Chrysler RAM TRUCK 2500 1994 Service Manual page 196

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ALL-WHEEL ANTILOCK BRAKE SYSTEM (ABS) 5 - 37
The sensors convert wheel speed into an electrical
signal. The signal is transmitted to the electronic
control module for processing.
The trigger mechanism for each front wheel sensor
is a gear type, tone wheel. The tone wheels are
mounted on the inboard side of the disc brake rotor
hub (Fig. 5).
Fig. 5 Front Tone Wheel Location (In Rotor)
A single sensor is used to monitor rear wheel speed
and rate of deceleration. The sensor is mounted at
the top of the rear axle housing (Fig. 6). The trigger
mechanism for the sensor is an exiter ring pressed
onto the differential case. The ring is pressed onto
the case next to the ring gear.
The teeth on the tone wheels and exiter ring inter­
rupt the sensor magnetic field as they rotate. The
rate of interruption is converted into speed signals
which are transmitted to the electronic control mod­
ule.
The front and rear sensors are fixed and not ad­
justable. A front or rear sensor must be replaced
when diagnosis indicates a fault has occurred.
Master Cylinder And Power Brake Booster
A standard master cylinder and vacuum power
brake booster are used with the ABS system. The
same master cylinder and dual or single diaphragm
boosters are used with each antilock system.
ALL WHEEL ANTILOCK SYSTEM (ABS) OPERATION
The all wheel ABS system is a three channel de­
sign. The front wheel brakes are controlled individu­
ally and the rear wheel brakes i n tandem (Fig. 7).
The ABS system is designed to retard wheel lockup
during periods of high wheel slip when braking. Re­
tarding wheel lockup is accomplished by modulating
fluid pressure to the wheel brake units.
The antilock electrical system is separate from
other electrical circuits i n the vehicle. A specially pro­
grammed electronic control module is used to operate
the system components.
Fig. 6 Rear Wheel Sensor Location
The front and rear antilock valves contain electri­
cally operated solenoid valves. The solenoid valves
modulate brake fluid apply pressure during antilock
braking. The valves are operated by the antilock elec­
tronic module.
The ABS valve provides two channel pressure con­
trol of the front brakes. Each front wheel brake unit
is controlled separately. Two solenoid valves are used
in each control channel.
The RWAL valve controls the rear wheel brakes i n
tandem. The rear brake valve contains two solenoid
valves.
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.
The pump/motor assembly on the front antilock
valve provides the fluid volume needed during an­
tilock braking. The pump is operated by an integral
electric motor. The DC type motor is controlled by
the ABS control module.
MBS OPERATION IN NORMAL BRAKING MODE
The ABS electronic control module monitors wheel
speed sensor inputs continuously while the vehicle is