Dodge 2000 DURANGO Service Manual page 195

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DESCRIPTION AND OPERATION (Continued)
During normal braking, the master cylinder, power
booster and wheel brake units all function as they
would in a vehicle without ABS. The HCU compo-
nents are not activated.
The purpose of the antilock system is to prevent
wheel lockup during periods of high wheel slip. Pre-
venting lockup helps maintain vehicle braking action
and steering control.
The antilock CAB activates the system whenever
sensor signals indicate periods of high wheel slip.
High wheel slip can be described as the point where
wheel rotation begins approaching 20 to 30 percent of
actual vehicle speed during braking. Periods of high
wheel slip occur when brake stops involve high pedal
pressure and rate of vehicle deceleration.
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.
CONTROLLER ANTILOCK BRAKES
DESCRIPTION
The CAB is mounted on the top of the hydraulic
control unit (Fig. 2). The CAB operates the ABS sys-
tem and is separate from other vehicle electrical cir-
cuits. CAB voltage source is through the ignition
switch in the RUN position.
OPERATION
The CAB contains dual microprocessors. A logic
block in each microprocessor receives identical sensor
signals. These signals are processed and compared
simultaneously.
The CAB contains a self check program that illu-
minates the ABS warning light when a system fault
Fig. 2 CAB/HCU
1 – CAB
2 – PUMP WIRING
3 – PUMP MOTOR
4 – HCU
is detected. Faults are stored in a diagnostic program
memory and are accessible with the DRB scan tool.
ABS faults remain in memory until cleared, or
until after the vehicle is started approximately 50
times. Stored faults are not erased if the battery is
disconnected.
NOTE: If the CAB needs to be replaced, the rear
axle type and tire revolutions per mile must be pro-
gramed into the new CAB. For axle type refer to
Group 3 Differential and Driveline. For tire revolu-
tions per mile refer to Group 22 Tire and Wheels. To
program the CAB refer to the Chassis Diagnostic
Manual.
HYDRAULIC CONTROL UNIT
DESCRIPTION
The hydraulic control unit (HCU) consists of a
valve body, pump, two accumulators and a motor
(Fig. 2). The assembly is mounted on the driverside
inner fender under the hood.
OPERATION
The pump, motor, and accumulators are combined
into an assembly attached to the valve body. The
accumulators store the extra fluid which had to be
dumped from the brakes. This is done to prevent the
wheels from locking up. The pump provides the fluid
volume needed and is operated by a DC type motor.
The motor is controlled by the CAB.
BRAKES 5 - 41