Base Braking Mode

Under normal operating conditions, the brake system will operate using conventional braking by means of brake pedal force, vacuum booster and the master cylinder. Each front channel consists of a motor with an Expansion Spring Brake (ESB), gear driven ballscrew, piston, check valve and solenoid. Under normal operating conditions, the piston is held in the up-most (home) position. This is accomplished by turning the screw via the motor to drive the nut upwards. Once at the home position, the piston is held by an ESB. While at the up-most position, the piston also unseats the check valve via a protrusion on the piston. Two paths are then available to transfer braking pressure to the wheel:

• Through the modulator, around the check valve and out to the wheel.
• Through the modulator, past the normally open solenoid and out to the wheel.

The solenoid in the front circuits provides an alternate brake pressure path to the wheel. With this arrangement, if the ABS were to lose power or malfunction with the piston not in its home position, a redundant brake fluid path is available.

The hydraulically isolated rear wheel channels operate in a similar manner, except both are controlled together with one motor and there is no solenoid. While at the home position, the piston unseats the check valve via a protrusion on the piston. With the rear wheel, only one path is then available to transfer braking pressure to the rear wheels: Through the modulator, around the check valve and out to the rear wheels.

There is no rear solenoid due to the nature of the braking system. The majority of braking is accomplished with the front brakes. If an ABS failure were to occur that affected the operation of the rear brakes, a DTC would be stored and the EBCM would turn on both the Amber ABS and the Red BRAKE indicator lights.