Antilock Braking Mode

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 Antilock Brake System (ABS) operation, the solenoids are turned on to eliminate that brake pressure path to the caliper (Left figure). The EBCM then provides controlled current to the motors to regulate the speed and amount of backward motion.
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. 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. Total brake pressure during ABS is limited to the brake pressure present when ABS was entered. Also, any time wheel brake pressure exceeds thy 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.
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.
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.

NOTE: In this manner, ABS installed to this model cannot increase brake pressure above master cylinder pressure applied by the driver and cannot apply the brakes by itself.