Raider 2WD V6-3.7L (2008)
Hydraulic Control Assembly - Antilock Brakes: Description and Operation
HCU (HYDRAULIC CONTROL UNIT)
DESCRIPTION
The HCU consists of a valve body, pump motor, low pressure accumulators, inlet valves, outlet valves and noise attenuators.
OPERATION
Accumulators in the valve body store extra fluid released to the system for ABS mode operation. The pump provides the fluid volume needed 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 three channel ABS or four channel With Traction Control pressure control to the front and rear brakes. One channel controls the
rear wheel brakes in tandem ABS. The two remaining channels control the front wheel brakes individually. During antilock braking, the solenoid
valves are opened and closed as needed.
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.
NOTE: The three modes mentioned below do occur but not necessarily in the order listed every time. 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 the brakes. This
cycle controls re-application of fluid apply pressure.
