Body Control System Description and Operation Rear Electrical Center
Rear electrical center (REC)
The Rear Electrical Center (REC) is a central module and is located in the luggage compartment on the left-hand side, behind the service flap. It contains a fuse and relay box and a control unit which communicates with the other bus users via the LS CAN bus. The REC controls, amongst other things, the rear lighting, the rear window wiper, the central locking and the electric windows. Various sensors (e.g. tank fill level) are read in and their values output as bus messages by the REC.
The following functions are actuated depending on the corresponding signal input:
Turn signals | Trailer lighting |
Hazard warning lights | Reversing lights |
Parking light left/right | Brake light |
Position light | Rear fog lamp |
Daytime running lamp | Follow Me Home lighting |
Emergency lighting | Bulb check |
Interior lighting | Central locking |
| • | Electric windows function (the REC is the function master for the electric windows. It actuates all servo motors in the doors directly). |
| • | Alarm system function (the REC communicates via a serial UART interface with the ultrasonic alarm system or with the alarm siren). |
Other functions
| • | Tank fill level, the REC reads in the value from the tank fill level sensor and transmits this on the bus. |
| • | Trailer module; a trailer module can be connected to the REC. The wiring harness for the trailer coupling is then connected to this. If the REC detects that a trailer is actually connected, it sends a corresponding message on the bus, actuates the lights on the trailer and will also monitor the bulbs in the trailer lighting. However, the rear fog lamp and the reversing light of the trailer will not be monitored, as most commercially-available trailers are not fitted with this function. |
Body Control System Description and Operation Steering Column Control Module
SCCM
The steering column control module (SCCM) is the module on the steering column which, amongst other things, contains the ignition lock and the turn signal and wiper switches. It combines the following sub-systems (some of which are optional).
Immobilizer, steering wheel controls, transponder aerial, ignition lock, turn signal stalk, main/dipped beam stalk, cruise control stalk, Info Display buttons, windscreen wiper stalk, radio receiver for the key remote control, steering angle sensor, horn button, PEPS start button (passive entry passive start system) and ESCL (electric steering column lock). The SCCM has been developed to reduce the complexity of the wiring and reduce the weight of the steering column.
SCCM Control Unit
The steering column control module (SCCM) control unit has two CAN interfaces, one to the low-speed CAN bus and one to the high-speed CAN bus. It is used as the interface between the high-speed bus and the low-speed bus to exchange information between the CAN buses. All components of the SCCM, except the ignition key with its RF receiver/transmitter (transponder), are directly connected to the SCCM control unit (plug connection) and therefore require no additional wiring.
Voltage Management, Voltage Supply, Ignition Lock
The SCCM control unit is responsible for voltage management within the vehicle. It is responsible for decoding the ignition lock information (ignition key position) and for forwarding this information as CAN information to the rest of the vehicle. In addition, the SCCM actuates, by hard wiring, two power supply relays in the UEC and REC. The ACC (ignition lock position 1) signal is also transmitted by hard wire to the ECM to power it up more quickly. The engine will fire more quickly as a result.
Bus Interface to the High-Speed CAN (HS CAN) and Low-Speed CAN (LS CAN)
| • | The SCCM has two CAN interfaces for data communication. |
| • | High-speed CAN for engine systems, drive train systems, immobilizer, etc. |
| • | Single wire low-speed CAN for body systems. |
| • | Interface between HS CAN and LS CAN. |
| • | The SCCM is the interface (transfer point) between the low-speed bus and the high-speed bus. Selected bus messages are transferred from the low-speed bus to the high-speed bus and vice versa. |
Diagnostics Interface
| • | The SCCM control unit has a CAN diagnostics interface to the HS CAN and the LS CAN. The HS and LS bus lines are looped through the SCCM to the diagnostics plug. Warning: If the SCCM plug is disconnected, the connection between the diagnostics plug and the HS and LS bus is interrupted. |
| • | To avoid damage to control units in the CAN network by external voltage shorts or ground shorts, the SCCM control unit has an internal protective circuit both for the HS CAN and the LS CAN. Diagnosis is also carried out by means of the CAN bus. |
Dimming the Steering Wheel Controls Illumination
The buttons for the steering wheel controls are illuminated. This illumination is dimmed in response to information which is transmitted by the IPC to the SCCM via the LS CAN.
Turn Signals Function
The SCCM control unit records 4 positions of the turn signal stalk in digital form. The turn signal stalk no longer engages mechanically and instead returns to its initial position, even when the turn signals are switched on. They are switched off either by the steering angle sensor or by moving the stalk in the opposite direction. This information is transmitted by the SCCM control unit via the LS CAN to the REC which then specifies the flash frequency and the duty factor.
Cruise Control Function
The SCCM control unit records the 3 positions of the cruise control switch in digital form. This information is transmitted by the SCCM control unit to the engine control module (ECM) control unit via the HS CAN.
Windscreen Wiper Function
The SCCM control unit records the 6 positions of the windscreen wiper stalk in digital form. This information is transmitted by the SCCM control unit to the UEC control unit via the LS CAN. The wiper stalk no longer engages mechanically; instead, it returns to its initial position even when the wipers are switched on. It is switched off by moving the lever in the opposite direction.
Horn Function
The SCCM control unit records the position of the horn button. This information is transmitted by the SCCM control unit to the UEC control unit via the LS CAN.
Immobilizer Function (Immobilizer 3)
An immobilizer function (immobilizer 3) is incorporated into the SCCM control unit. The SCCM control unit communicates with the transponder in the ignition key with the aid of the transponder aerial connected to it. It evaluates the transponder signal. Even if the ignition key is recognized as valid, the identifiers of the REC, UEC, instrument panel cluster (IPC) control units are still checked. If at least 2 are recognized as valid components, the release procedure continues and communication is carried out with the ECM control unit with ignition ON. The ECM control unit requests a start release from the SCCM control unit. If release fails, the ignition key must be removed from the ignition lock so that the Key IN status can be detected again and a new release procedure started by the transponder. The conversion from analogue to digital signals is carried out by the SCCM control unit.
Loudspeaker Function
At the request of the UEC, REC, PAS, PEPS, IPC, DIS, SDM, SLM or TPMS control units, the SCCM triggers an acoustic signal from the integral loudspeaker. This signal is specific for each function. For example, the REC control unit may send a message to the SCCM indicating the frequency at which the turn signal tone should be output (normal flash frequency or faster flash frequency if turn signal is faulty). Other functions involving the sending of a sound output request to the SCCM control unit include parking light activated, door, boot or tailgate open, Park Assist, adaptive cruise control, windscreen wiper switch off and deactivate transport mode.
Steering Wheel Controls Function (Optional Equipment)
The SCCM control unit records which button on the steering wheel controls has been actuated - if several buttons have been pressed at the same time, the button with the greatest priority is always evaluated. This information is transmitted by the SCCM control unit to the DIS control unit via the CAN. Some of this information is then forwarded to the radio control unit via the DIS interface.
Info Display Function (Optional Equipment)
The SCCM control unit records that the Info Display operating switch has been actuated. This information is transmitted by the SCCM control unit to the DIS control unit via the CAN.
Steering Wheel Position Function
The SCCM module has a steering angle sensor. This sensor records the current steering angle or the current steering movement and transmits this information to the SCCM control unit. The steering angle sensor signal is used internally by the SCCM control unit to reset the turn signal. The SCCM control unit transmits the steering angle information to the ABS/ESP control unit via the HS CAN if the vehicle has ESP (optional). The EHPS also uses this information. For the ESP version, the sensor is "multi-turn" capable, i.e. it can indicate the exact angular position across the entire steering range. Vehicles without ESP are fitted with a simplified version which, after 360° starts again at 0.
Air Bag Firing Signal
The air bag firing circuit is connected through to the driver's air bag (to the ignition pellet) in the steering wheel via the contact unit of the SCCM control unit.
Key Remote Control Function
The SCCM control unit has a receiver which converts the key remote control signal. The SCCM control unit evaluates this information and transmits it to the REC via the CAN.
Main/dipped Beam Function
The SCCM control unit records the 2 positions of the main/dipped beam switch. This information is transmitted by the SCCM to the UEC control unit via the CAN.
Key Locking Function
The key locking function is only active on vehicles with automatic transmission. The TCM control unit transmits the current selector lever position to the SCCM control unit via the HS CAN. If the current selector lever position is P, the SCCM deactivates the key lock and the ignition key can be removed.
Starter Control Function
If the SCCM detects the "engine start" ignition key position, it sends a CAN message to the ECM to actuate the starter. After receiving this message, the ECM sends another bus message to the UEC via the SCCM interface. The UEC then actuates the starter relay incorporated in it. Direct actuation of the start relay by the SCCM without the "diversion" via the ECM would not be sensible as the ECM decides whether the starter should be actuated (if the engine is already running, the ECM will not send a bus message to the UEC to actuate the starter, even if the driver has turned the key to the "Start" position).
Active/Select One-Touch Buttons Function
The SCCM records the actuation of the Active/Select one-touch buttons in the steering wheel and transmits the gearshift request to the TCM / MTA via the HS CAN.
Body Control System Description and Operation Underhood Electrical Center
Under hood Electrical Center (UEC)
The UEC is a central module in the engine compartment near to the battery. It contains a fuse and relay box as well as a control unit. The control unit communicates with other bus users via the LS CAN bus. The UEC controls, amongst other things, the front lighting, the windscreen wipers and the horn. Various sensors are read (e.g. brake fluid level sensor) and their values are output by the UEC as bus messages.
Wiper Function
The UEC is the centre of all actuated components:
| • | Front wiper relay |
| • | 4 wiper stages (one-touch wipe, intermittent, slow wipe, fast wipe). If the drivers door is opened when the fast stage is selected, the wiper is automatically reset to the slow stage. This function is programmable. |
| • | Rain/light sensor. |
| • | Intermittent function (with rain sensor fitted). |
| • | Wiper service position: After the ignition has been switched off and the key is in position "0", the wipers can be moved to a service position (for example to change the wipers) by actuating the wiper switch in the SCCM. |
| • | Rear wiper. |
| • | Windscreen wash system. |
| • | Rear window wash system (If the washer fluid level is too low, the rear window wash system will be deactivated). |
| • | Headlamp wash system (If the washer fluid level is too low, the headlamp wash system will be deactivated). |
Exterior Lighting Function
The following functions are actuated depending on the corresponding signal input:
| • | Turn signal function. |
| • | Parking light left/right. |
| • | Position light. |
| • | Dipped beam. |
| • | Daytime running lamps (programmable according to country). |
| • | Automatic light function (if fitted with rain/light sensor). |
| • | Lights-on warning (if position or dipped beam lights are on and the drivers door is opened with the key removed from the ignition, a warning buzzer will sound). |
| • | Emergency lighting (if, when the ignition is on, the UEC does not receive any bus messages or the bus messages are illegible, the dipped beam and position light are activated). |
| • | "Follow me home" functions (if the headlamp flash stalk is operated with the key removed from the ignition, the lights switched off and the driver's door open, the UEC will switch on the dipped beam, the reversing lights and the tail lights for 30 seconds). |
| • | Main beam: The UEC will only switch on main beam if the dipped beam is switched on. (On vehicles with rain/light sensors, the main beam can only be switched on if the sensor has reported that it is "night", the dipped beam is switched on and the engine is running). |
| • | There are two programmable options for main beam operation: A: The UEC switches off dipped beam when main beam is operated. B: Both main beam and dipped beam are on together. |
| • | Headlamp flash (warning: if the key is removed and the door open, the same stalk will switch on the "follow me home" function). |
| • | Front fog lamps (can only be activated if the ignition is switched on and the position light or main beam are switched on). |
| • | Bulb check. |
