Chrysler RAM TRUCK 2500 1994 Service Manual page 197

5 - 38 ALL-WHEEL ANTILOCK BRAKE SYSTEM (ABS)
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POWER
BRAKE
BOOSTER
FRONT
ANTILOCK
VALVE
RF
WHEEL
RR
WHEEL
Fig. TABS Hydraulic
in motion. However, the module will not activate any
ABS components as long as sensor inputs indicate
normal braking.
During normal braking, the master cylinder, power
booster and wheel brake units all function as they
would i n a vehicle without ABS. The solenoid valves
are not activated.
ABS OPERATION I N ANTILOCK B R A K I N G
MODE
The ABS module activates the system whenever
sensor signals indicate the onset of high wheel slip.
High wheel slip can be described as the point
where wheel rotation begins approaching zero (or
lockup) during braking. The antilock system retards
lockup during high slip conditions by modulating
fluid apply pressure to the wheel brake units.
Brake fluid apply pressure is modulated according
to wheel speed, degree of slip and rate of decelera­
tion. A sensor at each wheel converts wheel speed
into electrical signals. These signals are transmitted
to the module 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 i n tandem. A speed sensor input
signal indicating high slip conditions activates the
control module antilock program.
The solenoid valves are not static during antilock
braking. They are cycled continuously to modulate
pressure. Solenoid cycle time i n antilock mode can be
measured i n milliseconds.
LR
WHEEL
J9305-113
System Connections
COMBINATION VALWE OPERATION
Pressure Differential Switch
The pressure differential switch is connected to the
brake warning light. The switch is triggered by
movement of the switch valve. The purpose of the
switch is to monitor fluid pressure i n the separate
front/rear brake hydraulic circuits.
A decrease or loss of fluid pressure i n either hy­
draulic circuit will cause the switch valve to shuttle
forward or rearward i n response to the pressure dif­
ferential. Movement of the switch valve will push the
switch plunger upward. This closes the switch inter­
nal contacts completing the electrical circuit to the
warning light. The switch valve will remain i n an ac­
tuated position until repair restores system pressures
to normal levels.
Metering Valve
The metering valve is used to balance brake action
between the front disc and rear drum brakes. The
valve meters (holds-off) full apply pressure to the
front disc brakes until the rear brakeshoes are i n full
contact with the drums. The valve is designed to
maintain front brake fluid pressure at 21-207 kPa
(3-30 psi) until the hold-off limit of 807 kPa (117 psi)
is reached. At this point, the metering valve opens
completely permitting full fluid apply pressure to the
front disc brakes.