Anti-lock Brake System
The Anti-lock Brake System (ABS) is standard equipment on all U.S.
models and the Canadian six-
passenger model. It is optional on the
Canadian seven-passenger model.
The ABS works by measuring how fast the wheels are turning during
braking and comparing their speeds.
If any wheel is rotating much slower
than the others (on the verge of locking up and skidding), the systemreduces hydraulic pressure to that
wheel's brake caliper. When that wheel's speed matches the other
wheels, the system applies normal
hydraulic pressure. This can take
place several times per second at
each wheel. You feel the ABS
working as rapid pulsations in the
brake pedal.
Each wheel has a wheel speed
sensor assembly. As the wheel
rotates, the sensor sends electrical pulses to the ABS control unit. The
pulse frequency varies with the
wheel speed.
The electrical output of the ABS control unit is connected to the
modulator/solenoid unit. During
braking, the ABS control unit monitors the pulse frequencies from
the four wheels. When the control unit detects a wheel locking up, itenergizes the appropriate solenoid in
the modulator/solenoid unit. There are six solenoids: two for each front
wheel, and two for the rear wheels.
The energized solenoid reduces hydraulic pressure to one side of amodulator valve. This, in turn,
reduces hydraulic pressure in the
brake line going to the affected
wheel. When that wheel speeds up because of the reduced braking effort, the control unit de-energizes the solenoid. This builds hydraulic
pressure on the modulator valve.
The pressure increases in the hydraulic line to the wheel.
For the system to react quickly, the
modulator/solenoid unit must have
brake fluid under high pressure.
This is supplied by a piston-type accumulator that is pressurized by
an electric pump. A pressure-sensingswitch on the accumulator controls
this pump.
The control unit also contains error detection circuitry. It monitors the
operation of the wheel sensors,solenoids, pump, and electronics. If
the control unit detects any faults, it shuts off power to the pump motor
and solenoids, disabling the ABS.
The indicator on the instrument panel comes on. The brakes then
work like a conventional system
without anti-lock capabilities.
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