Design and function of electrical components in PARAMETER STEERING - RA4600FBL0020X(46-0020)

Models 140.0 with LH, 140.1 

Fig 2: Block Diagram - Models 140.0 With LH, 140.1

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Models 140.0 with HfM, 140.1 except for 

Fig 3: Block Diagram - Models 140.0 With HfM, 140.1

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig 4: Identifying Module Box

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Electronic control unit (N49/1) 

The electronic PML control unit is accommodated in the module box (F23) - exceptions: models 140 with engine 120 (F23) - , and engines 104, 119 California version (under the right rear seat bench).

Its functions are divided up as follows:

• voltage supply
• speed performance map control
• safety circuit and monitoring device and
• fault memory

Voltage supply 

The PML control unit (N49/1) is supplied with battery voltage via the base module (N16/1) fuse F3. The operating voltage is in the range of 11-14 volts.

Low voltages < 8.0 V cause the PML to cut out; they are recorded by the fault memory and can be read out for diagnosis via pulse output.

After starting the engine, the function of PML is only taken over by the control unit when the alternator voltage is > 8 V.

Fault memory 

The control unit is provided with a fault memory and is thus capable of diagnosis. Faults occurring remain stored even when the ignition is switched off and the battery is disconnected.

Fig 5: Identifying PML Control Unit

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

In this way the voltage supply and, due to the engine running, the drive of the hydraulic pressure supply is confirmed.

The voltage supply is permanently controlled by the monitoring device.

Speed performance map/control 

The vehicle speed is the input value for controlling parameter steering. The vehicle speed is assigned a nominal current value in a performance map which is controlled at the parameter steering (PML) P valve (Y10).

Safety circuit and monitoring device 

There are two separate alternatives for deactivating the parameter steering (PML) P valve (Y10) during fault detection by means of the monitoring device and safety circuit. It is possible to shut off the valve from the positive and negative side. The monitoring device also checks the voltage supply (refer to the section on voltage supply).

Control unit location under the rear seat bench on models 140 with engine 120 and engines 104, 119 Californian version.

Fig 6: Identifying Control Unit

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The parameter steering (PML) P valve (Y10) located on the steering housing controls the steering wheel torque. It is a pulsed current-controlled proportional valve and is actuated by the control unit (49/1) at approx. 6 V.

Fig 7: Identifying P Valve

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Depending on the current, the P valve controls the control orifice and thus the oil flow through the servo spool valves. The oil flow causes a pressure drop at the fixed orifice of the governor slide valve. This differential pressure occurs at the end face of the spool valve, thus producing the required steering wheel torque.

Fig 8: Electrical Actuation Graph

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

If the valve is fully activated (v=0km/h), the control orifice is closed.

No oil flows and the valve spool is only moved against the spring force of the basic load spring (12), this corresponds to the lowest steering wheel torque (parking situation).

Fig 9: Identifying Valve Closed

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

As a function of speed, the P valve progressively opens the control orifice in accordance with a defined characteristic which is stored in the control unit (refer to graph), and thus the oil flow.

By increasing the oil flow the steering wheel torque also increases progressively.

Fig 10: Identifying Valve Open

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

1. Model 140.0 with LH, 140.1 

   On vehicles with ABS 

   The parameter steering (PML) control unit (N49/1) receives a speed signal from the rear axle speed sensor (L6) via the ABS control unit (N30)

   Fig 11: Speed Recording Chart - On Vehicles With ABS

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

   On vehicles with ASR 

   The parameter steering (PML) control unit (N49/1) calculates a vehicle speed signal from the two speed signals.

   By calculating the mean value, the PML control unit can detect a failure between the left and right speed sensor.

   The ASR control unit (N30/1) is fitted in place of the ABS control unit (N30).

   Fig 12: Speed Recording Chart - On Vehicles With ASR

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

   Inductive road speed sensor signal, transmission 

   Fig 13: Inductive Road Speed Sensor Signal, Transmission

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

   A second speed signal is recorded by the inductive road speed sensor (L2) on the transmission and is transmitted directly to the PML control unit (N49/1).

   From the speed signals of the speed sensor and the inductive road speed sensor, transmission (L2), the vehicle speed is calculated and the steering force assistance then controlled.

   The signal from the rear axle is to be viewed as an operating signal and the signal from the transmission as a check signal.

   Two speed signals are necessary for safety reasons in order to differentiate between a vehicle being at a standstill and any failure of a speed signal.

   When the vehicle is stationary, no signal comes from either sensor, maximum steering force assistance is provided.

   In the event of the failure of a sensor, a signal is transmitted from the other sensor and the system is shut off (normal function of power steering).

   Fig 14: Identifying Inductive Road Speed Sensor, Transmission (L2)

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
2. Models 140.0 with HFM, 140.1 except for 

   The inductive road speed sensor (L2) on the transmission for the speed signal is omitted.

   The PML control unit (N49/1) receives a "status signal" for this from the ABS control unit (N30) or ASR control unit (N30/1), which indicates the vehicle status (vehicle stationary or travelling) and indicates that the speed sensors are operational.

   The status signal is derived from all speed sensors (3 or 4), and terminal 61 - engine on/off.

   The following conditions are specified as the status signal:

   • Vehicle stationary (v < 3km/h) 3 Hertz
   • Vehicle travelling (v > 3km/h) 25 Hertz
   • Fault signal (speed sensor defective) DC voltage 11-14 V
     Fig 15: Identifying Front Left Wheel Speed Sensor (L6/1)

     Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

   The following faults result in fault detection and the switching off of the parameter function by the status signal:

   • Speed sensor cable fracture
   • Speed sensor loose contact
   • Incorrect number of teeth - rear axle Different tire sizes -
   • axle
   • Different tire sizes - front/rear

   The front left wheel speed (L6/1), previously on ABS rear center (L6), or on ASR rear left and right (L6/3, L6/4) is used as the actual speed signal.

   Vehicles with ABS 

   Fig 16: Wheel Speed Status Signal (Vehicles With ABS)

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

   Vehicles with ASR 

   Fig 17: Wheel Speed Status Signal (Vehicles With ASR)

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
3. Design and function of speed sensors 

   Rear speed sensor with ABS

   Fig 18: Design Of Rear Speed Sensor With ABS

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

   Rear left and right speed sensor with ASR

   Fig 19: Design Of Rear Left And Right Speed Sensor With ASR

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

   Speed sensors comprise a magnetic core and a coil. They sense the wheel speed on toothed rotors. The speed sensors and rotor are at a set distance from each other.

   If the rotor turns then the magnetic field changes, whereby an alternating voltage is induced in the coil. Its frequency depends on the rotor speed.

   Fig 20: Function Of Speed Sensor

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
4. Design and function of inductive road speed sensor, transmission 

   The inductive road speed sensor, transmission (L2) is located on the rear transmission cover. It provides the signals for the speed check directly to the PML control unit.

   Fig 21: Identifying Inductive Road Speed Sensor, Transmission

   Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The inductive road speed sensor comprises a permanent magnet and a coil which is wound around the permanent magnet. A pulse disk with 4 segments is located on the transmission output shaft.

If a segment is located adjacent to the inductive sensor, the magnetic field over the slight gap is closed. If the segment moves away from the inductive sensor, the magnetic field collapses and an induction voltage is thus induced in the coil.

Fig 22: Function Of Inductive Road Speed Sensor, Transmission

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The next segment closes the magnetic field again and an induction voltage is once again induced in the coil.

This induction voltage has a sinusoidal shape. It is converted into a square-wave voltage in the PML control unit (49/1) and processed as a speed signal check signal in the computer.

Fig 23: Diagnosis Fault Memory On Vehicle

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The PML control unit (49/1) is provided with a fault memory and is thus capable of diagnosis. Faults occurring are stored even when the ignition is switched off.

The fault memory can be read out using an adaptor and a pulse counter or hand-held tester via a test coupling (X11/4) in the module box (engine compartment). The fault path under investigation is displayed with the fault readout. The diagnosis and electrical test program is listed in the Diagnosis Manual Chassis Volume 1, Index 7.1.

Diagnosis test coupling 

Assignment for diagnosis 38-pin (X11/4)

Fig 24: Assignment For Diagnosis 38-Pin (X11/4)

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Parameter steering valve 

The parameter steering valve must only be replaced when the steering gear is removed, paying great attention to cleanliness.

Chassis dynamometer and brake tester 

When driving on the chassis dynamometer and brake tester, there are no special instructions to observe since the parameter steering obtains both speed signals from the drive area (no fault is stored).

Checking fault memory 

The fault memory is to be checked or erased after repair work on the parameter steering.