Chrysler 2004 Concorde Service Manual page 179

LH
HCU (HYDRAULIC CONTROL
UNIT)
DESCRIPTION
The hydraulic control unit (HCU) is mounted to
the CAB as part of the ICU (Fig. 12). The ICU is
located in front of the driver's side front tire, behind
the inner fender splash shield. The HCU controls the
flow of brake fluid to the brakes using a series of
valves and accumulators. A pump/motor is mounted
on the HCU to supply build pressure to the brakes
during an ABS stop.
The HCU on a vehicle equipped with traction con-
trol has a valve block that is approximately one inch
longer than a HCU on a vehicle that is equipped
with ABS only in order to incorporate the additional
valves.
For more information, refer to the ICU. (Refer to 5
- BRAKES/HYDRAULIC/MECHANICAL/ICU (INTE-
GRATED CONTROL UNIT) - DESCRIPTION)
OPERATION
The operation of the HCU's hydraulic circuits can
be found in Hydraulic Circuits And Valve Operation.
(Refer to 5 - BRAKES - ABS/HYDRAULIC/ME-
CHANICAL - OPERATION)
The following topics explain how the different com-
ponents within the HCU operate.
VALVES AND SOLENOIDS
The valve block contains four inlet valves and four
outlet valves. The inlet valves are spring-loaded in
the open position and the outlet valves are spring-
loaded in the closed position during normal braking.
The fluid is allowed to flow from the master cylinder
to the wheel brakes.
During an ABS stop, these valves cycle to maintain
the proper slip ratio for each wheel. The inlet valve
closes preventing further pressure increase and the
outlet valve opens to provide a path from the wheel
brake to the HCU accumulators and pump/motor.
This releases (decays) pressure from the wheel brake,
thus releasing the wheel from excessive slippage.
Once the wheel is no longer slipping, the outlet valve
is closed and the inlet valve is opened to reapply
(build) pressure.
For information on the valves used with the trac-
tion control system, (Refer to 5 - BRAKES - ABS -
TRACTION CONTROL - OPERATION).
BRAKES - ANTILOCK BRAKE SYSTEM 5 - 75
BRAKE FLUID ACCUMULATORS
There are two fluid accumulators in the HCU, one
for the primary hydraulic circuit and one for the sec-
ondary hydraulic circuit. Each hydraulic circuit uses
a 5 cc accumulator.
The fluid accumulators temporarily store brake
fluid that is removed from the wheel brakes during
an ABS cycle. This stored fluid is used by the pump/
motor to provide build pressure for the brake hydrau-
lic system. When the antilock stop is complete, the
accumulators are drained by the pump/motor.
PUMP/MOTOR
There are two pump assemblies in the HCU, one
for the primary hydraulic circuit and one for the sec-
ondary hydraulic circuit. Both pumps are driven by a
common electric motor (Fig. 12). This DC-type motor
is integral to the HCU and is controlled by the CAB.
The pump/motor provides the extra amount of
brake fluid needed during antilock braking. Brake
fluid is released to the accumulators when the outlet
valve is opened during an antilock stop. The pump
mechanism consists of two opposing pistons operated
by an eccentric camshaft. In operation, one piston
draws fluid from the accumulators, and the opposing
piston pumps fluid to the master cylinder circuits.
When the antilock stop is complete, the pump/motor
drains the accumulators.
The CAB may turn on the pump/motor when an
antilock stop is detected. The pump/motor continues
to run during the antilock stop and is turned off after
the stop is complete. Under some conditions, the
pump/motor runs to drain the accumulators during
the next drive-off.
The pump/motor is not a serviceable item; if it
requires replacement, the HCU must be replaced.
REMOVAL - HCU
To remove the HCU, the ICU must be removed
from the vehicle and disassembled. (Refer to 5 -
BRAKES - ABS/HYDRAULIC/MECHANICAL/ICU
(INTEGRATED CONTROL UNIT) - REMOVAL)
INSTALLATION - HCU
To install the HCU, it must be attached to the
CAB, forming the ICU. (Refer to 5 - BRAKES - ABS/
HYDRAULIC/MECHANICAL/ICU
CONTROL UNIT) - ASSEMBLY)
(INTEGRATED