Opel Frontera UE. Manual - part 115

5A–6

BRAKE CONTROL SYSTEM

Pressure Isolation (Pressure Maintain)

The electro-hydraulic control unit is activated when the 
brakes are applied which sends a signal to the coil 
integrated module to prepare for a possible anti-lock 
stop.
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, further unnecessary 
increase in the brake pressure is therefore prevented.

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(1) Brake

(2) Master Cylinder

BRAKE CONTROL SYSTEM

5A–7

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. At this point, the brake 
pedal is isolated from the base brake system, the 
hydraulic control unit pumps are primed and the 
attenuators are ready to pump fluid.

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(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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(1) Brake

(2) Master Cylinder

BRAKE CONTROL SYSTEM

5A–9

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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(1) Brake

(2) Master Cylinder