Hydraulic Unit

WARNING: This page does not describe the selected car, but rather 34 other vehicles, including the 2004 BMW M3, 2004 BMW 330xi, 2004 BMW 330i, 2004 BMW 330Ci, and 2004 BMW 325xi. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: Hydraulic units are model specific and are not interchangeable.

Hydraulic unit has 4 channels allowing braking control at individual wheels. To control the braking system pressure, the brake hydraulic unit has a 3-way valve which permits 3 separate brake pressure conditions. Pressure build-up, pressure retention and pressure drop are the 3 pressure phases that adapt to the vehicle braking requirements.

As soon as wheel lock-up is indicated, brake fluid pressure is reduced. If the wheel still tends to lock, pressure will be further reduced until wheel acceleration or the slip limit is detected. The pressure is then raised and the control phases repeated. These pressure changes occur in milliseconds.

The return delivery pump returns brake fluid from the wheel brake cylinder, while dropping the pressure to the appropriate brake circuit. The pump is a 2-piston design so circuits of a dual brake circuit system remain separated.