump activates at 48.23 kPa (12 in-Hg) and shuts off at 68.32 kPa (17 in-Hg) of vacuum. If the brake pedal is applied with severely low vacuum in the booster, the ABS module activates the pump motor in the HCU to assist with braking at vehicle speeds above 2 km/h (1 mph).

Traction Control System

The traction control system utilizes the ABS and the PCM to minimize wheel spin during acceleration. When the ABS module detects a wheel spinning excessively, it commands the HCU to modulate brake pressure to the appropriate brake caliper. This is accomplished by opening and closing the appropriate solenoid valves inside of the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN that a traction control event is taking place. When the IPC receives this message, it flashes the stability/traction control indicator (sliding car icon). If the event is severe enough, the ABS module sends a message to the PCM over the HS-CAN to assist with traction control. When the PCM receives this message, it adjusts engine timing and decreases fuel injector pulses. Once the affected wheel returns to normal speed, the ABS module returns the solenoid valves in the HCU to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC turns off the stability/traction control indicator (sliding-car icon).

Once vehicle speed reaches or exceeds 144 km/h (89 mph), traction control is accomplished only through PCM intervention.

Certain DTCs in the ABS module disable the traction control system. Depending on the DTCs present, the red brake warning light, the yellow ABS warning indicator and the stability/traction control indicator (sliding car icon) illuminate.

Electronic Stability Control (ESC) and Roll Stability Control (RSC(R)) Systems

The ESC and RSC(R) systems are controlled by the ABS module and use the same wheel speed sensors and tone rings used for anti-lock braking. The ESC and RSC(R) systems also use input from the steering wheel rotation sensor, the stability control sensors (yaw rate, roll rate, longitudinal and lateral accelerometers) which are internal to the Restraints Control Module (RCM) and information from other modules sent over the HS-CAN to help maintain vehicle stability.

The ABS module uses the various sensors to continuously monitor vehicle roll motion and direction of travel relative to the driver's intended course. If the ABS module determines from all these inputs that the vehicle is unable to travel in the intended direction or that a condition exists for a potential rollover event, it modulates brake pressure to the appropriate brake caliper(s) by opening and closing the appropriate solenoid valves inside the HCU while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN that a stability event is taking place. When the IPC receives this message, it flashes the stability traction control indicator (sliding car icon). During a stability event, conditions may require the involvement of the PCM to limit engine torque like it does for traction control. Once the stability condition has been corrected, the ABS module sends another message over the HS-CAN indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC turns off the stability traction control indicator (sliding car icon).

Certain DTCs being present in the ABS module may disable the ESC and RSC(R) systems. Depending on the DTCs present, the yellow ABS warning indicator and/or the stability traction control indicator (sliding car icon) are illuminated.

Trailer Sway Control System

Trailer sway control is a unique function of the RSC(R) system that uses information from the steering wheel rotation sensor and the yaw information from the RCM to determine if a trailer sway event is taking place. Trailer sway is the undesirable yaw force a trailer can apply to the towing vehicle. If the ABS module determines that a trailer sway event is taking place, it takes action similar to the action taken for ESC and RSC(R). At the same time, the ABS module sends a message over the HS-CAN that a trailer sway event is taking place. When the IPC receives this message, it flashes the stability traction control indicator (sliding car icon) and displays TRAILER SWAY REDUCE SPEED in the message center. If necessary, the PCM reduces engine torque as it does for ESC and RSC(R). Once the trailer sway has been corrected, the ABS module sends another message over the HS-CAN indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC turns off the stability traction control indicator (sliding car icon).

If the vehicle is also equipped with a Trailer Brake Control (TBC) system, it receives the trailer sway message from the ABS module and takes appropriate action to assist in reducing the trailer sway. Refer to Auxiliary Brake System Testing and Inspection for information on the TBC system.

Trailer sway control only activates with vehicle speed greater than 64 km/h (40 mph). Any malfunction that disables the RSC(R) system also disables trailer sway control. Trailer sway control can also be disabled through the message center. Refer to the Owner's Literature for the disabling/enabling procedure. Regardless of the chosen state (enabled or disabled), trailer sway control is re-enabled at each ignition key cycle.

Stability/Traction Control Switch

Unlike conventional ABS, the individual systems of AdvanceTrac(R) with RSC(R) can be deactivated by the driver through the stability/traction control switch. The switch is hard-wired to the IPC and when pressed, sends a ground signal to the IPC. The IPC then sends a message over the HS-CAN indicating that the driver has requested that the system be deactivated and illuminates the stability traction control indicator (sliding car icon) Off. The circumstances present when the switch is pressed and the duration of the button press determines what systems are disabled. Refer to Description and Operation for information.

If the stability/traction control switch is pressed and held for more than 30 seconds, a DTC sets in the ABS module.

Once disabled, the system remains in that state until the driver presses the switch again, the ignition is cycled or the vehicle speed reaches or exceeds 56 km/h (35 mph). Once the vehicle speed reaches or exceeds 56 km/h (35 mph), RSC(R) and ESC are reactivated. Once the vehicle speed drops below 48 km/h (30 mph), traction control returns to the state the driver had originally selected, either on or off.

The systems cannot be disabled when a MyKey(R) restricted key is in use with the AdvanceTrac(R) always-on feature configured to on. If an attempt is made to disable the ESC system while a MyKey(R) restricted key is in use, ADVTRAC ON MY KEY SETTING displays in the IPC message center. For information about MyKey(R), refer to Anti-Theft - Passive Anti-Theft System (PATS) Testing and Inspection.

Stability/Traction Control Indicator (Sliding Car Icon)

Status of the traction control, ESC and RSC(R) systems, is indicated by a stability/traction control indicator (sliding-car icon) and a stability/traction control indicator (sliding car icon) Off located in the IPC. The stability/traction control indicator (sliding car icon) illuminates continuously when there are DTCs present in the ABS module directly related to the specific system. The stability traction control indicator (sliding car icon) flashes when one or more of the systems is currently active. When the driver disables the traction control system through the use of the traction control switch, the stability/traction control indicator (sliding car icon) Off illuminates continuously.

The IPC message center is a reconfigurable telltale which is capable of displaying a variety of different messages and indicators such as the stability traction control indicator (sliding car icon), stability/traction control indicator (sliding car icon) Off and the Four-Wheel Drive (4WD) mode/range or door open messages. For information about the IPC, refer to Instrumentation, Message Center, and Warning Chimes Testing and Inspection.

Hill Descent Control(TM) and Hill Descent Control(TM) Switch (SVT Raptor only)

Hill Descent Control(TM) uses the pump motor and HCU to provide vehicle speed control while descending a downhill grade, information from the RCM to determine whether or not the vehicle is on a downhill grade and wheel speed sensor information to determine vehicle speed. When the Hill Descent Control(TM) switch is pressed, a ground signal is sent to the PCM indicating that the driver has activated the system. The PCM then sends a message to the ABS module along the HS-CAN requesting system activation. The ABS module uses the input from the RCM and wheel speed sensors to determine the mode of operation (active, stand-by or deactivated). During a hill descent while the Hill Descent Control(TM) system is activated, the driver sets vehicle speed through the use of the accelerator and brake pedals. Once the desired speed has been set, the system maintains that speed until one of the following occur:

- The vehicle is no longer on a downhill grade.
- The driver presses the switch again.
- Vehicle speed exceeds 64 km/h (40 mph).
- The system is disabled for cooling.

During periods of sustained use, the Hill Descent Control(TM) system may automatically deactivate in order to cool the brake system. When this occurs, the system notifies the driver with a message in the message center and by sounding a warning chime. At this time the driver needs to provide brake and/or throttle control to maintain the desired descent speed. The system remains disabled until it has sufficiently cooled. The length of time needed for cooling varies depending on operating conditions.

Stability Control Sensors

The stability control sensors for the ESC system include the yaw rate sensor and the lateral and longitudinal accelerometers. The yaw rate sensor and the lateral and longitudinal accelerometers are able to detect and measure changes in vehicle direction that indicate the need for the ABS to make corrections that help prevent vehicle rollover or spin outs. The sensors are housed in the RCM which sends sensor information to the ABS module over the Controller Area Network (CAN). Install a new RCM if any of the sensors are damaged.

Steering Wheel Sensor Information

For the 6.2L engine, the steering wheel rotation sensor uses an optical sensor and a slotted wheel to measure the rate of rotation (angle) of the steering wheel. The steering wheel rotation sensor uses the HS-CAN to transmit information to the ABS module about whether the steering wheel is being turned left or right and how far it is being turned. The steering wheel rotation sensor does not indicate the absolute position of the steering wheel relative to straight-ahead. The ABS module learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The module confirms this position and modifies it as necessary during every new driving cycle. For all other applications, the steering wheel rotation sensor is part of the Electronic Power Assist Steering (EPAS) and is not serviced separately.

ABS Module Configuration

Module configuration is a scan tool process which configures specific vehicle settings in a new module. If installing a new ABS module and/or a new HCU, the ABS module must be configured. To configure the ABS module, refer to Programmable Module Installation (PMI) in Module Configuration Programmable Module Installation (PMI).