System Operation

Base Braking Mode:

BASE BRAKE MODE
Under normal operating conditions, the brake system will operate using conventional braking by means of brake pedal force, the vacuum booster and the master cylinder. Each ABS channel consists of a motor, solenoid, expansion spring brake (ESB), ball screw, piston, and check valve. Under normal operating conditions (base braking), the piston is held in the upmost or "home" position and the solenoid is open (not energized). This is accomplished by the ball screw via the motor to drive the nut upwards. Once at the upmost position, the piston is held by an ESB.

Brake Fluid Transfer Paths
Two paths are available to transfer brake fluid to the calipers: (1) into the modulator, around the open check valve and out to the caliper, and (2) into the modulator, through the normally open solenoid and out to the caliper. The solenoid in the front circuits provides an alternate brake fluid pressure path to the caliper. With this arrangement, if the ABS were to lose power or malfunction with the piston not at "home" position, a redundant brake fluid path is always available.

Antilock Braking Mode:

ANTILOCK BRAKING MODE
ABS has been designed to improve the controllability and steerablity of a vehicle during braking. ABS accomplishes this by controlling the hydraulic brake pressure applied to each front caliper and the rear wheel cylinders. Antilock braking occurs only when the stop lamp switch is closed and the Electronic Brake Control Module (EBCM) determines that at least one wheel is about to lose traction during braking. The EBCM will then allow the hydraulic modulator to change the brake pressures several times each second to keep the wheel(s) from locking and provide the driver with maximum vehicle controllability. ABS cannot increase the brake pressure above the master cylinder pressure applied by the driver and cannot apply the brakes by itself.

Result Of ABS Braking
The result is greatly improved braking, enabling the driver to more easily maintain steerability and to bring the vehicle to a controlled stop. ABS provides effective braking and directional control over a wide range of road surfaces and driving conditions. If any wheel begins to approach lockup, the EBCM will control the three motors and three solenoids, appropriately, to control brake pressure to the affected wheel(s). During ABS operation, the solenoids are turned on to isolate that brake pressure path to the caliper/wheel cylinder. The EBCM then provides controlled current to the motors to regulate the speed and amount of movement. As the motor moves backwards, the piston follows the nut downward, allowing the check valve to seat. The brake pressure to the caliper/wheel cylinder is now a function of the controlled volume within the piston chamber. To reduce brake pressure, the motor drives the nut further downward. To reapply or increase brake pressure, the motor drives the nut and piston upward. If ABS was entered during low brake pressure, such as on ice, and dry pavement is then encountered during reapply, the piston is driven all the way to the top. This results in the unseating of the check valve, and a return to base braking until sufficient brake pressure exists to cause the wheel to approach lockup again; at this point, the ABS cycle would start again. This process can occur in less than one second if the driver is pressing firmly on the brake pedal.

BRAKE PRESSURE
Total brake pressure during ABS is limited to the brake pressure present when ABS was entered. Also, any time wheel brake pressure exceeds the brake pressure at the master cylinder (caused by reduced force on the brake pedal), the check valve unseats and a small volume of brake fluid is returned to the master cylinder to equalize pressure. In this manner, ABS cannot increase brake pressure above master cylinder pressure applied by the driver and cannot apply the brakes by itself. When ABS is no longer required, the pistons are returned to their upmost (or home) position and are held by the ESBs. The solenoids are simultaneously opened to again provide a redundant base braking path.