RAM 3500 Truck 2WD V8-5.7L VIN D (2003)
Hydraulic Control Assembly - Antilock Brakes: Description and Operation
HCU (Hydraulic Control Unit)
HYDRAULIC CONTROL UNIT
DESCRIPTION
The HCU consists of a valve body, pump motor, low pressure accumulators, inlet valves, outlet valves and noise attenuators.
The HCU on the EBC 125 has an integral valve body for controlling the front and rear Brakes. The HCU on the EBC 325 has an integral valve body
for controlling the front and rear Brakes.
Within the HCU are solenoids, valves, check valves, and a reset switch necessary to apply and release brake pressure as required to avoid wheel
lockup, keep the wheels rolling, and maintain optimum deceleration.
The Isolation Valve(s) is/are normally open, allowing unrestricted flow from the master cylinder to the wheels. When the CAB determines antilock
intervention is required, the valve(s) close to isolate the master cylinder hydraulic circuit(s). Fluid is trapped in the circuit(s) and then prevented from
reaching the wheels.
The Dump Valve(s) is/are pulsed on and off by the CAB. The valve(s) cycles only if the isolation valve is closed. When dump is on, it allows fluid to
the low-pressure accumulator for temporary storage. This causes the pressure to the wheel to decrease. When the dump valve is off, fluid is allowed to
the wheel.
The Brake Return Check Valve allows the HCU to drain faster after the antilock activation, when the brake is released.
The Reset Switch is used in the EBC 125 (RWAL) system only. The switch is positioned in the HCU to monitor the master cylinder (input), rear brake
(output) and accumulator pressure. During normal braking, pressure is equal for input and output and the switch remains open. During an antilock stop
the switch will close as the isolation valve is cycled and pressure becomes unequal between input and output. At the end of an antilock stop the
isolation valve will open as master cylinder pressure and rear pressure equalize. The switch is used to monitor correct operation of the HCU and to set
a DTC if incorrect pressure is detected on the EBC 125 (RWAL) antilock system when not in an ABS event.
When the brakes are applied, fluid is forced from the master cylinder outlet port(s) to the HCU inlet ports(s). This pressure is transmitted through
normally open isolation valve(s) inside the HCU, then through the outlet port(s) of the HCU to the wheels. If the CAB senses that a wheel is about to
lock based on wheel speed sensor data, it pulls the normally open isolation valve closed for that circuit. This prevents any more fluid from entering
that circuit. The CAB continues to look at the wheel speed sensor(s) signal to determine if the wheel is still decelerating. If deceleration is still taking
place, the normally closed dump valve for that circuit is opened. This action dumps any pressure that is trapped between the normally open valve and
the brake back into an accumulator. Once the affected wheel comes back up to speed, the CAB returns the valves to their normal condition allowing
the affected brake to be reapplied. On the EBC 325 system, there is a pump/motor unit, two accumulators, three isolated valves, and three dump valves
which are used to provide rapid response during the reapply sequence and to minimize pedal feedback due to the increased hydraulic circuits.
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 CAB.
The valves modulate brake pressure during antilock braking and are controlled by the CAB.
The HCU provides three channel pressure control to the front and rear brakes. One channel controls the rear wheel brakes in tandem. 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 everytime.
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 CAB 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 CAB closes the outlet valve and begins a pressure increase or hold cycle as needed.
