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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 (Hydraulic Control Unit) 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 (Hydraulic Control Unit), through the outlet ports of the HCU (Hydraulic Control Unit) 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 (Hydraulic Control Unit) 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 (Hydraulic Control Unit) 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 (Instrument Panel Cluster) 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 (Hydraulic Control Unit) 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 (Electronic Brake Distribution) continues to function unless the DTCs are for wheel speed sensors or the HCU (Hydraulic Control Unit). When EBD (Electronic Brake Distribution) 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 brake booster indicates severely low vacuum, a DTC is set in the ABS module. The brake vacuum pump activates at 57.56 kPa (17 in-Hg) and shuts off at 71.11 kPa (21 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 (Hydraulic Control Unit) 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 (Hydraulic Control Unit) to modulate brake pressure to the appropriate brake caliper. This is accomplished by opening and closing the appropriate solenoid valves inside of the HCU (Hydraulic Control Unit) while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN (High Speed Controller Area Network) that a traction control event is taking place. When the IPC (Instrument Panel Cluster) 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 (High Speed Controller Area Network) 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 (Hydraulic Control Unit) to their normal position, deactivates the hydraulic pump motor and sends another message over the HS-CAN (High Speed Controller Area Network) indicating that the traction event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC (Instrument Panel Cluster) turns off the 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 sliding car icon illuminate.

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

The ESC (Electronic Stability Control) and RSC(R) (Roll Stability Control) systems are controlled by the ABS module and use the same wheel speed sensors and tone rings used for anti-lock braking. The ESC (Electronic Stability Control) and RSC(R) (Roll Stability Control) 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 (High Speed Controller Area Network) 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 (Hydraulic Control Unit) while the hydraulic pump motor is activated. At the same time, the ABS module sends a message over the HS-CAN (High Speed Controller Area Network) that a stability event is taking place. When the IPC (Instrument Panel Cluster) 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 (High Speed Controller Area Network) indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC (Instrument Panel Cluster) turns off the sliding car icon.

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

Trailer Sway Control System

Trailer sway control is a unique function of the RSC(R) (Roll Stability Control) system that uses information from the steering wheel rotation sensor and the yaw information from the RCM (Restraints Control Module) 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 (Electronic Stability Control) and RSC(R) (Roll Stability Control). At the same time, the ABS module sends a message over the HS-CAN (High Speed Controller Area Network) that a trailer sway event is taking place. When the IPC (Instrument Panel Cluster) 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 (Electronic Stability Control) and RSC(R) (Roll Stability Control). Once the trailer sway has been corrected, the ABS module sends another message over the HS-CAN (High Speed Controller Area Network) indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IPC (Instrument Panel Cluster) 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. for information on the TBC (Trailer Brake Control) system.

Trailer sway control only activates with vehicle speed greater than 64 km/h (40 mph). Any malfunction that disables the RSC(R) (Roll Stability Control) 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.

Hill Start Assist

The hill start assist system engages automatically when the following conditions are met:

- The driver applies the vehicle brake and the vehicle is at a complete stop.

- Road inclination of approximately 4 degrees or more is detected (as indicated by the stability control sensor information).

- The vehicle transmission information indicates that the driver selects the correct gear, indicating the intention to drive up the hill.

- The parking brake is not engaged.

Once all the above criteria have been met and the driver releases the brake pedal, hill start assist utilizes the valves in the Hydraulic Control Unit (HCU) to hold the car stationary for up to 2 seconds giving the driver time to accelerate the vehicle. As the vehicle accelerates hill start assist gradually releases the brake pressure to make sure that the vehicle is neither rolling back nor driving off until there is sufficient driving torque to accelerate the vehicle forward. Hill start assist also functions if the vehicle is facing downhill if the reverse gear is selected.

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

Hill Descent Control(TM) uses the pump motor and HCU (Hydraulic Control Unit) to provide vehicle speed control while descending a downhill grade, information from the RCM (Restraints Control Module) 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 (High Speed Controller Area Network) requesting system activation. The ABS module uses the input from the RCM (Restraints Control Module) 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/Traction Control Switch

Unlike conventional ABS, the individual systems of AdvanceTrac(R) with RSC(R) (Roll Stability Control) can be deactivated by the driver through the stability/traction control switch. The switch is hard-wired to the IPC (Instrument Panel Cluster) and when pressed, sends a ground signal to the IPC (Instrument Panel Cluster).

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) (Roll Stability Control) and ESC (Electronic Stability Control) 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 (Electronic Stability Control) system while a MyKey(R) restricted key is in use, ADVTRAC ON MY KEY SETTING displays in the IPC (Instrument Panel Cluster) message center. For information about MyKey(R), refer to Antitheft and Alarm Systems.

Stability/Traction Control Indicators

Both the stability/traction control indicator (sliding car icon) and the stability/traction control disabled indicator (sliding car OFF icon) alert the driver of stability/traction control system status.

When certain vehicle dynamic system DTC(s) are present, the ABS module sends a message to the Instrument Panel Cluster (IPC) over the High Speed Controller Area Network (HS-CAN). Depending on the nature of the DTC(s), the IPC (Instrument Panel Cluster) responds by either solidly illuminating 1 or both icons or by flashing the icon(s) once every 2 seconds.

When a stability/traction control event is taking place, the ABS module sends a message to the IPC (Instrument Panel Cluster) over the HS-CAN (High Speed Controller Area Network) to flash the sliding car icon twice per second.

When the driver disables the stability/traction control system by using the traction control switch, the IPC (Instrument Panel Cluster) receives a ground signal from the switch and then sends a message over the HS-CAN (High Speed Controller Area Network) indicating that the driver has requested the system be deactivated and illuminates the sliding car OFF icon.

Stability Control Sensors

The stability control sensors provide information to the ABS module for the operation of the ESC (Electronic Stability Control) system, RSC(R) (Roll Stability Control) system (if equipped), hill start assist system and the Hill Descent Control(TM) system (if equipped). The stability control sensors are housed in the Restraints Control Module (RCM) and are serviced with the module. The sensors used for stability control include the yaw rate sensor, the lateral accelerometer, the longitudinal accelerometer and (if equipped with RSC(R) (Roll Stability Control) ) the roll rate sensor. Sensor information is sent to the ABS module over a private HS-CAN (High Speed Controller Area Network).

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 (High Speed Controller Area Network) 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) system and is located inside the PSCM (Power Steering Control Module). The sensor is serviced with the PSCM (Power Steering Control Module). The PSCM (Power Steering Control Module) uses the HS-CAN (High Speed Controller Area Network) 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 PSCM (Power Steering Control Module) 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 ABS module confirms this position and modifies it as necessary during every new driving cycle.

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 (Hydraulic Control Unit), the ABS module must be configured. To configure the ABS module, refer to Programmable Module Installation (PMI) in Information Bus Programming and Relearning.
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