Dodge RAM TRUCK 1500 1995 Service Manual page 200

5 - 28 REAR WHEEL ANHLOCK BRAKE (RWAL) SYSTEM
B R
Fig. 5 Rear Wheel Speed Sensor Location
ANTILOCK WARNING LAMP
The amber warning lamp is located in the instru­
ment cluster adjacent to the standard brake warning
lamp. The antilock lamp alerts the driver if a system
fault occurs.
The antilock lamp is also used for troubleshooting
purposes. The lamp is in circuit with the control
module which has a self test program. If a system
fault occurs, the test program will flash the lamp
when the diagnostic connector is grounded. The flash
codes are used to identify a problem component.
REAR WHEEL ANTILOCK SYSTEM OPERATION
During light brake application, rear wheel deceler­
ation and/or slip is not sufficient to activate the an­
tilock system components. Brake fluid apply pressure
to the rear wheels remains normal and is not modu­
lated. However, when braking effort, degree of wheel
slip, and rate of deceleration approach programmed
limits, sensor inputs will cause the module to acti­
vate the system.
Normal Braking Mode
In normal braking mode, the antilock solenoid
valves are inactive. The valves are open allowing nor­
mal fluid flow to the rear wheel cylinders.
Start Of Antilock Modi Braking
When high pedal effort braking occurs, the de­
crease in exciter wheel rotating speed is noted and
converted into an electronic signal by the sensor. The
signal is transmitted to and processed by the control
module. The module determines that acceptable lim­
its of wheel slip/deceleration are about to be ex­
ceeded. The module activates the antilock solenoid
valves at this point.
The module transmits a signal that activates the
solenoid valves. The valves then decrease, or isolate
fluid apply pressure to the rear wheel cylinders as
needed.
Closing the valves prevents further fluid flow to
the rear wheel cylinders. This action isolates the rear
brakes from the master cylinder. The net effect is to
decrease rear brake apply pressure to the point
where the wheels will continue to rotate but not lock.
When rear brake pressure requirements return to
normal levels, the solenoid valves are inactivated.
This action allows the resumption of normal fluid
flow to the wheel cylinders.
Solenoid Valve Cycle Times
Activation (opening/closing) of the solenoid valves
is not static during antilock mode braking. Valve op­
eration is continuous as they are rapidly cycled in re­
sponse to sensor inputs and control module
commands. Cycle times are measured in millisec­
onds.
As the demand for antilock mode brake operation
is decreased, the module deactivates the hydraulic
valve components to restore normal brake operation.
COMBINATION WALVE 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 in the separate
front/rear brake hydraulic circuits.
A decrease or loss of fluid pressure in either hy­
draulic circuit will cause the switch valve to shuttle
forward or rearward in 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 in 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 in full