Principles Of Operation
Anti-Lock Control
Principles of Operation
Anti-Lock Control
The standard ABS module manages anti-lock braking to maintain vehicle control during deceleration.
When the ignition switch is in the RUN position, the ABS module does a preliminary electrical check and, at approximately 40 km/h (25 mph), the hydraulic pump motor is turned on for approximately one half second. During this time, a buzzing or humming noise may be heard and a vibration may be felt in the brake pedal. This is a normal condition. Any malfunction of the ABS disables the stability control and the ABS warning indicator illuminates. However, the power-assist braking system functions normally.
The ABS module monitors and compares the rotational speed of each wheel. Wheel speeds are measured by the wheel speed sensor, which electrically senses each tooth of the sensor ring as it passes through the magnetic field of the sensor. When the ABS module detects an impending wheel lock, the ABS module commands the hydraulic pump motor on and commands the Hydraulic Control Unit (HCU) to open and close the appropriate solenoid valves to modulate the brake pressure to the individual brake caliper(s). Once the affected wheel(s) returns to the desired speed, the ABS module commands the HCU to return the solenoid valves to their normal position and normal base brake operation is restored.
The ABS module must be configured when a new ABS module is installed.
Electronic Brake Distribution (EBD)
The ABS incorporates a strategy called Electronic Brake Distribution (EBD). EBD uses the HCU as an electronic proportioning valve. On initial application of the brake pedal, full pressure is applied to the rear brakes. The ABS module uses wheel speed input to calculate an estimated rate of deceleration. Once vehicle deceleration exceeds a certain 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.
Some drivers may feel a slight bump sensation in the brake pedal when EBD is active.
If anti-lock braking is disabled due to DTCs being present in the ABS module, EBD will continue to function unless the DTCs are for 2 wheel speed sensors on the same axle or HCU faults.
ABS With Traction Control, Electronic Stability Control (ESC) and Roll Stability Control (RSC(R))
The ABS module with traction control, Electronic Stability Control (ESC) and Roll Stability Control (RSC(R)) performs the same functions as the standard ABS module. It also performs additional functions specific to traction control, ESC and RSC(R).
Traction Control
When the drive wheels lose traction and begin to spin, with vehicle speed under 100 km/h (62 mph), the ABS module commands the hydraulic pump motor on and commands the HCU to open and close the appropriate solenoid valves to modulate the brake pressure to the brake caliper(s) while simultaneously sending a request to the PCM over the High Speed Controller Area Network (HS-CAN) bus to reduce engine torque to maintain vehicle traction. The PCM accomplishes this by minor incremental timing changes and fewer fuel injector pulses until the ABS module ends the request. The request ends when the driven wheel speed returns to the desired speed. After the vehicle speed exceeds 100 km/h (62 mph), the traction control is accomplished only through the PCM torque control.
Electronic Stability Control (ESC)
The ESC system constantly monitors the vehicle motion relative to the intended course. This is done by using sensors to compare the steering input and the yaw rate sensor with that of the actual vehicle motion. The ESC system monitors information from the throttle position sensor, the steering wheel rotation sensor (steering wheel angle and rate of change) and a yaw rate sensor (integral to the stability control sensor cluster) that measures changes in vehicle direction. If the ESC system determines from all these inputs that the vehicle is unable to travel in the intended direction, the system adjusts the brake torque at specific wheels in response to direct measurement of the vehicle motion and reduces engine torque to allow the vehicle to follow the intended course.
When the ESC system activates, any of the following can occur:
- A rumble or grinding sound much like ABS or traction control
- 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 will be felt in the pedal much like ABS
Roll Stability Control (RSC(R))
The RSC(R) system constantly monitors the vehicle roll motion relative to the intended course. This is done by using sensors to compare the steering input and brake application with that of the actual vehicle motion. The system does not activate when the vehicle is traveling in reverse; however, the standard ABS and traction control continue to function as usual. The RSC(R) system monitors information from the wheel speed sensors, the Throttle Position (TP) sensor, the steering wheel rotation sensor (steering wheel angle and rate of change) and a roll-rate sensor (integral to the stability control sensor cluster) that measures changes in vehicle motion. If the RSC(R) system determines from all these inputs that conditions exist for a potential roll-over event, the system adjusts the brake torque at specific wheels in response to direct measurement of the vehicle roll motion and reduces engine torque to make the vehicle more stable. By adjusting brake torque, the system can reduce the cornering forces and, therefore, the total roll motion of the vehicle.
When the RSC(R) system activates, any of the following can occur:
- A rumble or grinding sound much like ABS or traction control
- 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 will be felt in the pedal much like ABS
- If the event is severe enough and the driver's foot is not on the brake, the brake pedal will move to apply higher brake forces. A whooshing sound can also be heard from under the instrument panel in an event this severe
Some drivers may notice a slight movement of the brake pedal when the system checks itself. The brake pedal moves when an active test of the brake booster is run. During this test a small amount of pressure is generated at the master cylinder, but no pressure is generated in the brake calipers.
Trailer Sway Control
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 stability control sensor cluster 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 it is determined that an event is taking place, the ABS module will modulate 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 bus that a trailer sway event is taking place. When the PCM receives this message, it will assist with trailer sway by adjusting engine timing and decreasing fuel injector pulses. When the Instrument Cluster (IC) receives this message, it will flash the stability/traction control message center indicator "sliding-car icon" and display TRAILER SWAY, REDUCE SPEED in the message center. Once the trailer sway has been corrected, 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 bus indicating that the event has ended. The PCM returns engine timing and fuel injectors to normal operation and the IC extinguishes the "sliding-car icon".
Trailer sway control will only activate with vehicle speed greater than 64 km/h (40 mph). Any malfunction that disables the RSC(R) system will also disable trailer sway control. Trailer sway control can also be disabled by entering the setup menu in 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 will be re-enabled at each ignition key cycle.
Stability/Traction Control Switch
Unlike the standard anti-lock control system, the individual systems of ESC with RSC(R) can be deactivated by the driver through the stability/traction control switch. The switch is hard-wired to the ABS module and when pressed sends a voltage signal to the module. The ABS module then sends a message over the HS-CAN bus indicating that the driver has requested that the system be deactivated and illuminates the stability/traction indicator "sliding-car icon". The circumstances present when the switch is pressed and the duration of the button press will determine what systems are disabled.
- Button pressed momentarily - engine traction control, ESC and RSC(R) systems are disabled and the "sliding-car icon" will illuminate solidly. Brake traction control will continue to function.
- Button pressed and held for more than 5 seconds with vehicle speed under 40 km/h (25 mph) - ESC, RSC(R), engine traction control and brake traction control systems are all disabled and the "sliding-car icon" will flash momentarily and then illuminate solidly.
- Vehicle speed exceeds 40 km/h (25 mph) after the button is pressed and held for more than 5 seconds - engine traction control, brake traction control, RSC(R) and ESC will be enabled, but the "sliding-car icon" will be illuminated solidly.
- Button pressed after deactivation - all systems will be enabled and the "sliding-car icon" will not be illuminated.
Once disabled, the systems will remain in that state until the driver presses the switch again, the ignition key is cycled or the vehicle speed reaches or exceeds 40 km/h (25 mph). Once the vehicle speed reaches or exceeds 40 km/h (25 mph), RSC(R) and ESC will re-activate but traction control will remain deactivated.
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" located in the Instrument Cluster (IC). The indicator will illuminate solidly when the driver disables the one or more of the systems through the use of the stability/traction control switch or if there are DTCs present in the ABS module directly related to the specific system. The indicator will flash when one or more of the systems is currently active.
Stability Control Sensor Cluster
The sensor cluster (also called the accelerometer) consists of the yaw rate sensor, roll rate sensor, lateral accelerometer and longitudinal accelerometer. The lateral accelerometer measures the acceleration that corresponds to the force involved when the vehicle slides sideways. This acceleration has 2 forms. The first is the centrifugal acceleration that is generated when the vehicle travels around in a circle. The second is the acceleration due to gravity. On level ground there is no contribution from this acceleration. However, if the vehicle is parked sideways on a bank or incline, the sensor measures some lateral acceleration due to gravity, even though the vehicle is not moving.
The longitudinal accelerometer measures the acceleration corresponding to the force involved when the vehicle moves forward and rearward in the horizontal plane, along the centerline of the vehicle's front and rear wheels.
The yaw rate sensor measures the relative vehicle motion about the vertical axis through its center of gravity.
The roll rate sensor measures the rate of rotation along the front to rear horizontal axis.
Steering Wheel Rotation Sensor
The steering wheel rotation sensor measures the rate of rotation of the steering wheel by monitoring the steering wheel rotation ring as it passes through the sensor gap. The rotation sensor uses 2 signal lines 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 rotation sensor does not indicate the position of the steering wheel relative to straight-ahead. The ESC/RSC(R) systems learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The ESC/RSC(R) systems confirm this position and modifies it as necessary during every new driving cycle.
Active Brake Booster
The active brake booster functions like a conventional brake booster with the added feature that it can be actuated electrically by the ABS module. The actuation of the brake booster by the ABS module is necessary in severe stability assist events as it makes sure that the HCU can generate enough brake pressure to maintain vehicle stability. The brake assist from the brake booster is especially useful in cold weather because the HCU cannot draw the brake fluid from the reservoir quickly at cold temperatures.
The ABS module controls a solenoid/air valve attached to the active brake booster to actuate the booster. The ABS module uses the brake booster pedal force switch (integral to the booster) and the brake pressure transducer (integral to the HCU) to determine if booster actuation is necessary. When actuation is necessary, the ABS module applies increasing current to the solenoid which opens the air valve which allows more air in and increases the boost provided by the brake booster. When actuation is no longer necessary, the ABS module decreases the current to the solenoid which closes the air valve and decreases the boost provided by the brake booster. When the ABS module is actuating the brake booster, the brake pedal will be pulled toward the front of the vehicle as the brake booster input rod moves. The ABS module will continue to actuate the brake booster until it receives brake pedal input from the driver.
Brake pedal input from the driver is determined using the brake booster pedal force switch and the stoplamp switch when all of the following conditions exist:
- The ignition key is in the RUN position
- The ABS module is not actuating the active brake booster
- There are no DTCs present in the ABS module
Brake pedal input from the driver is determined using only the stoplamp switch when one or more of the following conditions exist:
- The ignition key is in the OFF position
- The ABS module is actuating the active brake booster
- There are DTCs present in the ABS module
When the ignition switch is in the OFF position, failures cannot be detected. If there are failures, several layers of redundancy attempt to keep the driver brake pedal input signal accurate. Control of the stoplamp operation is also required because the stoplamp switch can change states during a stability assist event when the driver is not applying the brake pedal. The switch can also change states during the system check of the brake booster. Only the stability assist system can interpret these changed states and determine which are actually due to driver brake pedal input. Similarly, the stability assist system can take advantage of other sensors to detect stoplamp switch failures.
Stability Control Sensor Calibration
When an ABS system fault has been corrected or an ABS component has been replaced, it will be necessary to calibrate the ABS module. The calibration procedure is required for the stability control sensors to learn the "zero-position" of the vehicle which means the vehicle must be on a level surface and must not be moving.
If a DTC is logged for any component of the stability/traction control system, the DTC must be cleared or repaired before carrying out the calibration procedure except those DTCs indicating a need for calibration. The need to calibrate is also indicated by the stability/traction control indicator "sliding-car icon" flashing once every 2 seconds.
To carry out the calibration procedure, connect the scan tool, proceed to the IVD Initialization sequence and follow the scan tool instructions.
ABS Module Configuration
When a new ABS module is installed it must be configured. To configure the ABS module, refer to Programmable Module Installation (PMI) in Information Bus. Programming and Relearning