Hydraulic Control Unit (HCU)

The EBB HCU is located in the left rear corner of the engine compartment. The EBB HCU consists of the brake fluid reservoir, and the HCU, which contains the valve body with pressure and temperature sensors, pump motor, low pressure accumulators, inlet valves, outlet valves and noise attenuators. Accumulators in the valve body store extra fluid released from the calipers during ABS mode operation. The motor is controlled by the BSCM. The valves modulate brake pressure during antilock braking and are controlled by the BSCM. During antilock braking, the solenoid valves are opened and closed as needed. They are cycled rapidly and continuously to modulate pressure and control wheel slip and deceleration. Traction Control System (TCS) and Electronic Stability Program (ESP) modulate pressure on each wheel individually without any driver input to the brake pedal. The valves are all contained in the valve body portion of the HCU.

The BPTS provides the ABS system a linear signal representing the position of the brake pedal, from fully pressed to fully released. This signal is used by the ABS module to determine if there is driver input to the system resulting in fluid pressure buildup in the hydraulic system.

The BPTS is internal to the EBB and is not serviced separately from the EBB.

The data from the ABS Brake Pressure Sensor, also integral to the EBB, is used to help in determining if the driver is intentionally applying the brakes or if there is system generated pressure during autonomous braking events to illuminate the brake lights.

During antilock braking, solenoid valve pressure modulation occurs in three stages; pressure increase, pressure hold, and pressure decrease. A brief description of each pressure modulation follows:

PRESSURE INCREASE 

• The inlet valve is open and the outlet valve is closed during the pressure increase cycle. The pressure increase cycle is used to reapply the brakes. This cycle controls re-application of fluid apply pressure.

PRESSURE HOLD 

• Both solenoid valves are closed in the pressure hold cycle but only the inlet valve is energized. Fluid apply pressure in the control channel is maintained at a constant rate. The BSCM maintains the hold cycle until sensor inputs indicate a pressure change is necessary.

PRESSURE DECREASE 

• The outlet valve is opened and the inlet valve is closed during the pressure decrease cycle. A pressure decrease cycle is initiated when speed sensor signals indicate high wheel slip at one or more wheels. At this point, the BSCM closes the inlet then opens the outlet valve, which also opens the return circuit to the accumulators. Fluid pressure is allowed to bleed off (decrease) as needed to prevent wheel lock. Once the period of high wheel slip has ended, the BSCM closes the outlet valve and begins a pressure increase or hold cycle as needed.

The HCU is powered by the BSCM, which receives fused B(+) from two 50 amp fuses in the Power Distribution Center (PDC). The BSCM receives an ignition RUN/START feed from a 10 amp fuse in the PDC.