Speed-sensitive power steering (SPS), function (Version A) - GF46.50-P-0001CE

MODEL 204.0/2 with CODE (213) Speed-sensitive power steering as of Model Year 2009/modification year 08 

MODEL 204.901/956/981/983/987/988/992 with CODE (213) Speed-sensitive power steering 

MODEL 204.302/303/347/349/357 with CODE (213) Speed-sensitive power steering 

Function requirements general 

• 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.

The Electronic Stability Program control unit (N30/4) will be introduced into service as of 1.3.2011 (model 204.0/2) or as of 1.6.2011 (model 204.3) in 2 variants:

• ESP control unit, basic variant (except code (233) DISTRONIC PLUS)
• ESP control unit, premium variant (with code (233) DISTRONIC PLUS)

Up to 28.2.2011 or 31.5.2011 only the ESP control unit will be used in the basic version.

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 204.041/046/052/054/056/241/246/252/254/256/302/303/347/349/357 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.

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
• Function sequence for SPS [PML] electrically controlled reaction
• 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 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 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 ≤ 100 km/h
• Function with vehicle speed v > 100 km/h

Shown on left wheel lock 

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

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.

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

Function with vehicle speed 0 km/h < v ≤ 100 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 led to the reaction balls via the regulating valve, causing 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.

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

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.

Function sequence for SPS [PML] electrically controlled reaction 

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.

Rack-and-pinion steering straight-ahead driving function sequence 

When driving straight ahead (steering wheel in center position) the rotary slide of the rotary slide valve is in the neutral position. The control grooves of the rotary valve and the pilot bushing are aligned such that the oil flow supplied by the power steering pump is routed straight back via 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.

	Electrical function schematic for speed-sensitive power steering (PML)		PE46.50-P-2050-97FAA
	Speed-sensitive power steering (SPS [PML]), location of components	Model 204.007/008/022/077/081/085/087/089	GF46.50-P-0001-01CE
		Model 204.041/046/052/054/056 up to 31.8.08
		Model 204.207/208/222/277/289
		Model 204.241/246/252/254/256 up to 31.8.08
		Model 204.956/981/983/987
		Model 204.006/044/045/065	GF46.50-P-0001-01CF
		MODEL 204.041/046/052/054/056/241/246/252/254/256 as of 1.9.08
		Model 204.245
		Model 204.001/002/003/025/047/048/049/092/201/202/203/225/247/248/249/292 as of model year 2010/YoM 09
		Model 204.302/303/347/349/357
	Speed-sensitive power steering (SPS [PML]), block diagram		GF46.50-P-0001-02CE
	Overview of system components for speed-sensitive power steering (SPS [PML])		GF46.50-P-9999CE