Opel Frontera UE. Manual - part 1377

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

SECTION

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

SECTION

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

SECTION

5A–10

BRAKE CONTROL SYSTEM

System Components

Electronic Hydraulic Control Unit (EHCU), three Wheel 
Speed Sensors, Warning Light, and G-sensor.

Electronic Hydraulic Control Unit (EHCU)

The EHCU consists of ABS control circuits, fault 
detector, and a fail-safe.  It drives the hydraulic unit 
according to the signal from each sensor, cancelling 
ABS to return to normal braking when a malfunction has 
occurred in the ABS.
The EHCU has a self-diagnosing function which can 
indicate faulty circuits during diagnosis.
The EHCU is mounted on the engine compartment rear 
right side.  It consists of a Motor, Plunger Pump, 
Solenoid Valves.
Solenoid Valves: Reduces or holds the caliper fluid 
pressure for each front disc brake or both rear disc 
brakes according to the signal sent from the EHCU.
Reservoir: Temporarily holds the brake fluid that returns 
from the front and rear disc brake caliper so that 
pressure of front disc brake caliper can be reduced 
smoothly.
Plunger Pump: Feeds the brake fluid held in the 
reservoir to the master cylinder.
Motor: Drives the pump according to the signal from 
EHCU.
Check Valve: Controls the brake fluid flow.

ABS Warning Light

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Vehicles equipped with the Anti-lock Brake System have 
an amber “ABS"  warning light in the instrument panel.  
The “ABS" warning light will illuminate if a malfunction  
in the Anti-lock Brake System is detected by the 
Electronic Hydraulic Control Unit (EHCU).   In case of 
an electronic malfunction, the EHCU will turn “ON" the 
“ABS" warning light  and disable the Anti-lock braking 
function.
The “ABS" light will turn “ON" for approximately three 

seconds after the ignition  switch is to the “ON" position.
If the “ABS" light stays “ON" after the ignition switch is 
the “ON" position,  or comes  “ON" and stays “ON" while 
driving, the Anti-lock Brake System should be inspected  
for a malfunction according to the diagnosis procedure.

Wheel Speed Sensor

It consists of a sensor and a rotor.   The sensor is 
attached to the knuckle on the front wheels and to the 
rear axle case on the rear differential.
The rotor is press-fit in the axle shaft.
The flux generated from electrodes magnetized by a 
magnet in the sensor varies due to rotation of the rotor,  
and the electromagnetic induction generates alternating 
voltage in the coil.   This voltage draws a “sine curve"  
with the frequency proportional to rotor speed and it 
allows detection of wheel speed.

G-Sensor

The G-sensor installed inside the EHCU detects the 
vehicle deceleration speed  and sends a signal to the 
EHCU.  In 4WD operation, all four wheels may  be 
decelerated in almost the same phase, since all wheels 
are connected mechanically.
This tendency is noticeable particularly on roads with 
low friction coefficient, and the ABS control is  adversely 
affected.
The G-sensor judges whether the friction coefficient of 
road surface is low or high, and  changes the EHCU's 
operating system to ensure ABS control.

Normal and Anti-lock Braking

Under normal driving conditions, the Anti-lock Brake 
System functions the same as a  standard power 
assisted brake system. However, with the detection of 
wheel lock-up,  a slight bump or kick-back will be felt in 
the brake pedal. This pedal “bump" will be followed by a 
series of short pedal pulsations which occurs in rapid 
succession.  The brake pedal pulsation will continue 
until there is no longer a need for the anti-lock function  
or until the vehicle is stopped. A slight ticking or popping 
noise may be heard during brake applications  when the 
Anti-lock features is being used.
When the Anti-lock feature is being used, the brake 
pedal may rise even as the brakes are being applied.  
This is also normal.  Maintaining a constant force on the 
pedal will provide the shortest stopping distance.

Brake Pedal Travel

Vehicles equipped with the Anti-lock Brake System may 
be stopped by applying normal force  to the brake pedal. 
Although there is no need to push the pedal beyond the 
point where it stops or holds the vehicle, by applying 
more force the pedal will continue to travel toward the 
floor.
This extra brake pedal travel is normal.

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