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

MODEL 164.120/122 as of model year 2009 up to model year 2010/YoM 08 model refinement package,/YoM 09 

MODEL 164.822/825 as of model year 2009 up to model year 2010/YoM 08,/YoM 09 model refinement package 

MODEL 164.128/172/177/186 as of model year 2009/YoM 08 model refinement package 

MODEL 164.828/871/886 as of model year 2009/YoM 08 

MODEL 164.156/182 

MODEL 251.020/022/026/122/125/126 as of model year 2009 up to model year 2010/modification year 08,/modification year 09 

MODEL 251.054/056/065/072/077/154/156/165/172 as of model year 2009/modification year 08 

MODEL 251.157 as of model year 2012/YoM 11 

Function requirements, general 

• Circuit 61 ON

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

Speed-sensitive power steering (SPS), general 

The steering wheel's rotary motion is changed to 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 at the steering wheel is increased as per a specified characteristic from 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 is adapted to the respective requirement via the speed-sensitive power steering solenoid valve (Y10). The electronic control takes place through the ESP control unit (N47-5).

The speed-sensitive power steering consists of the following subfunctions:

• Function sequence for SPS [PML] with steering angle 
• Function sequence for SPS [PML] electrically controlled reaction 
• Function sequence for SPS [PML] when driving straight-ahead 

Function sequence for 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 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 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 in a speed-dependent manner.

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 at vehicle speed of v = 0 km/h 
• Function with vehicle speed from 0 km/h < v ≤ 100 km/h
• Function at speed of v > 100 km/h 

Shown on left wheel lock 

Function at vehicle speed of v = 0 km/h 

Shown on left wheel lock 

The SPS [PML] solenoid valve is energized to a maximum and as a result 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 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. Via the constant constriction orifice the oil flow is passed on to the power steering pump reservoir virtually depressurized.

Function with vehicle speed from 0 km/h < v ≤ 100 km/h

Shown on left wheel lock 

As the vehicle speed increases the energizing of the SPS [PML] solenoid valve decreases continuously, and it 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 SPS solenoid valve. This is then passed on as an almost depressurized return flow pressure to the power steering pump reservoir.

Function at speed of v > 100 km/h 

Shown on left wheel lock 

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 regulating valve which causes the pressure applied to the surface of the rotary valve to increase to the greatest degree.

The constant constriction orifice reduces the regulated pressure from the SPS solenoid valve. This is then passed on as an almost depressurized return flow pressure to the power steering pump reservoir.

Function sequence for SPS [PML] electrically controlled reaction 

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 vehicle speed determined by the ESP control unit is assigned a specified current in a performance characteristics map

This is then set in the SPS [PML] solenoid valve and influences the oil flow (hydraulic reaction) 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 for SPS [PML] when driving straight-ahead 

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.

Shown in straight-ahead driving 

	Speed-sensitive power steering (SPS [PML]), location of components		GF46.50-P-0001-01GM
	Electrical function schematic for speed-sensitive power steering (PML)	Model 164	PE46.50-P-2050-97MAA
		Model 251	PE46.50-P-2050-97RAA
	Speed-sensitive power steering (SPS [PML]), block diagram		GF46.50-P-0001-02GM
	CDI control unit, component description	N3/9 With engine 629.9 in model 164.1/8	GF07.16-P-6000OAG
		With engine 642.8 in model 164.1/8, model 251.1	GF07.16-P-6000OGU
		With engine 642.9 in model 164.1/8, model 251.0/1	GF07.16-P-6000OHG
	ME-SFI [ME] control unit, component description	N3/10 With engine 156.980 in model 164.177	GF07.61-P-6000MAG
		With engine 272.9 in model 164.1, 251.0/1	GF07.61-P-6000MIG
		With engine 273.9 in model 164.1/8, 251.0/1	GF07.61-P-6000MLG
		With engine 276.9 in model 251.1	GF07.61-P-6000MMH
	Component description for ESP control unit	N47-5	GF42.45-P-5118GM