ABS General System Description
ABS System Location View:
THE ABS STOPPING ADVANTAGE
A brake system needs tire traction to function. When wheel lock-up (tire skids, rather than rolls) occurs during braking, the amount of traction force between the tire and road surface decreases dramatically. Since traction force is necessary to slow or steer a vehicle, wheel lock-up during braking is extremely undesirable. Antilock brakes prevent wheel lock-up from occurring, providing shorter stopping distances on some surfaces, with greater steering control during braking than a non-antilock brake equipped vehicle. Stopping distance is dependant on many variables including but not limited to brake conditions, tire conditions, road surface conditions, and steering input.
ABS COMPONENTS
The Saturn antilock brake system consists of two subsystems: a conventional hydraulic brake system and the antilock brake control system. The antilock brake control system consists of the following components: four wheel speed sensors, a motor pack assembly, a modulator assembly, two solenoid valves, an antilock brake control module, and interconnecting wiring.
^ Wheel Speed Sensors: These sensors provide a frequency and voltage signal that is proportional to the wheel speed. The sensor consists of wire windings wrapped around a magnet. A toothed (47 teeth) speed sensor ring (spinning at road wheel speed) interrupts the magnetic field created by the wheel speed sensor, alternately increasing and decreasing the strength of the field. This increasing and decreasing magnetic field creates a voltage and frequency signal in the sensor wire windings that varies proportionally to road wheel speed. This signal is used by the antilock brake control module to determine the road wheel speed. Typical sensor output signals vary from 100 millivolts at low road wheel speeds to 90 volts at high speeds.
On the front wheels, the speed sensor is fastened to the steering knuckle. The front wheel speed sensor toothed ring is pressed onto the end of the axle shaft assembly. On the rear wheels, both the wheel speed sensor and toothed ring are contained within the wheel hub/bearing assembly.
^ Motor Pack Assembly/Modulator Assembly/Solenoid Valves: The purpose of these components is to modulate brake hydraulic pressure when the antilock brake control module determines wheel lock-up (based on wheel speed sensor inputs). Together with the brake master cylinder, these components make up the brake control assembly.
^ Antilock Brake Control Module: The control module is an electronic device that controls the antilock brake system operation. In addition to the wheel speed sensor input, other inputs to the module include the brake switch, ignition switch, and battery voltage through the ABS enable relay. A serial data link is provided for communication with various diagnostic tools.
BRAKE SYSTEM WARNING LIGHTS
^ Amber ANTILOCK Telltale: The antilock brake control module continuously monitors itself and other ABS components. If it detects a system problem, the amber ANTILOCK telltale (located in the vehicle instrument cluster) will either flash or light continuously.
The amber ANTILOCK telltale will flash if the problem does not immediately affect ABS operation. However, if a flashing light does occur, repairs must be made to the ABS system as soon as possible.
The amber ANTILOCK telltale will light continuously (solid light) if a problem that will affect the operation of the ABS system is detected. Should this occur, antilock brake control will not be available. Although conventional (non-ABS) brakes will remain operational, repair should be made to the ABS system as soon as possible.
^ Red BRAKE Telltale: Located in instrument cluster, it will light if any of the following conditions are met:
a. Low brake fluid level in master cylinder
b. Parking brake switch is closed
c. It is turned On by the antilock brake control module
Certain antilock brake malfunctions may degrade base brake performance. Should this occur, both the amber ANTILOCK and red BRAKE telltale will turn On.
ANTI LOCK BRAKE SYSTEM OPERATION
The antilock brake system operates in two distinct modes. When the antilock brake control module determines that no wheel lock-up is occurring, the brake system operates in the base brake mode. When impending wheel lock-up is detected, the brake system switches to the antilock brake system (ABS) mode.
In the ABS mode, brake fluid pressure is controlled by the modulator assembly. In the base brake mode the brake fluid pressure delivered to the brakes is simply the pressure created in the master cylinder; no ABS components affect the brake fluid pressure.
When brakes are applied in the base brake mode, brake fluid is provided with two unrestricted paths from the master cylinder to the front brakes, and one path to the rear brakes. A piston, located within the modulator assembly, is extended by the motor pack assembly to its home position, opening a check valve. By opening the check valve, a path is provided for brake fluid to travel from the master cylinder to the brake caliper. Also, while in the base brake mode, a solenoid valve (attached to the modulator assembly for the front brake only) is kept open, allowing a second path for brake fluid to travel to the front brake caliper. Expansion spring brakes (ESB) are located within the motor pack to prevent the motors from back-driving due to brake fluid pressure on the ball screws.
When in the ABS mode, the antilock brake control module commands the motor pack assembly to close the solenoid and lower the piston. Now, the brake fluid pressure created by the master cylinder no longer acts on the brake calipers. Instead, brake pressure to the calipers is now controlled by the piston; downward or upward motion of the piston will decrease or increase the brake fluid pressure (providing less or more braking force), as determined by the wheel speed sensors and antilock brake control module.
Each front wheel is provided with a separate piston, solenoid valve and motor, while both rear wheels share a single motor that controls two pistons.
DIAGNOSTICS
The antilock brake control module stores diagnostic information that, when accessed with a Scan tool, will provide detailed diagnostic information, should a fault be present. Forty-seven Codes and six Information Flags are provided to assist with ABS system diagnosis. Diagnostics will also reveal whether or not the fault is intermittent With the ABS diagnostic system, fault codes can only be read and cleared with a Scan Tool. However, if no faults occur for 100 drive cycles (a drive cycle occurs if the ignition is turned On and the vehicle is driven faster than 17 km/h [10 m.p.h.), fault codes are cleared from the antilock brake control module.
Should multiple ABS system malfunctions occur, the first five fault codes will be stored in Malfunction History in the order in which they occurred (least recent to most recent). This can be useful, since one primary fault can often cause other secondary faults to occur. In such cases, knowing the order in which these faults occurred provides useful diagnostic information.
ABS INITIALIZATION
ABS equipped vehicles perform an initialization process at 13 km/h (8 mph) by commanding the ABS motors to drive the pistons to the "home position." When the vehicle exceeds 5 km/h (3 mph), ABS braking will be available, the but system testing will not occur until 13 km/h (8 mph). During this 13 km/h (8 mph) initialization process, noise may be heard and pedal movement may be felt if the driver's foot is on the brake pedal.