The solenoid valves are controlled by the ECU, which receives signals from four sensors that control the speed of each wheel. By comparing these signals, the BEU can determine the speed at which the vehicle is moving. It can then use this speed value to determine when the wheel starts to decelerate at an abnormal rate compared to the vehicle's speed, and thus predicts when the wheel tends 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 has a tendency to lock up, it closes the appropriate exhaust solenoid valves in the hydraulic unit, which then separates the brake of the wheel that has a tendency to lock up from the master cylinder, effectively containing the hydraulic pressure.
If the wheel speed continues to decrease at an abnormal rate, the ECU opens the intake solenoid valves on the corresponding brake and activates the return electric pump, which pumps the brake fluid back to the master cylinder, releasing the brake. After the wheel speed returns to an acceptable value, the pump stops; the solenoid valves switch again, allowing brake pressure to be applied from the master cylinder to the caliper or wheel cylinder, which then reactivates the brake. This cycle can be executed several times per second.
The operation of the solenoid valves and the return pump creates pressure pulses in the hydraulic actuator. When the ABS system is operating, these pulses can be felt through the brake pedal.
The operation of the ABS system is completely dependent on electrical signals. To prevent the system from responding to any inaccurate signals, a built-in protection circuit monitors all signals received by the ECU. If an abnormal signal or low battery voltage is detected, the ABS system will automatically turn off and the warning light on the instrument panel will turn on to inform the driver that the ABS system is not working. However, normal braking is still possible.
On later models (starting around 2001) the ABS system is also complemented by other safety systems. It's an electronic brake force distribution system (EE3FD), which automatically distributes the braking force between the front and rear wheels, and (on some models) electronic stability program (ESP), which controls the forces generated during cornering and the steering angle, and then applies the braking force to the corresponding wheel to improve vehicle stability.
If a problem occurs in any of the above systems, you should contact your Peugeot/Citroen dealer or another person who has the appropriate equipment to diagnose and repair the vehicle.