Body Control Module: Description and Operation
Footwell Module
The footwell module (FRM) is an electrical nodal point in the footwell on the drivers side. The footwell module activates the lighting, the front power windows and the outside mirrors. It picks up the door signals, the driver's door switch block and the ride height sensors. The footwell module also controls the adaptive headlights. The footwell module is also the interface to the dashboard.
Brief component description
The following components deliver signals for the footwell module:
- Ride height sensors
- Brake lamp switch
- Reversing light switch
- Hazard warning flasher switch
- Light switch
- Assist systems operating facility
- Driver's door switch block
- Door contacts of the front and rear doors
- Driver's door lock
A number of control units are involved in the lighting system. In a stricter sense, the following control units are involved in the lighting (in alphabetical order):
- ACSM: Crash safety module
ACSM stands for "Advanced Crash Safety Module", referred to as the crash safety module.
The footwell module is connected to the Crash Safety Module via the K-CAN. In the event of an accident with corresponding severity, the footwell module switches on the interior light and the hazard warning lights automatically.
The crash safety module sends a message regarding the front passenger seat occupation detection on the K-CAN. The belt hand-over on the passengers side is only activated when the front passenger seat is occupied.
- AHM: Trailer module
The trailer module delivers the signal indicating whether a trailer is attached to the vehicle. The trailer module also controls the trailer lighting.
During trailer towing, the trailer module automatically disables e.g. the rear Park Distance Control (PDC) and the rear fog lamp on the vehicle.
If a trailer is detected, the automatic parking function is disabled. (Automatic parking function: To improve the view of the kerb, the outside mirror glass is folded downwards when reverse gear is engaged. This brings the lower area immediately around the vehicle, i.e. the kerb, into the field of vision.)
The trailer module (AHM) is connected to the footwell module via the K‐CAN.
- DSC: Dynamic Stability Control
When cruise control is in operation, the brake lights are actuated during automatic braking (legal requirement). To do this, a signal must be sent from the DSC to the accelerator pedal module via the PT‐CAN.
- EGS: Electronic Gearbox Control
If reverse gear is selected with an automatic gearbox, this information is sent to the footwell module via the vehicle busses. The footwell module then switches on the reversing lights.
In vehicles with a manual gearbox, the reverse gear switch is directly connected to the footwell module.
- FLA: High-beam assistant
In accordance with the traffic situation, the high-beam assistant sends a switch-on recommendation or a switch-off recommendation for the high-beam headlights to the footwell module (FRM). On the basis of this recommendation and various other the input variables, the footwell module decides whether the main-beam headlights should be switched on or off.
- FRM: Footwell module
The footwell module is connected to the vehicle with 3 plug connections. Two 51 pin connector connect the main wiring harness. Another 26 pin connector is for the connection to the dashboard.
- FZD: Roof function centre
The roof function centre is responsible for the components of the interior lighting in the roof area.
The Basic variant of the roof function centre has its own control unit. The front interior light and the rear-compartment interior light are activated via the footwell module (FRM).
The High variant of the roof function centre has its own control unit, the FZD control unit.
The footwell module is connected to the FZD control unit via the K‐CAN.
- JBE: Junction Box Electronics
The lighting for the luggage compartment and the glove box lighting are connected to the junction box electronics.
- LDM: Longitudinal dynamics management
In conjunction with the LDM, the option "Active Cruise Control" uses the "turn signal" from the footwell module to help when changing lanes. In other words, if a left turn indicator is given before overtaking, the distance to the vehicle in front is reduced. The vehicle to be overtaken is then "lost" more quickly. On the other hand, when changing to the right-hand driving lane, a vehicle moving there is picked up more quickly.
- RLSS: Rain-light solar sensor
The rain/light sensor measures the ambient brightness outside the vehicle. Depending on the ambient lighting conditions, the footwell module will switch the driving lights on or off. To do this, the automatic driving lights control must be activated (light switch in switch position "A"). At dusk, the rain light sensor sends the message "dusk". The footwell module switches the low-beam headlight on. The automatic headlight-range adjustment for the dipped-beam headlights is actuated. In darkness, the rain/light sensor transmits the message "Darkness". The adaptive headlights are then activated when the vehicle is cornering.
- SZL: Steering column switch cluster
The switch signals from the turn-signal/high-beam switch are picked up and evaluated by the SZL. The evaluated switch signals are forwarded as resistance-coded by the SZL via direct lines to the footwell module
- Driver's door switch block
The driver's door switch block is connected to the footwell module via a LIN bus.
The driver's door switch block contains the buttons for the power windows, the outside mirror adjusters and the folding function for the outside mirrors (only with outside mirrors that are equipped with this feature as an optional extra) as well as the buttons for the child locks and the rear window roller sunblind.
- Assist systems operating facility
The driver assistance systems operating unit is connected to the footwell module by a LIN bus.
The operating unit contains button for the following driver assistance systems:
- Lane departure warning
- Head-up display
- Night vision
- Dynamic Braking Control
- Lane departure warning
The following components are activated by the footwell module:
- Exterior mirrors
- Front power window drives
- Headlight
- Tail lights
- Fog lights
- Centre high-mount stop lamp
- Auxiliary turn indicator light
- Front courtesy lighting
- Rear courtesy lighting
- Licence plate lamps
- 2 headlight driver modules for the step motors of the adaptive headlights
- Electric steering column adjustment
System functions
The following functions are executed by the footwell module:
- Gateway between LIN bus and K‐CAN
- Waking by means of various signals
- Other functions
Gateway between LIN bus and K‐CAN
The footwell module (FRM) enables communication to take place between the LIN bus and the K‐CAN. The footwell module transfers the messages to the relevant recipient bus.
Components on the LIN bus:
- Special equipment exterior mirrors
- Driver's door switch block
- Assist systems operating facility
- Light switch
- Operating unit for seat memory, driver and front passenger
- 2 headlight driver modules for the step motors of the adaptive headlights
Waking by means of various signals
The footwell module (FRM) can be wakened via the following signals:
- K-CAN active
- Terminal 15 ON
- Hazard-warning switch ON
- Change in status of door contacts
- Anti-theft alarm system triggered
Other functions
The footwell module (FRM) influences various functions in the vehicle. The following components are controlled by the footwell module:
- Exterior mirrors
- Exterior lighting
- Interior lighting
- Central locking system
- Power window regulator
Exterior mirrors
The requests to adjust the mirror are sent via the LIN bus. The footwell module (FRM) receives and processes the signals via the LIN bus. Every 20 milliseconds, the footwell module queries whether there are requests from the switch block.
In the Sleep mode, the power supply is switched off. The outside mirrors cannot be actuated.
The mirror heating on the outside mirrors (optional extra) is activated by the footwell module. The corresponding information is forwarded to the evaluation electronics in the mirror via the LIN bus.
If a mirror memory is installed, the footwell module saves the memory position of the mirrors.
Exterior lights
The lighting function are integrated in the footwell module (FRM).
These lighting function are:
- Parking lamps
- High-beam headlamps
- Headlamp flasher
- Turn signals
- Hazard warning flashers
- Brake lamp
- Low-beam headlights
- Reversing light (signal for reversing light comes from electronic transmission control via bus or from reverse gear switch)
- Fog lights
- Rear foglamp
- Parking lamps
Besides the lighting functions, other exterior lighting functions are also integrated in the footwell module:
- Lighting system monitoring
- Headlight vertical aim control
- Emergency operating mode if the footwell module fails
- (important lighting functions are substituted by other lights being actuated in the event of individual lights failing.)
- "Follow-me-home" lighting
- Visual alarm after triggering of the anti-theft alarm system (the visual alarm is issued according to the coding by hazard warning lights, flashing with low-beam headlights or flashing with high-beam headlights.)
- Actuation of bi-xenon headlamps
- Activation of the step motor controller for the adaptive headlights
- Turning lights The cornering light is coupled to the Adaptive Headlight.
All lighting functions except the additional brake light are supplied with a pulse-modulated signal by the footwell module (FRM). This PWM signal enables a uniform brightness of the exterior lights. Interior lights
On vehicles without roof function centre (FZD), the footwell module actuates the interior lighting in the roof area directly. All footwell module outputs for interior lighting are pulse-width modulated. This ensures that the interior lighting maintains constant brightness despite voltage fluctuations.
With standard equipment, the interior lighting comprises the following components:
- Front interior light
- Luggage compartment lights
- Glove compartment lighting
- Footwell lighting
The following components can also be installed as special equipment for the interior lights:
- Rear interior light
- Courtesy lighting
Central locking system
The footwell module evaluates the status of the Hall effect sensors in the door contacts. When the vehicle is locked or unlocked with the mechanical key element, the footwell module will recognize this request. The footwell module transmits a message on the K‐CAN to the CAS (Comfort Access System):
Power window regulator
The footwell module (FRM) and the junction box electronics (JBE) control the power window drives.
The relays for the front power windows motors are located in the footwell module. The relays for the rear power window drives are fitted in the junction box electronics.
Electric steering column adjustment
The electrical steering column adjustment enables the driver to set the optimized position of the steering wheel. There is a switch on the left of the steering column for operation. The inclination and length of the steering column are adjusted electrically.
Power for the electrical steering column adjustment is provided via terminal 30 switched. The footwell module (FRM) activates the electrical steering column adjustment (FRM).
When the steering column is adjusted, a Hall sensor on the steering column adjustment motor sends a signal to the FRM control unit. The position of the steering column is registered by and stored in the FRM control unit.
For the key memory, this means that the position of the steering column can be called up.
To support entry and exit, the tilt adjustment moves to the highest position. For Easy Exit, the steering column adjustment is automatically activated under the following conditions:
- Terminal R OFF
- Terminal R ON and driver's door opened (signal via door contact)
- Terminal 15 OFF with driver's door open (signal via door contact)
The steering column memory is activated under the following conditions:
- One of the two memory buttons is pressed (from terminal R ON).
- Another identification sensor is used (personalization). Precondition: The selection "Last seat position after unlocking" is activated in the "Settings" menu in the Central Information Display (CID).
In both cases, the steering column memory only moves to the corresponding memory position after terminal 15 On.
Notes for Service department
Nearly all Car and Key Memory functions are programmed inside the vehicle itself. (Please refer to the "Personal profile" section of the Owner's Handbook: Individual settings for up to 3 remote controls via the display in the instrument panel or via the Central Information Display)
Switch-on conditions
The footwell module (FRM) receives a large number of input signals that switch on the interior lighting. The input signals are read in directly by the footwell module or received via the K-CAN.
The following signals switch the interior lighting ON:
- Interior lighting button pressed
- Door opened
- Unlocking using drivers door cylinder lock
- Unlocking using remote control
- Terminal 30B OFF if terminal 58g was ON for a maximum of 2 minutes beforehand
- Crash signal present
- Lock button on remote control pressed when central locking system has been in central double-locking for at least 10 seconds
The interior lighting is switched off under the following preconditions:
- Central locking system set to thief-proof mode, all doors and the tailgate closed
- Interior lighting button pressed for longer than 3 seconds
- Terminal 58 ON and terminal 30B OFF
- Terminal 30B ON with closed doors
- The vehicle is unlocked with the remote control and no door is opened with the space of 20 seconds
- Terminal 30B OFF and the door opened for longer than 1 minute
- Via diagnosis "Power Down"
- After 8 minutes terminal 30B OFF
When terminal R is switched OFF, the footwell module (FRM) will switch the interior lighting off after 8 minutes. To do so, the footwell module sends the message for consumer shutdown via the K-CAN. The roof control panel (FZD) receives this message and switches off the interior lighting in the roof area. Interior lights that are switched on directly by the footwell module are also switched off.
The footwell module provides terminal 58 g via the K-CAN or via conventional wiring. Terminal 58 g is pulse-width modulated and has 2 brightness stages.
- Brightness stage for locating lights. The brightness for the locating lights can be set individually with the rocker switch on the steering-column stalk.
- Brightness stage for function indicator lamps. The brightness stage for the function indicator lamps is not dimmed and is switched on with full brightness.
As soon as the hazard warning switch has been pressed, the lighting of the hazard warning switch is switched to full brightness by the footwell module. If terminal 58 g is activated, the hazard-warning lights switch will no longer be illuminated with full intensity. The brightness depends on the setting for the locator lighting.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.