Jeep Cherokee 2000, Service Manual

Page: 204 / 1528

microprocessors. A logic block in each microprocessor
receives identical sensor signals. These signals are
processed and compared simultaneously. The CAB
contains a self check program that illuminates the
ABS warning light when a system fault is detected.
Faults are stored in a diagnostic program memory
and are accessible with the DRB scan tool. ABS
faults remain in memory until cleared, or until after
the vehicle is started approximately 50 times. Stored
faults are not erased if the battery is disconnected.
HYDRAULIC CONTROL UNIT
DESCRIPTION
The HCU consists of a valve body, pump motor,
and wire harness (Fig. 1).
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 oper-
ated 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. The valves are not
static. They are cycled rapidly and continuously to
modulate pressure and control wheel slip and decel-
eration.
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 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.
PRESSURE HOLD
Both solenoid valves are closed in the pressure
hold cycle. Fluid apply pressure in the control chan-
nel is maintained at a constant rate. The CAB main-
tains 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 pres-
sure increase cycle is used to counteract unequal
wheel speeds. This cycle controls re-application of
fluid apply pressure due to changing road surfaces or
wheel speed.
WHEEL SPEED SENSORS AND TONE WHEEL
DESCRIPTION
A speed sensor is used at each wheel. The front
sensors are mounted to the steering knuckles. The
rear sensors at the outboard end of the axle.
OPERATION
The sensors convert wheel speed into a small AC
electrical signal. This signal is transmitted to the
CAB. The CAB converts the AC signal into a digital
signal for each wheel. This voltage is generated by
magnetic induction when a tone wheel passes by the
stationary magnet of the wheel speed sensor.
A gear type tone ring serves as the trigger mecha-
nism for each sensor. The tone rings are mounted at
the outboard ends of the front and rear axle shafts.
Different sensors are used at the front and rear
wheels (Fig. 2). The front/rear sensors have the same
electrical values but are not interchangeable. The
Fig. 1 Controller Antilock Brakes
1 – HCU
2 – MOTOR
3 – CAB
5 - 34 BRAKES
XJ
DESCRIPTION AND OPERATION (Continued)