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

GF46.50-P-0001RI Speed-sensitive power steering (SPS), function
- refinement package
- as of model year 09

General function requirements
^ 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 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 SPS [PML] solenoid valve (Y10). Control takes place by the ESP, SPS [PML] and BAS control unit (N47-5).

The system is made up of the following component functions:
^ Function sequence on steering lock
^ Function sequence, electrically controlled feedback
^ Function sequence when driving straight ahead

Function sequence on steering lock
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 to match vehicle speed. Regulation takes place via the SPS solenoid valve which is actuated accordingly by the ESP, SPS [PML] and BAS control unit.

The speed is determined by the ESP, SPS [PML] and BAS control unit by evaluating the signals from the following components: - Left front rpm sensor (L6/1)
- Right front rpm sensor (L6/2)
- Left rear rpm sensor (L6/3)
- Right rear rpm sensor (L6/4)

The conditions described in the following differ on steering lock:
^ Function with vehicle speed v = 0 km/h
^ Function at vehicle speed v > 0 km/h and ≤ 100 km/h
^ Function with vehicle speed v > 100 km/h

Component Identification:

Function with vehicle speed v = 0 km/h
The SPS [PML] solenoid valve is energized to a maximum and as a result closed against the force of the integrated pressure 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 at vehicle speed v > 0 km/h and ≤ 100 km/h
As the vehicle speed increases the flowing of the SPS [PML] 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 SPS 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 > 100 km/h
The SPS [PML] solenoid valve is energized to the minimum extent. As a result the highest possible pressure is led from the SPS [PML] solenoid valve to the reaction balls via the control valve which 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 SPS solenoid valve. This is then passed on as an almost depressurized backflow to the power steering pump reservoir.

Function sequence, electrically controlled feedback
The electronic control system of the speed-sensitive power steering is integrated in the ESP and BAS control unit. A certain manual force is set at the steering wheel 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 ESP, SPS [PML] and BAS control unit is assigned a specified current in a performance map. This is then set in the SPS [PML] 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.

The actuation of the SPS [PML] solenoid valve is pulsed. The current is regulated by pulse-width modulation of these pulses.

Component Identification:

Function sequence when driving straight ahead
When driving straight ahead (steering wheel in center position) the rotary valve of the rotary slide shift 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 with the rotary valve and on the other side with the drive pinion.