Function

POWER STEERING PUMP
Power steering assists the driver during steering and must therefore apply a large part of the steering moment

Any power steeling requires an energy source for this

The hydraulic energy is produced by a power steering pump which is driven by the engine

The power steering pump is designed as a vane-type pump

It meets the requirements which are placed on a power steering system in the vehicle particularly well

Higher volumetric efficiency at idle speed, low output at high speeds, low noise and low pulsation delivery as well as producing higher pressures are the most important points which characterize the vane-type pump

The flow rate of the vane-type pump is designed so that full power steering assistance is achieved at engine idle speed

The flow rate is controlled via the flow rate control valve

The max. pump pressure is controlled via the pressure limiting valve which is integrated in the flow rate control valve

STEERING GEAR
The steering gear is designed as a recirculating ball steering system

The servo section in the steering gear comprises a power piston, which is located as a modular design around the mechanical transmission part in a cylindrical housing and represents the connection to the steering wheel and the wheels

The oil circuit between the power steering pump and steering gear is created by a high pressure and return hose
When the engine is running, oil is delivered through the oil circuit via circulation pressure

The oil pressure required for steering assistance is supplied by a servo valve spool which throttles the oil flow rate and at the same time directs the pressure oil to the correct side of the power piston

The other side of the power piston is simultaneously connected in a Repressurized condition to the power steering pump reservoir via the spool valve

PARAMETER STEERING (SPEED SENSITIVE POWER STEERING)
Parameter Steering (PML) also known as Speed Sensitive Power Steering (SPS) is a speed responsive power steering system
It incorporates conventional servo assistance into its basic function

Vehicle speed serves as the parameter
With the aid of electronics, the hydraulic steering force assistance is precisely adapted to the respective requirement in each defined speed range

Thus when parking high steering force assistance is ensured which then decreases as speed increases

The steering force assistance is no longer reduced at speeds above 80 km/h
The result is that parameter steering does not become excessively difficult to operate as speed increases further

The components for controlling the parameter steering are constantly monitored
If there is a fault in the electronic system, the parameter function is switched off and the basic steering force assistance of power steering is retained

Speed is recorded via the PML electronic control unit which receives the speed signals from the speed sensors
By means of speed detection, a performance map stored in the PML control unit used to control the PML P valve

An electronically controlled control orifice in the PML P valve (in the steering housing) controls the differential pressure at the end faces of the spool valve depending on the actual effective operating pressure and the respective speed
The torque to be applied by the driver at speeds above 80 km/h is therefore approximately 5-6 Nm

At speeds below 80 km/h, the torque at the steering wheel is reduced in accordance with the stored performance map in the PML control unit so that it is only approx. 2-3 Nm during parking (refer to diagram below)