Dodge Dakota 2001, Service Manual

Page: 198 / 2307

The antilock system prevents 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. Sensors at each front wheel convert 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 con-
trol 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.
OPERATION
When the brakes are applied, hydraulic fluid is
routed from the master cylinder’s secondary circuit,
through the combination valve, to the RWAL valve.
From there hydraulic fluid is routed to the rear
brake wheel cylinders. The Controller Antilock Brake
monitors rear wheel speed through the rear wheel
speed sensor. If a wheel is about to lock-up, the CAB
signals the RWAL valve. The RWAL valve modulates
the hydraulic brake pressure to the rear wheels to
prevent wheel lock-up.
NORMAL BRAKING
During light brake application, rear wheel deceler-
ation is not sufficient to activate the antilock system
components. During a normal stop hydraulic brake
fluid flows unrestricted to the rear wheel cylinders to
stop the vehicle. The antilock solenoid valves are
inactive. The isolation valve is open and the dump
valve is closed allowing normal fluid flow to the rear
wheel cylinders.
REAR WHEEL ANTILOCK BRAKING
If the CAB senses impending rear wheel lock-up, it
will energize the isolation solenoid. This prevents a
further increase of driver induced brake pressure to
the rear wheels. If this initial action is not enough to
prevent rear wheel lock-up, the CAB will momen-
tarily energize a dump solenoid. This opens the
dump valve to vent a small amount of isolated rear
brake pressure to an accumulator. The action of fluid
moving to the accumulator reduces the isolated brake
pressure at the wheel cylinders. The dump (pressure
venting) cycle is limited to very short time periods
(milliseconds). The CAB will pulse the dump valve
until rear wheel deceleration reaches the desired slip
rate programmed into the CAB. The system will
switch to normal braking once wheel locking tenden-
cies are no longer present.
DIAGNOSIS AND TESTING - ANTILOCK
BRAKES
The ABS brake system performs several self-tests
every time the ignition switch is turned on and the
vehicle is driven. The CAB monitors the systems
input and output circuits to verify the system is oper-
ating correctly. If the on board diagnostic system
senses that a circuit is malfunctioning the system
will set a trouble code in its memory.
NOTE: An audible noise may be heard during the
self-test. This noise should be considered normal.
NOTE: The MDS or EDT with J-1850 is used to diag-
nose the ABS system. For additional information
refer to the Antilock Brake section.
DIAGNOSIS AND TESTING - REAR WHEEL
ANTILOCK
Diagnosis of base brake conditions which are
mechanical in nature should be performed first. This
includes brake noise, lack of power assist, parking
brake, or vehicle vibration during normal braking.
The RWAL brake system performs several self-
tests every time the ignition switch is turned on and
the vehicle is driven. The CAB monitors the system
inputs and outputs circuits to verify the system is
operating properly. If the CAB senses a malfunction
in the system it will set a DTC into memory and trig-
ger the warning lamp.
NOTE: The MDS or EDT with J-1850 is used to diag-
nose the RWAL system.
STANDARD PROCEDURES - RWAL SERVICE
PRECAUTIONS
The RWAL uses an electronic control module, the
CAB. This module is designed to withstand normal
current draws associated with vehicle operation.
Care must be taken to avoid overloading the CAB
circuits. In testing for open or short circuits, do
not ground or apply voltage to any of the cir-
cuits unless instructed to do so for a diagnostic
procedure. These circuits should only be tested
AN
BRAKES - ABS 5 - 35
BRAKES - ABS (Continued)