The anti-lock brake system (ABS) consists of the following components:
Anti-Lock Brake System (ABS) With Stability Assist
The anti-lock brake system with traction control and stability assist consists of these additional components:
Principles Of Operation
Active Brake Booster (ABS With Stability Assist)
The active brake booster functions like a conventional brake booster with the added feature that it can be actuated electrically by the stability assist module. The electrical brake booster actuation is necessary in severe stability assist events and ensures that the hydraulic control unit can generate enough brake pressure to maintain vehicle stability. The brake assist from the brake booster is especially useful in cold weather when the hydraulic control unit cannot draw the brake fluid from the reservoir quickly at cold temperatures. A solenoid within the active brake booster has a release switch to indicate when the driver is stepping on the brake pedal. The solenoid provides electrical actuation of the brake booster. Without the force of the input rod, the air valve is directly opened due to the movement of the energized solenoid. With increasing current applied to the solenoid, the air valve opens and output force is created. With decreasing current applied to the solenoid, the air valve is closed and the vacuum valve opens, reducing output force. The release switch indicates when the brake pedal has been applied. It is integrated into the booster key that normally sits against the rear shell of the boot in its rest position and it adjoins the valve body when in its balance position. The switch itself provides indication from two positions. In its rest position, it supplies two signals, normally open and normally closed. Upon application of sufficient force to initiate movement of the key away from the valve body, the brake booster switch changes state, signifying a driver application.
When the stability assist module is activating the brake booster, the brake pedal is pulled forward as the valve body moves. The release switch is held in position by the input rod spring and remains against the valve body. Although the stop lamp switch changes state when the brake pedal drops, the release switch remains inactive until it encounters an external force from the driver or a stability assist event controlled by the stability assist module.
The brake pedal force switch sends a brake application input to the stability assist module. This signal is called driver brake application. This signal only changes state when the driver has applied the brake. If the ignition switch is in the RUN position, the brake booster is not electrically actuated, and no failure has been detected, the driver brake application signal will indicate a brake application when either the BPP switch or the release switch (built into the brake booster) indicates that the driver has applied the brake pedal. If the ignition switch is in the OFF position, the driver brake application signal will indicate a brake apply only when the BPP switch indicates that the driver is applying the brake pedal. When the ignition switch is in the OFF position, failures cannot be detected. If there are failures, several layers of redundancy will attempt to keep the driver brake application signal accurate. Control of the stop lamp 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 application. Similarly, the stability assist system can take advantage of its other sensors to detect BPP switch failures.Anti-Lock Brake System
The anti-lock brake control module manages anti-lock braking, while the anti-lock brake control module with stability assist manages anti-lock braking, traction control, and engine control systems to maintain vehicle control during deceleration and acceleration.
When the ignition switch is put in the RUN position, the module will do a preliminary electrical check, and at approximately 12 mph (20 km/h), the hydraulic pump motor is turned on for approximately one half-second. Any malfunction of the anti-lock brake control system will disable the stability assist and the anti-lock brake warning indicator will illuminate. However, the power-assist braking system will function normally.Electronic Brake Distribution (Ebd)
The EBD controls rear brake pressure and acts as an electronic proportioning valve. It is controlled by the ABS module. When EBD is disabled, the amber ABS warning indicator will illuminate.Panic Assist Braking (ABS With Stability Assist)
Panic assist braking functions by fully applying the brakes during a panic stop, using a specially developed pressure/pedal movement mechanism. Depending on vehicle speed, if pedal stroke is more rapid than normal, the brake booster automatically applies full brake boost. Panic assist will be disabled when the driver releases the brake pedal.Sensor Cluster (ABS With Stability Assist)
The sensor cluster (also called the accelerometer) consists of the yaw rate sensor, lateral accelerometer and longitudinal accelerometer (4WD vehicles only). The lateral accelerometer measures the acceleration which corresponds to the force involved when the vehicle slides sideways. This acceleration has two forms. The first is the centrifugal acceleration which 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 will measure some lateral acceleration due to gravity even though the vehicle is not moving.
The longitudinal accelerometer (4x4 vehicles only) 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.Stability Assist
With the ignition in the START or RUN positions, the stability assist module functions similarly to a conventional anti-lock brake module by monitoring and comparing the rotational speed of each wheel. Wheel speeds are measured by the wheel speed sensor which electrically senses each tooth of the anti-lock sensor ring as they pass through the sensor's magnetic field. When the stability assist module detects an impending wheel lock, wheel spin or vehicle motion that is inconsistent with the driver commands, brake pressure is modulated to the appropriate brake caliper(s). The stability assist module triggers the HCU to open and close the appropriate solenoid valves. Once the affected wheel(s) return to the desired speed, the stability assist module returns the solenoid valves to their normal position, and normal base brake operation is restored.
The ABS module communicates with the powertrain control module (PCM) to assist with traction control. At speeds under 62 mph (100 km/h), when the drive wheels lose traction and begin to spin, the stability assist module requests the PCM to reduce engine torque while simultaneously applying and releasing the appropriate brake caliper(s) to maintain traction. The PCM accomplishes this by minor incremental timing changes and fewer fuel injector pulses until the stability assist module ends the request. The request ends when the driven wheel speed returns to the desired speed. After the vehicle speed exceeds 62 mph (100 km/h), the traction control is accomplished only through the PCM torque control. The stability assist system constantly monitors vehicle motion relative to the driver's intended course. This is done by using sensors to compare the driver's 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, ABS and traction control will continue to function as usual.
The stability assist control switch allows the driver to control use of the stability assist system. This is independent of the ABS function, which cannot be switched off by the driver. The stability assist system status is indicated by a stability assist warning lamp in the stability assist switch. An illuminated stability assist switch indicates that the stability assist system is switched off. In vehicles with a message center, the message AdvTrac Off, T/C ON is displayed. The anti-lock brake function continues to work as designed unless the yellow anti-lock brake warning indicator is also illuminated. Normal braking function always occurs, unless the red brake warning indicator is illuminated.
When stability assist activates, any of the following can occur:
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 will be generated at the master cylinder, but no pressure will be generated in the brake calipers.Steering Wheel Rotation Sensor (ABS With Stability Assist)
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 steering wheel rotation sensor uses two signal lines to transmit information to the stability assist module, 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 position of the steering wheel relative to straight-ahead. The stability assist system learns this position by comparing the steering wheel position with other signals and storing the position it has learned. The stability assist system confirms this position and modifies it as necessary during every new driving cycle.