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

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

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 rack-and-pinion steering gear has a constant gear ratio, i.e. the tooth pitch is identical across the entire toothed rack.

Steering assistance is executed hydraulically by a belt driven power steering pump.

On model 204.001/002/003/006/025/044/045/047/048/049/ 065/092/201/202/203/225/245/247/248/249/292 and as of 1.9.08 on model 204.041/046/052/054/056/241/246/252/ 254/256 the volume flow is regulated to suit the requirement via the power steering pump quantity control valve fitted to the power steering pump (Y10/1). As a result, the flow pressure in the servo system is reduced if power-assisted steering is not required, thereby reducing energy consumption. The volume flow is regulated by the Electronic Stability Program control unit (N30/4).

To cool the power steering fluid, an oil cooler in the form of a coiled pipe is fitted immediately upstream of the radiator. The oil cooler version depends on the engine mounted in each particular vehicle.

The steering assistance is regulated by the rack-and-pinion steering gear itself. The hand force to be applied to the steering wheel is increased as per a specified characteristics map from a vehicle standstill up to a speed of v = 100 km/h. The hydraulic reaction assembly is adapted to the respective requirement via the speed-sensitive power steering solenoid valve (Y10). Electronic control is provided by the Electronic Stability Program control unit.

The speed-sensitive power steering system consists of the following subfunctions:
^ Speed-sensitive power steering turn operation function sequence
^ SPS [PML] electrically controlled reaction function sequence
^ Rack-and-pinion steering straight-ahead driving function sequence

Speed-sensitive power steering turn operation 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 according to the vehicle speed. Regulation is effected by the speed-sensitive power steering solenoid valve, which is actuated by the Electronic Stability Program control unit according to 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 < v s100 km/h
^ Function with vehicle speed v > 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 exerted on the reaction balls, no additional torque acts 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 < v s100 km/h
As the vehicle speed increases, energization of the speed-sensitive power steering solenoid valve decreases continuously; the valve 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 > 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
Electronic control of the speed-sensitive power steering is integrated in the Electronic Stability Program 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).

In the Electronic Stability Program control unit, the determined vehicle speed is assigned a nominal current via 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 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.