Jeep Cherokee (XJ): Description and operation
DESCRIPTION The purpose of the antilock system is to prevent
wheel lockup during periods of high wheel slip. Preventing
lockup helps maintain vehicle braking action
and steering control.
The antilock CAB activates the system whenever
sensor signals indicate periods of high wheel slip.
High wheel slip can be described as the point where
wheel rotation begins approaching 20 to 30 percent of
actual vehicle speed during braking. Periods of high
wheel slip occur when brake stops involve high pedal
pressure and rate of vehicle deceleration.
Battery voltage is supplied to the CAB ignition terminal
when the ignition switch is turned to Run position.
The CAB performs a system initialization
procedure at this point. Initialization consists of a
static and dynamic self check of system electrical
components.
The static check occurs after the ignition switch is
turned to Run position. The dynamic check occurs
when vehicle road speed reaches approximately 30
kph (18 mph). During the dynamic check, the CAB
briefly cycles the pump and solenoids to verify operation.
If an ABS component exhibits a fault during initialization,
the CAB illuminates the amber warning
light and registers a fault code in the microprocessor
memory. OPERATION During normal braking, the master cylinder, power
booster and wheel brake units all function as they
would in a vehicle without ABS. The HCU components
are not activated.
During antilock braking fluid pressure is modulated
according to wheel speed, degree of slip and
rate of deceleration. A sensor at each wheel converts
wheel speed into electrical signals. These signals are
transmitted to the CAB for processing and determination
of wheel slip and deceleration rate.
The ABS system has three fluid pressure control
channels. The front brakes are controlled separately
and the rear brakes in tandem. A speed sensor input
signal indicating a high slip condition activates the
CAB antilock program. Two solenoid valves are used
in each antilock control channel. The valves are all
located within the HCU valve body and work in pairs
to either increase, hold, or decrease apply pressure as
needed in the individual control channels. The solenoid
valves are not static during antilock braking.
They are cycled continuously to modulate pressure.
Solenoid cycle time in antilock mode can be measured
in milliseconds. DESCRIPTION The CAB is mounted to the HCU and operates the
ABS system (Fig. 1) separate from other vehicle electrical
circuits. OPERATION The CAB voltage source is through the ignition
switch in the RUN position. The CAB contains dual 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. 1 - HCU 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 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. The valves are not
static. They are cycled rapidly and continuously to
modulate pressure and control wheel slip and deceleration.
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 channel
is maintained at a constant rate. The CAB 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 counteract unequal
wheel speeds. This cycle controls re-application of
fluid apply pressure due to changing road surfaces or
wheel speed. 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 mechanism
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 sensors have a resistance
between 900 and 1300
ohms.
1 - FRONT SENSOR FRONT SENSOR AIR GAP Front sensor air gap is fixed and not adjustable.
Only rear sensor air gap is adjustable.
Although front air gap is not adjustable, it can be
checked if diagnosis indicates this is necessary. Front
air gap should be 0.36 to 1.5 mm (0.014 to 0.059 in.). If
gap is incorrect, the sensor is either loose, or damaged. REAR SENSOR AIR GAP A rear sensor air gap adjustment is only needed
when reinstalling an original sensor. Replacement
sensors have an air gap spacer attached to the sensor
pickup face. The spacer establishes correct air gap
when pressed against the tone ring during installation.
As the tone ring rotates, it peels the spacer off
the sensor to create the required air gap. Rear sensor
air gap is 0.92-1.275 mm (0.036-0.05 in.).
Sensor air gap measurement, or adjustment procedures
are provided in this section. Refer to the front,
or rear sensor removal and installation procedures as
required. DESCRIPTION The combination valve contains a pressure differential
valve and switch and a rear brake proportioning
valve. The valve is not repairable and must be
replaced as an assembly if diagnosis indicates this is
necessary.
OPERATION PRESSURE DIFFERENTIAL VALVE The pressure differential switch is connected to the
brake warning light. The switch is actuated by movement
of the switch valve. The switch monitors fluid
pressure in the separate front/rear brake hydraulic
circuits.
A decrease or loss of fluid pressure in either
hydraulic circuit will cause the switch valve to shuttle
to the low pressure side. Movement of the valve
pushes the switch plunger upward. This action closes
the switch internal contacts completing the electrical
circuit to the red warning light. The switch valve will
remain in an actuated position until repairs to the
brake system are made. PROPORTIONING VALVE The proportioning valve is used to balance frontrear
brake action at high decelerations. The valve
allows normal fluid flow during moderate braking.
The valve only controls fluid flow during high decelerations
brake stops. DESCRIPTION The G-switch is located under the rear seat. The
switch has directional arrow and must be mounted
with the arrow pointing towards the front of the
vehicle. OPERATION The switch (Fig. 3), provides an additional vehicle
deceleration reference during 4x4 operation. The
switch is monitored by the CAB at all times. The
switch reference signal is utilized by the CAB when
all wheels are decelerating at the same speed. DESCRIPTION The amber ABS warning lamp is located in the
instrument cluster. The lamp illuminates at start-up
to perform a self check. The lamp goes out when the
self check program determines the system is operating
normal. OPERATION If an ABS component exhibits a fault the CAB will
illuminate the lamp and register a trouble code in
the microprocessor. The lamp is controlled by the
CAB. The lamp is illuminated when the CAB sends a
ground signal to the ABS relay. The ABS relay then
grounds the lamp circuit and illuminates the lamp.
1 - SWITCH PART NUMBERAntilock brake system
Controller antilock brakes
Fig. 1 Controller Antilock Brakes
2 - MOTOR
3 - CABHydraulic control unit
Wheel speed sensors and tone wheel
Fig. 2 Wheel Speed Sensors
2 - REAR SENSOR
3 - PICKUP FACECombination valve
G-switch
Abs warning lamp
Fig. 3 G-Switch
2 - ARROW INDICATES FRONT OF SWITCH FOR PROPER
MOUNTING
Diagnosis and testing. Service procedures. Removal and installation
Other materials:
Cleaning and inspection
Caliper
CLEANING
Clean the caliper components with clean brake
fluid or brake clean only. Wipe the caliper and piston
dry with lint free towels or use low pressure compressed
air.
CAUTION: Do not use gasoline, kerosene, thinner,
or similar solvents. These products may leave a
residue that cou ...