GF46.50-P-0001FL Speed-Sensitive Power Steering (SPS), Function

GF46.50-P-0001FL Speed-sensitive power steering (SPS), function
- with CODE (213) Speed-sensitive power steering

Function requirements for speed-sensitive power steering (PML) in general
^ Status "Circuit 61 ON"

Speed-sensitive power steering (PML) in general
The steering wheel's rotary motion is converted into horizontal movement through the steering gear of the rack-and-pinion steering.

The steering gear of the rack-and-pinion steering has a variable gear ratio.

The gear ratio increases continuously from the center and reaches its maximum value at a steering wheel angle of 90 degrees.

The steering assistance is regulated by the rack-and-pinion steering gear itself.

It is performed hydraulically by a belt driven power steering pump.

The manual effort required to turn the steering wheel is increased from vehicle standstill up to a speed of v = 100 km/h in accordance with a given characteristic.

To cool the power steering fluid an oil cooler in the form of a coiled pipe is installed immediately upstream of the A/C system's condenser.

The hydraulic reaction assembly is adapted to the respective requirements via the speed-sensitive power steering solenoid valve (Y10). The volumetric flow is regulated to match demand through the quantity control valve installed on the power steering pump (Y10/1) (except model 212.077). The electronic control is provided by the Electronic Stability Program control unit (N30/4) (except code (233) DISTRONIC PLUS) or the Premium Electronic Stability Program control unit (N30/7) (with code (233) DISTRONIC PLUS).

The system is made up of the following component functions:
^ Speed-sensitive power-steering steering angle, function
^ SPS electrically controlled centering, function
^ Rack-and-pinion steering, function when driving straight ahead

Speed-sensitive power-steering steering angle, function
If the steering wheel is turned, then the drive pinion is supported on the toothed rack. The rotary valve is twisted against the pilot bushing surrounding it via the stabilizer bar and this changes the position of the control grooves in relation to one another. The pressure oil flow now reaches the right working cylinder (left wheel lock) or the left working cylinder (right wheel lock) and causes the axial movement of the gear rack to the right or left.

The oil from the unpressurized working cylinder is displaced. It flows back into the power steering pump reservoir via the open non-return grooves of the pilot bushing.

Steering force support is regulated according to the vehicle speed. This is controlled by the speed-sensitive power steering solenoid valve, which is actuated to match the speed by the Electronic Stability Program control unit (except code (233) DISTRONIC PLUS) or by the Premium Electronic Stability Program control unit (with code (233) DISTRONIC PLUS).

A differentiation is made here between the following conditions:
^ Function with vehicle speed v = 0 km/h
^ Function with vehicle speed 0 km/h less than v ≤ 100 km/h
^ Function with vehicle speed v greater than 100 km/h

Component Identification:

Function with vehicle speed v = 0 km/h
The speed-sensitive power steering solenoid valve receives maximum energy and as a result it is closed against the force of the integrated compression spring. The pressure ratios in the reaction chambers are identical since the supply of oil is stopped. No force is applied to the reaction balls, no additional moment arises on the surfaces of the rotary valve.

The steering is light, as the steering torque applied by the driver, consisting of overcoming the force of the torsion bar, is very low. Via the constant constriction orifice the oil flow is passed on to the power steering pump reservoir virtually depressurized.

Component Identification:

Function with vehicle speed 0 km/h less than v ≤ 100 km/h

As vehicle speed increases the supply to the speed-sensitive power steering solenoid valve decreases continuously, this is opened by the force of the integrated compression spring. The regulated pressure is routed to the reaction balls, which causes the pressure applied to the surfaces of the rotary valve to increase.

The force on the torsion bar is increased further as a result and the driver has to operate the steering wheel with a higher steering torque. The constant constriction orifice reduces the regulated pressure from the speed-sensitive power steering solenoid valve. This is then passed on as an almost depressurized backflow to the power steering pump reservoir.

Component Identification:

Function with vehicle speed v greater than 100 km/h
The speed-sensitive power steering solenoid valve receives minimum energy. This in turn causes the maximum possible pressure to be routed by the speed-sensitive power steering solenoid valve to the reaction balls, which causes a maximum increase in pressure to be applied to the surface of the rotary valve.

The constant constriction orifice reduces the regulated pressure from the speed-sensitive power steering solenoid valve. This is then passed on as an almost depressurized backflow to the power steering pump reservoir.

SPS electrically controlled centering, function
The electronic control for the speed-sensitive power steering is integrated into the Electronic Stability Program control unit (except code (233) DISTRONIC PLUS) or in the Premium Electronic Stability Program control unit (with code (233) DISTRONIC PLUS). The steering sensitivity of the steering wheel is adjusted to match the vehicle speed. As vehicle speed increases, the manual effort required to turn the steering wheel increases at the steering wheel (up to v = 100 km/h).

The vehicle speed determined by the Electronic Stability Program control unit (except code (233) DISTRONIC PLUS) or by the Premium Electronic Stability Program control unit (with code (233) DISTRONIC PLUS) is assigned to a specified current in a performance map. This is then set in the speed-sensitive power steering solenoid valve and influences the oil flow (hydraulic reaction assembly) within the rack-and-pinion steering gear and thus the manual force required to turn the steering wheel. Actuation of the speed-sensitive power steering solenoid valve is pulsed. The flow control results from the pulse width modulation (PWM) of this cycling.

Rack-and-pinion steering, function when driving straight-ahead
When driving straight ahead (steering wheel in center position) the rotary valve in the pilot bushing is in neutral.

The control grooves of the rotary valve and the pilot bushing are interdependent to the extent that the oil flow, as supplied by the power steering pump, is routed straight back over the return flow to the reservoir. The working cylinders are therefore unpressurized.

The stabilizer bar is pinned on one side to the rotary valve and on the other side to the drive pinion.