Isuzu Amigo / Axiom / Trooper / Rodeo / VehiCross. Manual — part 157

5A–6

BRAKE CONTROL SYSTEM

Pressure Isolation (Pressure Maintain)
The electro-hydraulic control unit is activated when the
brakes are applied.
If the information from the wheel speed sensors indicates
excessive wheel deceleration (imminent lockup), the first
step in the anti-lock sequence is to isolate the brake
pressure being applied by the brake pedal.
The microprocessor in the coil integrated module sends a
voltage to the coil to energize and close the isolation
valve. This prevents any additional fluid pressure applied
by the brake pedal from reaching the wheel. With the
isolation valves closed, unnecessary increase in the
brake pressure is prevented.

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Legend

(1) Brake
(2) Master Cylinder

5A–7

BRAKE CONTROL SYSTEM

Pressure Reduction
Once the brake pressure is isolated, it must be reduced to
allow the wheels to unlock. This is accomplished by
dumping a portion of the brake fluid pressure into a low
pressure accumulator.
The microprocessor activates the normally closed dump
valve to open, allowing fluid from the wheels to be
dumped into the accumulator. This is done with very short
activation pulses opening and closing the dump valve
passageway. Brake pressure is reduced at the wheel and
allows the wheel to begin rotating again. The fluid from the
brake piston is stored in the accumulator against spring
pressure and a portion of this fluid also primes the pump.
The dump valves are operated independently to control
the deceleration of the wheel.

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Legend

(1) Brake
(2) Master Cylinder

5A–8

BRAKE CONTROL SYSTEM

Pressure Increase (Re-apply)
The re-apply sequence is initiated to achieve optimum
braking. The isolation valve is momentarily opened to
allow master cylinder and pump pressure to reach the
brakes. This controlled pressure rise continues until the
wheel is at optimum brake output or until the brake
pressure is brought up to the master cylinder output
pressure.
If more pressure is required, more fluid is drawn from the
master cylinder and applied to the brakes. The driver may
feel slight pedal pulsations, or pedal drop, this is normal
and expected.

As fluid is re-applied to the brakes, the wheel speed will
reduce. If the wheels approach imminent lockup again,
the module will isolate, dump and re-apply again. This
cycle occurs in millisecond intervals, allowing several
cycles to occur each second. It is a much faster and more
controlled way of “pumping the pedal”.

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Legend

(1) Brake
(2) Master Cylinder

5A–9

BRAKE CONTROL SYSTEM

Brake Release
At the end of the anti-lock stop, when the brake pedal is
released, the pump will remain running for a short time to
help drain any fluid from the accumulators. As this fluid
returns into the system, the spring forces the piston back
to its original position.
The isolation valve opens and fluid may return to the
master cylinder. Conventional braking is then resumed.

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Legend

(1) Brake
(2) Master Cylinder