Steering Column: Description and Operation
Steering Column Switch Cluster
Steering column switch cluster
The steering column switching centre (SZL) has a conventional structure. The steering column switching centre is connected to the attachments by cables. There is no optical signal transfer as on e.g. the BMW 3 Series.
The steering column switching centre has the following tasks:
- Measuring the steering angle and steering-angle speed
- Transferring signals from the control elements in the multifunction steering wheel across the buffer spring cassette
- Picking up signals from the steering column levers
- Providing output of data as CAN messages on the bus or transferring it via direct lines
- Forwarding signals, e.g. driver's airbag and horn
If the vehicle is equipped with optional extra 210 "Dynamic Stability Control", the steering column switching centre is also connected to the chassis CAN (F-CAN).
On vehicles without DSC, the steering column switching centre has no steering-angle sensor.
Brief description of components
The following components are described for the steering column switching centre (SZL):
Light switch and turn indicator / main-beam switch
The steering-column lever on the left of the steering column has the following functions:
- Light switch
- High beam and headlight flasher
- Indicators
- Onboard computer button
The lines for the lighting are routed through the steering column switching centre. From the SZL, the lines lead to the footwell module (FRM), which activates the lighting.
The signal of the onboard computer button is sent on the PT-CAN to the instrument cluster.
Wipe-wash switch
The steering-column lever on the right of the steering column has the following functions:
- Windscreen wiper
- Rear window wiper
- Windscreen washer system, windscreen
- Windscreen washer system, rear window
The signals of the wipe-wash switch are sent by the steering column switching centre across the PT-CAN to the junction box electronics (JBE). The JBE activates the wipe-wash system.
Headlight vertical aim control
The knurled wheel for manual headlight vertical aim control (3 positions) is on the left-hand side trim panel of the steering column.
The signal from the knurled wheel is routed through the steering column switching centre. The footwell module (FRM) activates the headlight vertical aim control.
Steering angle sensor
The steering-angle sensor is in the steering column electronics of the steering column switching centre.
The steering-angle sensor picks up the following variables:
- Steering angle
- Steering-angle speed
- Steering-wheel turns
These variables are required e.g. by the dynamic stability control (DSC) or electromechanical power steering (EPS). The signals are sent from the SZL on the PT-CAN as well as the F-CAN.
Multifunction steering wheel
The multifunction steering wheel is connected via cables and a K bus with the SZL.
The multifunction steering wheel has the following functions:
- MFL button pad on the left with the functions:
station-search audio functions, telephone as well as voice input
Voice input only in conjunction with Car Communication Computer (CCC)
- MFL button pad on the right with the function:
cruise control
The cable from the switch for the horn is routed through the SZL. The signal is forwarded to the junction box electronics.
The cables for the driver's airbag run through the SZL. From the SZL, the cables lead to the multiple restraint system (MRS control unit).
Rocker switches for automatic gearbox
With optional extra 205 "Automatic gearbox", the rocker switches are also integrated on the multifunction steering wheel.
The two rocker switches are identical in structure and switched in parallel.
Pulling one or both of the rocker switches shifts a gear upwards. Pressing one or both of the rocker switches shifts a gear downwards.
The following control units are involved in the functions of the SZL:
FRM: Footwell module
The footwell module (FRM) receives the signals of the turn indicator / main-beam switch. The FRM thus activates the exterior lighting, including the headlight vertical aim control.
JBE: Junction Box Electronics
The junction box electronics activate the two horns via a relay in the JBE. In addition, the JBE supplies the steering column switching centre with terminal 30.
DSC: Dynamic Stability Control
The DSC requires the signals of the steering-angle sensor. These signals are sent on the F-CAN.
EPS: electromechanical power steering
The electromechanical power steering (EPS) also uses the signals of the steering-angle sensor. The signals are sent on the PT-CAN.
In the case of a steering column switching centre without steering-angle sensor, the EPS calculates the steering angle from other measured variables.
MRS: Multiple restraint system
The multiple restraint system activates the driver's airbag. To this end, the lines are routed through the steering column switching centre to the multifunction steering wheel.
EGS: Electronic gearbox control
The Electronic Gearbox Control (EGS) receives the signals of the rocker switches from the steering column switching centre. This enables the EGS to shift the gears of the automatic gearbox.
KOMBI: Instrument cluster
The instrument cluster receives the signals for activation of warning and indicator lamps on the body CAN.
System functions
The following system functions are described for the steering column switching centre:
- Design of the variants
- Signal transmission
- Undervoltage detection and overvoltage detection
Equipment variations
The variants for the steering column switching centre (SZL) depend on the options fitted. The SZL is available without steering-angle sensor (basic version) and with steering-angle sensor with dynamic stability control (High equipment).
The following optional extras expand the variants:
- Rain / driving light sensor
- Bi-xenon light with automatic headlight vertical aim control
- Dynamic Stability Control (DSC)
- Multifunction steering wheel
- Automatic gearbox with rocker switches on the steering wheel
Signal transmission
The steering column switching centre can be connected to the following bus systems:
- Powertrain-CAN (PT-CAN) already in the basic version
- Chassis CAN (F-CAN) with dynamic stability control
- Body bus (K-Bus = sub-bus) for the cruise-control system with multifunction steering wheel
The K bus runs from the steering column switching centre to the right-hand MFL button pad. The left-hand MFL button pad is connected via the right-hand MFL button pad.
The buffer spring cassette transfers electrical signals inside the steering wheel and from the steering wheel.
Undervoltage detection and overvoltage detection
The undervoltage detection and overvoltage detection range from 9 Volts to 16 Volts.
Outside of this range, no fault is stored in the fault code memory. Exceptions are fault code memory entries that refer directly to undervoltage or overvoltage.
Notes for Service department
General information
The steering-angle sensor in the steering column switching centre can be calibrated for the following angular range: ± 15°.
The steering-angle sensor is fixed in the SZL due to its design.
Diagnosis instructions
A service function is available in the BMW diagnosis system for the steering-angle sensor adjustment.
Diagnosis for the multifunction steering wheel and cruise-control system is implemented in the SZL (fault code memory entries in the event of faults).
If the SZL receives no information from the multifunction steering wheel or cruise-control system (e.g. on starting up):
the messages on the PT-CAN are filled with "Signal invalid".
Notes on coding / programming
The steering column switching centre (SZL) cannot be coded.
In the event of replacement, the steering column switching centre is delivered programmed. The steering-angle sensor in the SZL must be calibrated after replacement.
Switch-on conditions
The steering column switching centre is active under the following conditions:
- Wake-up line switched on, terminal 15 wake-up
No liability can be accepted for printing or other errors. Subject to changes of a technical nature