(3) Push sensor connector fastener clip into mount-
ing hole in frame rail.
(4) Connect vehicle wiring harness to sensor con-
nector.
(5) Install tire and wheel assembly (Refer to 22 -
TIRES/WHEELS - INSTALLATION). Progressively
tighten the wheel mounting nuts in the proper
sequence to 122 N·m (90 ft. lbs.) torque.
(6) Lower vehicle.
(7) Road test vehicle to ensure proper operation of
base brakes and ABS.
HCU (HYDRAULIC CONTROL
UNIT)
DESCRIPTION - HYDRAULIC CONTROL UNIT
(HCU)
The hydraulic control unit (HCU) is mounted to
the CAB as part of the ICU (Fig. 6). The HCU con-
trols 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.
VALVES AND SOLENOIDS
The valve block contains three inlet valves and
three outlet solenoid 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.
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 3 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. 6). 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, these pistons
are used to purge fluid from the accumulators back
into the master cylinder circuits. When the antilock
stop is complete, the pump/motor drains the accumu-
lators.
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.
OPERATION - HYDRAULIC CIRCUITS AND
VALVES
The hydraulic fluid control valves within the HCU
control the flow of pressurized brake fluid to the
wheel brakes during the different modes of ABS
braking. The following paragraphs explain how this
works. For purposes of explanation only, it is
assumed that only the right front wheel is experienc-
ing antilock braking; the following diagrams show
only the right front wheel in an antilock braking
operation.
ZB BRAKES - ABS 5 - 53
WHEEL SPEED SENSOR - REAR (Continued)
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