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

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


General function requirements
^ Circuit 61 ON

The circuit 61 status is sent to the chassis CAN (CAN E) by the CDI control unit (N3/9) (with diesel engine) or by the ME-SFI [ME] control unit (N3/10) (with gasoline engine).

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 used at the steering wheel is increased as per a specified characteristic from a vehicle standstill up to a vehicle speed of v = 100 km/h.

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 requirement via the speed-sensitive power steering solenoid valve (Y10). Electronic control is performed by the ESP control unit (N47-5).

As of 1.9.08 direct steering is used, which exhibits the following features:
- More responsive steering behavior, especially in urban traffic, when maneuvering or on twisty roads, has been achieved by modification of the toothed rack and as a result of the associated increase in gear ratio from the center position as of approx. 6 degrees steering angle
- Improved comfort, especially when parking, by reducing the steering wheel turns from 2.82 to 2.16 from end stop to end stop.

The SPS [PML] system is comprised of the following partial functions:
^ Function sequence - SPS [PML] with steering angle
^ SPS [PML] electrically controlled reaction function sequence
^ Function sequence - SPS [PML] when driving straight ahead

Function sequence - SPS [PML] with steering angle
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 flows into the right working cylinder (left wheel lock) or the left working cylinder (right wheel lock) and induces axial movement in the gear 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 according to the vehicle speed. It is controlled via the SPS [PML] solenoid valve, which is actuated by the ESP control unit according to vehicle speed.

The vehicle speed is calculated by the ESP control unit by evaluating 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)

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








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.





Function with vehicle speed 0 km/h < v ≤ 100 km/h
As the vehicle speed increases the current supply to the speed-sensitive power steering solenoid valve decreases continuously, and 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.





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.

SPS [PML] electrically controlled reaction function sequence
The electronic control of the SPS [PML] is integrated in the ESP 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 driving speed determined by the ESP 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 flow control results from the pulse width modulation (PWM) of this cycling.

Function sequence - SPS [PML] 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 to the rotary valve and on the other side to the drive pinion.