DaimlerChrysler LH New Yorker 1994, Электронное руководство по обслуживанию

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traction control). The inlet valves are spring loaded
in the open position and the outlet valves are spring
loaded in the closed position. During an anti-lock
stop these valves are cycled to maintain the proper
slip ratio for each channel. If a wheel locks, the inlet
valve is closed to prevent any further pressure in-
crease. Then the outlet valve is opened to release the
pressure back to the reservoir until the wheel is no
longer slipping. Once the wheel is no longer slipping
the outlet valve is closed and the inlet valve is
opened to reapply pressure. If the wheel is decelerat-
ing within its predetermined limits (proper slip ra-
tio), both valves will close to hold the pressure con-
stant. During the stop, the build pressure is initially
supplied by the master cylinder. When the build
pressure is reapplied, fluid comes from the master
cylinder which causes the brake pedal to drop. The
pedal continues to drop until the Pedal Travel Sen-
sor signals the (CAB) to turn on the pump/motor.
The pump/motor then pumps fluid from the reservoir
into the brake system. The master cylinder pistons
and brake pedal are then moved back until the
Pedal Travel Sensor signals the (CAB) to turn off the
pump/motor.
On vehicles equipped with traction control, the
valve block assembly also contains the two Traction
Control (Isolation) valves and a pressure switch. The
valves are spring loaded in the open position. During
traction control operation the traction control valves
are closed by the (CAB). This hydraulically isolates
the front brake channels from the master cylinder,
and the rear brake channels. The (CAB) activates the
motor/pump to run continuously by grounding the
motor/pump relay coil. By opening and closing the
inlet and outlet valves the (CAB) can apply and re-
lease the drive wheel calipers as needed to control
wheel spin. Two pressure relief valves allow unused
fluid volume/pressure to return to the reservoir. The
relief valves are needed because the pump can supply
more volume than the system requires.
The pump/motor assembly (Fig. 4) consists of an
electric motor with a rotation sensor and a dual pis-
ton pump. The motor shaft inside the pump is de-
signed as a cam in order to drive both pistons. The
rotation sensor is an inductive pick-up used by the
(CAB) to monitor pump/motor operation.
The reservoir at the H.C.U. (Fig. 4) includes a fluid
level switch (FLS2). This reservoir is connected to the
master cylinder reservoir by a low pressure hydraulic
tube. The (CAB) monitors the fluid level via the fluid
level switch (FLS2). When the reservoir is properly
filled the fluid level switch is closed. If the fluid level
falls below an acceptable level, the fluid level switch
will open. When the (CAB) detects a low fluid condi-
tion (FLS2 OPEN) it will inhibit the anti-lock and
traction control function, and turn on the ABS and
Traction Control warning lamps.
WHEEL SPEED SENSORS
A Wheel speed sensor (Fig. 5) is located at each
wheel to transmit road wheel speed information to
the Controller Anti-Lock Brake (CAB).
ANTI-LOCK BRAKES OPERATION AND
PERFORMANCE
This Anti-Lock Brake System represents current
state-of-the-art in vehicle braking systems. It offers
the driver increased safety and vehicle control during
hard braking. This is accomplished by a sophisticated
system of electrical and hydraulic components. As a
result, there are a few performance characteristics
that may at first seem different but should be consid-
ered normal. These characteristics are discussed be-
Fig. 3 ABS Master Cylinder And Fluid Reservoir
FLS-1
Fig. 4 Hydraulic Control Unit (HCU)
ANTI-LOCK BRAKE SYSTEM 5 - 77