Schematic Of The Brake Hydraulic System And Control Circuit; Wheel Sensors; Electronic Controller; Diagnosis And Repair - Jeep cherokee 1989 Owner's Manual

booster unit, an electric booster pump and motor assembly (which con-
tains an accumulator), a pressure modulator and an accumulator and
pressure switch assembly (see illustration).
accumulator, which supplies pressure to the braking system. The
pump and accumulator are mounted t o the firewall on the right-
hand side.
and power booster unit and modulates brake line pressure dur-
ing ABS operation. The valve body contains three valves
for each front wheel and one for the both of the rear wheels
combined.

Wheel sensors

These sensors are located at each wheel and generate small elec-
trical pulsations when the toothed sensor rings are turning, sending
a signal to the electronic controller, indicating wheel rotational speed.
The front wheel sensors (see illustration) are mounted to the steer-
ing knuckles in close relationship to the tone wheels, which are integral
with the front axleshafts.
The rear wheel sensors bolt to the drum brake backing plates (see
illustration). The tone wheels are integral with the rear axleshafts.

Electronic controller

The electronic controller is mounted under the rear seat and is the
"brain" for the ABS system. The function of the control module
consisting of microprocessors, a mercury switch and the related circuits
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needed for their operation is to accept and process information
received from the wheel speed sensors in conjunction with the signal
AND MOTOR
BOOSTER PUMP

2.2 Schematic of the brake hydraulic system and control circuit

The Motor Manual Guy
Chapter 9 Brakes
from the internal mercury switch to control the hydraulic line pressure,
avoiding wheel lock-up. The controller also constantly monitors the
system, even under normal driving conditions, to find faults within the
system.
If a problem develops within the system, a yellow or red light will
glow on the dashboard. A diagnostic code will also be stored in the
controller, which, when retrieved by a service technician, will indicate
the problem area or component.

Diagnosis and repair

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one
If a dashboard warning light comes on and stays on while the vehicle
is in operation, the ABS system requires attention. Although a special
electronic ABS diagnostic tester is necessary to properly diagnose the
system, the home mechanic can perform a few preliminary checks
before taking the vehicle t o a dealer who is equipped with this tester.
a) Check the brake fluid level in the reservoir.
connector is securely connected.
pressure switch and pressure modulator.
nections are secure and that the wiring is not damaged.
If the above preliminary checks do not rectify the problem, the vehicle
should be diagnosed by a dealer service department. Due to the rather
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complex nature of this system and the high operating pressures in-
volved, all actual repair work must be done by a dealer service depart-
ment.
MASTER CYLINDEWPOWER BOOSTER BOOST PRESSURE SWITCH
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ABS-equipped models