Combination Switch: Description and Operation
61 07 04 (103)
Steering Column Switch
E70, E71, E81, E82, E87, E90, E91, E92, E93
Introduction
For the first time on a BMW, the E87 steering column switch cluster (SZL) will optically record the steering angle and the positions of the steering column stalks.
With a conventional steering column stalk the electrical contacts are mechanically switched and thus influence the pressure points felt by the driver.
The benefit of optical monitoring is that it allows these pressure points to be more precisely defined. In other words, optical monitoring greatly reduces the level of mechanical influence.
> E70, E71:
> E81, E82, E87, E90, E91, E92, E93:
The steering column switch cluster (SZL) has the following tasks:
- Measuring the steering angle and steering-angle speed
- Picking up signals from controls in the multi-function steering wheel via coil springs
- Picking up signals from the steering column stalks
- Sending data as CAN messages on the BUS or via direct wires
- Looping signals through (e.g. driver's airbag and horn)
Brief description of components
The steering column switch cluster (SZL) is connected to the vehicle wiring harness via 2 plug connectors.
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The steering column switch cluster consists of the following components:
- Evaluation electronics
The evaluation electronics analyse the signals from the components and send them as CAN messages on the F-CAN.
The evaluation electronics include the following components:
- Microprocessor
- Power supply
- F-CAN connection
- Interfaces for optical and electrical input signals
- Steering-angle sensor
The steering-angle sensor optically detects the steering angle turned at the steering wheel by means of an encoded disc.
The data is read and analysed by the evaluation electronics. The data is transmitted directly to the DSC via the F-CAN. (F-CAN = Chassis CAN; DSC = Dynamic Stability Control.)
Signal path:
SZL -> F-CAN -> DSC
- Steering column stalk
The lever positions of all steering column stalks are optically monitored.
The actions of pressing the buttons, turning the thumbwheels and pressing the rocker switches are detected by switching mats. The switching mats forward the position of the buttons and thumbwheels as voltage signals.
Depending on the vehicle's equipment, the steering column switch cluster (SZL) could have the following steering column stalks:
- Wiper/washer switch
The switch signals are read into the steering column switch cluster where they are evaluated. The evaluated switch signals are emitted onto the F-CAN as a CAN message (It is only possible to check the switch positions with the BMW diagnosis system via DSC as the steering column switch cluster is not directly connected to the diagnosis system).
- Cruise-control system steering column stalk
The switch signals are read into the steering column switch cluster where they are evaluated. The evaluated switch signals are emitted onto the F-CAN as a CAN message (It is only possible to check the switch positions with the BMW diagnosis system via DSC as the steering column switch cluster is not directly connected to the diagnosis system).
- Turn-signal/main-beam switch
The switch signals are read into the steering column switch cluster where they are evaluated. The evaluated switch signals are resistance-encoded and transmitted from the steering column switch cluster by direct wires to the FRM (footwell module).
The signals for the on-board computer functions are transmitted by a direct wire to the instrument cluster (KOMBI).
- Coil spring cassette
The coil spring cassette has the task of transferring electrical signals to or from the steering wheel.
The signals are transferred as follows:
- Airbag unit (signals to steering wheel)
Signal path:
MRS control unit -> SZL (signal simply looped through) -> Coil spring -> Airbag unit
- Horn (signal from steering wheel)
Signal path:
Horn switch -> SZL (signal simply looped through) -> Junction box
- Multifunction buttons (signal from steering wheel)
Signal path:
Multifunction buttons -> SZL -> F-CAN -> Junction box electronics (signal simply looped through) -> F-CAN -> DSC control unit (serves as gateway) ->
PT-CAN -> Junction box electronics (serves as gateway) -> K-CAN -> Audio system
(check switch positions with BMW diagnostic system using DSC)
The control units involved in the functions of the steering column switch cluster are as follows:
- DSC: Dynamic Stability Control
The steering column switch cluster (SZL) does not have its own fault memory. All fault data for the steering column switch cluster are stored in the DSC control unit.
The DSC is connected to the steering column switch cluster via the F-CAN (chassis CAN).
- JBE: Junction box electronics
The junction box electronics unit is the data interface (= gateway) between the K-CAN and the PT-CAN. (K-CAN = Body CAN; PT-CAN = Powertrain CAN).
Signals from the F-CAN (Chassis CAN) are simply looped through. This means: Signals from the F-CAN are not received or evaluated by the junction box electronics. The F-CAN cables are simply routed through the junction box housing.
The junction box consists of the junction box electronics and the electrical distribution centre.
- FRM: Footwell module
The footwell module controls the vehicle lighting. The footwell module is connected to the steering column switch cluster (SZL) by direct wires.
- CAS: Car Access System
The Care Access System reactivates the control units on the PT-CAN via the wake-up wire (terminal 15 wake-up).
- KOMBI: Instrument cluster
Information on the on-board computer functions can be called up via the on-board computer button (BC button) and the rocker switch. (The BC button and the rocker switch are integrated into the turn-signal/main-beam switch).
The contents are displayed in the LCD display in the instrument cluster.
The signals for the on-board computer functions are transmitted by a direct wire from the SZL to the instrument cluster.
System functions
The steering column switch cluster (SZL) comprises the following functions:
- Monitoring of steering angle and steering-wheel turns
- Monitoring of signals from coil spring cassette
- Reading of signals from wiper/washer switch and cruise-control system steering column stalk and output of signals on the F-CAN
- Reading of signals from turn-signal/main-beam switch and resistance-encoded output of signals to the FRM
- Reading of signals from the BC switch for on-board computer function and from the rocker switch (on turn-signal/main-beam switch) and resistance-encoded output of signals to the instrument cluster
- Self-diagnosis (fault memory in DSC)
Monitoring of steering angle and steering-wheel turns
The steering column switch cluster (SZL) records the following information for the DSC functions:
- Steering angle
- Steering-angle rate
- Steering-wheel turn
The relative steering angle gives the angular position of the steering wheel. A steering angle of ±180° can be measured. The information always remains stored, even when there is no current applied to the steering column switch cluster.
Zero calibration must be performed with the BMW diagnosis system in the following cases:
- Replacement of SZL (steering column switch cluster)
- Coil spring cassette replacement
- Replacement of DSC control unit
- Working on the steering
Information about steering-wheel rotations
The signal from the steering-angle sensor is repeated every 360°. These signal repetitions are used to count the number of turns of the steering wheel.
The steering column switch cluster has a permanent power source via terminal 30. This allows steering wheel movements to be detected even when the ignition is OFF.
Following a power interruption, the information about the number of turns of the steering wheel is lost.
When a journey is started, the steering column switch cluster will again measure the number of turns of the steering wheel.
If this process has not been completed when a speed of approximately 25 km/h has been reached, the DSC indicator and warning light will light up (repeat the process by restarting).
During measurement, the steering column switch cluster uses various pieces of information, including data from the wheel-speed sensors on the front wheels.
Not all signals are processed in the steering column switch cluster. Some signals are simply looped through the steering column switch cluster (e.g.: driver's airbag, on-board computer function, horn).
Information is transmitted via the F-CAN (Chassis CAN) or via direct wires to the systems or affected control units.
The steering column switch cluster (SZL) is activated:
- From the outside via the wake-up wire (terminal 15 wake-up) with bus activation (e.g. when the driver's door is opened)
- From the inside by steering wheel movements
(without bus activation, only for internal evaluation of the steering angle)
- From the inside by movement of the turn-signal/main-beam switch
(without bus activation, SZL is only active as interface to FRM, e.g. when park lights are switched on)
Notes for service staff
Service staff should note the following points:
- General notes:
- Diagnosis: ---
- Encoding/programming:
Subject to change.