Principles of Operation

Principles of Operation

Standard ABS
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 20 km/h (12 mph), the hydraulic pump motor is turned on for approximately 1/2 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 assist 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) return 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 HCU is used as an electronic proportioning valve for electronic brake distribution (EBD). The HCU controls the brake fluid pressure to the rear brakes to prevent them from locking up during vehicle braking. 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 wheel speed sensors or the HCU. Wheel speed sensor DTCs and HCU DTCs will disable the EBD function of the HCU. When EBD is disabled, the ABS warning indicator and (if equipped) the roll stability control (RSC) warning indicator will illuminate.

ABS With Traction Control, Interactive Vehicle Dynamics (IVD) and Roll Stability Control (RSC)
The ABS module with traction control and roll stability control (RSC) performs the same functions as the standard ABS module. It also performs additional functions specific to traction control and RSC.

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.

Interactive Vehicle Dynamics (IVD)
The interactive vehicle dynamics (IVD) 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 IVD 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 IVD 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 IVD 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 IVD indicator 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)
The RSC system constantly monitors the vehicle 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 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 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 moment acting on the vehicle.

When the RSC 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 RSC indicator 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.

Stability/Traction Control Switch
The stability/traction control switch allows the driver to control use of the RSC and the traction control systems. This is independent of the standard ABS function, which cannot be switched off by the driver. The RSC and traction control system status is indicated by a RSC warning lamp in the message center. When the RSC and traction control system is switched off, this indicator will illuminate. The standard ABS functions continue to operate as designed unless the yellow ABS warning indicator is also illuminated. Normal braking function always occurs, unless the red brake warning indicator is illuminated.

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 RSC system learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The RSC system confirms 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 pedal travel sensor (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 pedal travel sensor and the brake pedal position (BPP) 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 BPP 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 BPP 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 BPP switch failures.