Operation

OPERATION

Accumulators in the valve body store extra fluid released from the calipers during ABS mode operation. The pump is used to clear the accumulator of brake fluid and is operated by a DC type motor. The motor is controlled by the ABM. The valves modulate brake pressure during antilock braking and are controlled by the ABM. The HCU provides individual pressure control to each front and rear brake.


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. Brake Traction Control and Electronic Stability Program modulate pressure on each wheel individually without any driver brake input.


During normal braking, the HCU solenoid valves and pump are not activated. The master cylinder and power booster operate the same as a vehicle without an ABS brake system. During antilock braking, solenoid valve pressure modulation occurs in three stages; pressure increase, pressure hold, and pressure decrease. The valves are all contained in the valve body portion of the HCU.


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 ABM 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 ABM closes the outlet valve and begins a pressure increase or hold cycle as needed.


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 ABM maintains the hold cycle until sensor inputs indicate a pressure change is necessary.


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 thew brakes. This cycle controls re-application of fluid apply pressure.