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

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

Function requirement
^ Status "Circuit 61 ON"

Speed-sensitive power steering (SPS), 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 to each side 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 and occurs hydraulically via the power steering pump which is driven by a belt.

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 performance map.

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 volume flow is regulated according to requirements by the power steering pump quantity control valve (Y10/1) attached to the power steering pump. Electronic control is performed by the Electronic Stability Program control unit (N30/4) (except code (233) DISTRONIC PLUS) or by the Electronic Stability Program Premium control unit (N30/7) (with code (233) DISTRONIC PLUS).

The SPS [PML] function is comprised of the following subfunctions:
^ SPS [PML] while steering function sequence
^ SPS [PML] electrically controlled reaction function sequence
^ Rack-and-pinion steering straight-ahead driving function sequence

SPS [PML] while steering function sequence
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 pressurized oil flow is now fed into the right working cylinder (left steer) or into the left working cylinder (right steer) and causes the axial movement of the toothed rack to the right or left. The oil is pushed out of the unpressurized working cylinder and flows back into the power steering pump reservoir via the open return grooves of the pilot bushing.

Steering force support is regulated to match vehicle speed. It is regulated via the speed-sensitive power steering solenoid valve, which is actuated by the Electronic Stability Program control unit (except code (233) DISTRONIC PLUS) or by the Electronic Stability Program Premium control unit (with code (233) DISTRONIC PLUS) according to the vehicle speed. 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 (i.e. the force required to overcome the force of the stabilizer 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 controlled pressure is lead to the reaction balls via the regulating valve 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. As a result maximum pressure is routed from the speed-sensitive power steering solenoid valve to the reaction balls via the regulating valve, which in turn causes the pressure applied to the surface of the rotary valve to increase to a maximum.

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 [PML] electrically controlled reaction function sequence
The electronic control of the SPS [PML] is integrated in the Electronic Stability Program control unit (except code (233) DISTRONIC PLUS) or in the Electronic Stability Program Premium control unit (with code (233) DISTRONIC PLUS).

The sensitivity of the steering wheel is adjusted according to the speed of the vehicle. 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 the Electronic Stability Program Premium control unit (with code (233) DISTRONIC PLUS) is assigned a nominal 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.

Component Identification:

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

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.