System Operation

BASE BRAKING
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 front channel consists of a motor, solenoid, Expansion Spring Brake (ESB), ball screw, piston, and check valve.

Piston Position
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.

Fluid Paths
Two paths are available to transfer brake fluid to the calipers. Through the modulator, around the open check valve and out to the caliper, and through the modulator, through the normally open solenoid and out to the caliper.

Fail-Safe Function
The solenoid in the front circuits provides an alternate brake fluid pressure path to the caliper. With this arrangement, if the Antilock Brake System (ABS) were to lose power or malfunction with the piston out of the "home" position, a redundant brake fluid path is available. The rear channel operates in a similar manner except solenoid is used.





ANTILOCK BRAKING
If any wheel begins to approach lock-up, the Electronic Brake Control Module (EBCM) will control the three motors and two solenoids, appropriately, to control brake pressure to the affected wheel(s). During front wheel ABS operation, the solenoids are turned on to eliminate that brake pressure path to the caliper. The EBCM then provides controlled current to the motors to regulate the speed and amount of backward motion.

Reducing And Applying Fluid Pressure
As the motor moves backwards the piston follows the nut downward, allowing the check valve to seat. 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.

Return To Base Braking
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 lock-up again.

Brake Pedal Force And ABS Pressure
Total brake pressure during ABS is limited to the brake pressure present when ABS was entered. Also, anytime wheel brake pressure exceeds the brake pressure at the master cylinder (caused by reduced force on the brake pedal), the check valve unseats and this volume of brake fluid is returned to the master cylinder. When ABS is no longer required the pistons are returned to their upmost (or home) position and are held by the ESBs.

NOTE: ABS installed on this model cannot increase brake pressure above master cylinder pressure applied by the driver and can not apply the brakes by itself.

Front And Rear Channels
The solenoids on the front channels are simultaneously opened to again provide a redundant base braking path. The rear channels operate in a similar manner except: (1) both rear circuits are controlled together and (2) no solenoid exists. Both rear channels are controlled by one motor. If either rear wheel begins to lock, brake pressure to both rear wheels is reduced to maximize vehicle stability.

ABS Failure
If an ABS failure were to occur that affected the operation of the rear base brakes, a Diagnostic Trouble Code (DTC) would be stored and the EBCM would illuminate both the "ABS" and "BRAKE" warning lights.