The solenoid valves are controlled by the BEU, which receives signals from four sensors that monitor the speed of rotation of each wheel. By comparing these signals, the ECU can determine the speed at which the vehicle is moving. It can then use this speed value to determine when the wheel begins to slow down at an abnormal rate compared to the vehicle's speed, and thus predict when the wheel is likely to lock up. During normal operation, the system operates in the same way as a conventional non-ABS braking system.
If the ECU senses that a wheel is prone to locking, it closes the corresponding outlet solenoid valves in the hydraulic unit, which then separates the brake of the wheel that is prone to locking from the master cylinder, effectively containing the hydraulic pressure.
If the wheel speed continues to decrease at an abnormal rate, the ECU opens the inlet solenoid valves on the corresponding brake and activates the return electric pump, which pumps brake fluid back to the master cylinder, releasing the brake. Once the wheel speed returns to an acceptable value, the pump stops; the solenoids switch again, allowing brake pressure to be applied from the master cylinder to the caliper or wheel cylinder, which then re-applies the brake. This cycle can be executed several times per second.
The operation of the electromagnetic valves and the return pump creates pressure pulses in the hydraulic drive. When the ABS system is operating, these pulses can be felt through the brake pedal.
The operation of the ABS system depends entirely on electrical signals. To prevent the system from responding to any inaccurate signals, a built-in protection circuit monitors all signals received by the electronic control unit. If an inaccurate signal or low battery voltage is detected, the ABS system is automatically disabled, and a warning light on the instrument panel illuminates to inform the driver that the ABS system is not functioning. However, normal braking is still possible.
On later models (starting around 2001) the ABS system is also complemented by other safety systems. This is an Electronic Brake-force Distribution (EE3FD) system that automatically distributes the braking force between the front and rear wheels, and (on some models) electronic Stability Program (ESP), which monitors cornering forces and steering wheel angle, and then applies braking force to the appropriate wheel to improve vehicle stability.
Text taken from the website (www.peugeotbook.ru)
If any of the above systems is faulty, you should contact your Peugeot/Citroen dealer or another specialist who has the appropriate equipment to diagnose and repair the vehicle.