Antilock 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.