Dodge 2000 DURANGO Service Manual page 196

5 - 42 BRAKES
DESCRIPTION AND OPERATION (Continued)
During normal braking, the HCU solenoid valves
and pump are not activated. The master cylinder and
power booster operate the same as a vehicle without
an ABS brake system.
The valve body contains the solenoid valves. The
valves modulate brake pressure during antilock brak-
ing and are controlled by the CAB.
The HCU provides three channel pressure control
to the front and rear brakes. One channel controls
the rear wheel brakes in tandem. The two remaining
channels control the front wheel brakes individually.
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.
During antilock braking, solenoid valve pressure
modulation occurs in three stages, pressure decrease,
pressure hold, and pressure increase. The valves are
all contained in the valve body portion of the HCU.
PRESSURE DECREASE
The inlet valve is closed and the outlet valve is
opened during the pressure decrease cycle.
A pressure decrease cycle is initiated when speed
sensor signals indicate high wheel slip at one or
more wheels. At this point, the CAB closes the inlet
to prevent the driver from further increasing the
brake pressure and locking the brakes. The CAB
then opens the outlet valve, which also opens the
return circuit to the accumulators. Fluid pressure is
allowed to bleed off (decrease) as needed to prevent
wheel lock.
Once the period of high wheel slip has ended, the
CAB closes the outlet valve and begins a pressure
increase or hold cycle as needed.
PRESSURE HOLD
Both solenoid valves are closed in the pressure
hold cycle. Fluid apply pressure in the control chan-
nel is maintained at a constant rate. The CAB main-
tains the hold cycle until sensor inputs indicate a
pressure change is necessary.
PRESSURE INCREASE
The inlet valve is open and the outlet valve is
closed during the pressure increase cycle. The pres-
sure increase cycle is used to counteract unequal
wheel speeds. This cycle controls re-application of
fluid apply pressure due to changing road surfaces or
wheel speed.
WHEEL SPEED SENSOR
DESCRIPTION
The ABS brake system uses 3 wheel speed sensors.
A sensor is mounted to each front steering knuckles.
The third sensor is mounted on top of the rear axle
differential housing.
OPERATION
The WSS consists of a magnet surrounded by
windings from a single strand of wire. The sensor
sends a small AC signal to the CAB. This signal is
generated by magnetic induction. The magnetic
induction is created when a toothed sensor ring
(exciter ring or tone wheel) passes the stationary
magnetic WSS.
When the ring gear is rotated, the exciter ring
passes the tip of the WSS. As the exciter ring tooth
approaches the tip of the WSS, the magnetic lines of
force expand, causing the magnetic field to cut across
the sensor's windings. This, in turn causes current to
flow through the WSS circuit (Fig. 3) in one direc-
tion. When the exciter ring tooth moves away from
the sensor tip, the magnetic lines of force collapse
cutting the winding in the opposite direction. This
causes the current to flow in the opposite direction.
Every time a tooth of the exciter ring passes the tip
of the WSS, an AC signal is generated. Each AC sig-
nal (positive to negative signal or sinewave) is inter-
preted by the CAB. It then compares the frequency of
the sinewave to a time value to calculate vehicle
speed. The CAB continues to monitor the frequency
to determine a deceleration rate that would indicate
a possible wheel-locking tendency.
The signal strength of any magnetic induction sen-
sor is directly affected by:
• Magnetic field strength; the stronger the mag-
netic field, the stronger the signal
• Number of windings in the sensor; more wind-
ings provide a stronger signal
• Exciter ring speed; the faster the exciter ring/
tone wheel rotates, the stronger the signal will be
• Distance between the exciter ring teeth and
WSS; the closer the WSS is to the exciter ring/tone
wheel, the stronger the signal will be
The rear WSS is not adjustable. A clearance speci-
fication has been established for manufacturing toler-
ances. If the clearance
specifications, then either the WSS or other compo-
nents may be damaged. The clearance between the
WSS and the exciter ring is 0.005 – 0.050 in.
DN
is not within these