Speed-sensitive power steering (SPS), function - GF46.50-P-0001FL

MODEL 212 (except 212.095/098/298) with CODE 213 (Speed-sensitive power steering) 

Function requirements for speed-sensitive power steering (PML) in general 

• Circuit 61 ON

Speed-sensitive power steering, 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°.

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 speed-sensitive power steering solenoid valve (Y10). The oil flow is regulated to meet demand through the power steering pump quantity control valve (Y10/1) (except CODE 990) installed on the power steering pump. The electronic control is provided by the Electronic Stability Program control unit (N30/4) (except CODE (233) (DISTRONIC PLUS), except CODE (990) AMG vehicles) or by the Premium Electronic Stability Program control unit (N30/7) (with CODE (233) (DISTRONIC PLUS) or with CODE (990) AMG vehicles).

The system is made up of the following subfunctions:

• Speed-sensitive power-steering steering angle, function 
• SPS electrically controlled centering, function 
• Rack-and-pinion steering, function when driving straight-ahead 

Speed-sensitive power-steering steering angle, function 

If the steering wheel is turned, then the drive pinion is supported on the toothed rack. The rotary valve is rotated over the stabilizer bar against the pilot bushing surrounding it, thus changing the position of the control grooves in relation to one another. The pressurized 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 toothed rack to the right or left.

The oil from the unpressurized working cylinder is displaced. It flows back into the power steering pump expansion reservoir through the open non-return grooves of the pilot bushing.

Steering force support is regulated according to the vehicle speed. Control is through the speed-sensitive power steering solenoid valve, which is actuated by the Electronic Stability Program control unit (except CODE (233) (DISTRONIC PLUS), except CODE (990) AMG vehicles) or by the Premium Electronic Stability Program control unit (with CODE (233) (DISTRONIC PLUS) or with CODE 990 (AMG vehicles) to match the 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 ≤100 km/h 
• Function with vehicle speed v > 100 km/h 

  Shown on left wheel lock 

  Fig 1: Speed-Sensitive Power Steering (SPS) Flow Diagram - Shown On Left Wheel Lock

  Courtesy of MERCEDES-BENZ USA

Shown with left steer and vehicle speed v = 0 km/h 

Fig 2: Speed-Sensitive Power Steering (SPS) Flow Diagram - Shown With Vehicle Speed V = 0 Km/H

Courtesy of MERCEDES-BENZ USA

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 because the oil flow supply has stopped. No force is applied to the reaction balls, no additional moment arises on the surfaces of the rotary valve.

Steering is easy since the steering torque that needs to be applied by the driver to overcome the force of the torsion bar is very low. The oil flows through the constant constriction orifice to the power steering pump expansion reservoir almost depressurized.

Shown with left steer and vehicle speed 0 km/h < v ≤100 km/h 

Fig 3: Speed-Sensitive Power Steering (SPS) Flow Diagram - Shown With Vehicle Speed 0 Km/H < V ≤ 100 Km/H

Courtesy of MERCEDES-BENZ USA

Function with vehicle speed 0 km/h < 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 regulated pressure is routed to the reaction balls, 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 routed practically depressurized as a backflow to the power steering pump expansion reservoir.

Shown with left steer and vehicle speed v >100 km/h 

Fig 4: Speed-Sensitive Power Steering (SPS) Flow Diagram - Shown With Vehicle Speed V >100 Km/H

Courtesy of MERCEDES-BENZ USA

Function with vehicle speed v > 100 km/h 

The speed-sensitive power steering solenoid valve receives minimum energy. This in turn causes the maximum possible pressure to be routed by the speed-sensitive power steering solenoid valve to the reaction balls, which causes a maximum increase in pressure to be applied to the surface of the rotary valve.

The constant constriction orifice reduces the regulated pressure from the speed-sensitive power steering solenoid valve. The oil flow is routed as an almost depressurized backflow to the power steering pump expansion reservoir.

SPS electrically controlled centering, function 

The electronic control of the speed-sensitive power steering is integrated into the Electronic Stability Program control unit (except CODE 233 (DISTRONIC PLUS), except CODE 990 (AMG vehicles) or in the Premium Electronic Stability Program control unit (with CODE 233 (DISTRONIC PLUS) or with CODE 990 (AMG vehicles). The steering sensitivity of the steering wheel is adjusted to match the vehicle speed. 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 speed determined by the Electronic Stability Program control unit (except CODE 233 (DISTRONIC PLUS), except CODE 990 (AMG vehicles) or by the Premium Electronic Stability Program control unit (with CODE 233 (DISTRONIC PLUS) or with CODE 990 (AMG vehicles) is assigned a specified current in a characteristics 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.

Rack-and-pinion steering, function when driving straight-ahead 

When driving straight ahead (steering wheel in center position) the rotary valve in the pilot bushing is in neutral.

The control grooves of the rotary valve and the pilot bushing are aligned such that the oil flow from the power steering pump is routed straight back via the return flow to the expansion 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.