Power steering operation (BMW 7 Series E32)

BMW cars are equipped with a SERVOTRONIC steering system, which consists of an electronic tachometer, a control unit mounted at the left front pillar and an electro-hydraulic converter (on the steering gear).

Operating principle

The speed signal from the tachometer is evaluated by the ECU and transmitted to the converter, which controls the process of draining the pressurized oil from the feedback cavities into the return circuit.

Functioning

When the steering wheel is turned clockwise, the right piston (6) of the valve moves. The oil under pressure is thus discharged into the right section (12) of the cylinder, and also through the right check valve (8) into the right section (4) of the feedback.

In the PARKING state (in this case, no speed signals are received from the tachometer and the converter valve (3) remains closed), oil flows through throttles (10) and (11) into the left compartment (5) of the feedback. The closed left check valve (9) prevents oil from flowing into the left compartment (13) of the cylinder, which is connected to the return circuit and is not under pressure.



Since there is the same pressure in both feedback compartments (4) and (5), there is no counter-torque on the steering wheel. This ensures easy steering.

When driving at medium speeds, the converter valve (3) does not open completely. The reduced pressure is transmitted to the left feedback compartment (5), while the right feedback compartment (4) is under full pressure.

The counteracting moment acting in pistons (6) and (7) of the valve is correspondingly smaller. The steering is strengthened by increasing the clearance.