Hydraulic unit

The hydraulic unit (2) of the anti-lock brake system is located in the engine compartment at the rear left. If the protective cover (1) is removed, the solenoid valve relay (3) and the return pump relay (4) are visible.

It is located in the engine compartment at the rear left: the hydraulic unit is between the brake line from the master cylinder and the brake lines to the wheel brakes. According to commands from the electronic control unit, brake pressure is held constant, reduced or built up again. However, this pressure cannot be higher than that generated in the master cylinder after the driver presses the brake pedal. Pressure is regulated by four fast-switching solenoid-operated valves, one per wheel.

If no power is supplied to the valves, pressure increases. At maximum power supply pressure is reduced, at medium supply pressure is held constant. The pressure reduction phase is particularly interesting: since brake fluid coming from the master cylinder cannot simply be allowed to flow into the reservoir (otherwise the brake pedal would simply drop), a powerful return pump pumps the brake fluid back to the master cylinder. You notice this process through the brake pedal, which begins to pulsate slightly when the return pump is running, i.e. when ABS is active. If you listen carefully you can also hear the pump noise. Sound insulation in the hydraulic unit slightly dampens the pump noise.

On the hydraulic unit, at the top, there are two relays in addition to the plug connector. The larger of them supplies power to the return pump; the smaller relay carries the control for the solenoid valves.