- Regular BMW owners who need to diagnose a faulty BMW FRM module
- DYIers who want to develop their own device for BMW
- Electrical engineers who are interested in reverse engineering BMWs
The BMW FRM at a glance
The BMW Footwell module (FRM) functions as an electrical hub or a node in the footwell on the drivers side. The FRM receives signals from the doors, it controls the lighting, it commands the the adaptive headlights and it also interfaces with the dashboard.
The BMW FRM receives signals from the following sources:
- Height sensors
- Reversing light switch
- Brake light switch
- Hazard warning flasher switch
- Light switch
- Driver’s door switch block
- Door contacts in front doors
- Driver’s door lock
The BMW FRM controls the following mechanisms:
- Exterior mirrors
- Power window drive
- Tail lights
- Fog lights
- Centre high-mount stop lamp
- Auxiliary turn indicator light
- Front courtesy lighting
- Rear courtesy lighting
- Number-plate light
- 2 stepper motor controllers for the adaptive headlight stepper motors
- 2 belt feeder controllers (only on E92)
- Exterior mirrors
- Exterior lighting
- Interior lighting
- Central locking system
- Power window regulator
BMW FRM Footwell connectivity
The BMW FRM footwell module is connected to the vehicle through 3 connectors. There are two 51-pin connectors for the main wiring harness. There is another 26-pin connector that goes to the dashboard.
BMW FRM system functions
The BMW FRM Footwell module performs the following system functions:
- Gateway between LIN bus and K-CAN
- Waking by means of various signals
- Storing vehicle order
- Other functions
Control units involved in the lighting system
The following is a list of all the control units involved in the lighting system, listed in alphabetical order:
- ACSM or MRS: Crash safety module / multiple restraint system
- AHM: Trailer module
- DSC: Dynamic Stability Control
- DSC: FLA: High-beam assistant
- FZD: Roof function center
- JBE: Junction Box Electronics
- LDM: Longitudinal dynamics management
- RLS / RLSS: Rain/light sensor or rain/light solar sensor
- SZL: Steering column switch cluster
E81, E82, E87, E89, E90, E91, E92: Multiple restraint system
E70, E71, E93: ACSM (”Advanced Crash Safety Module”)
The Footwell module is connected to the crash safety module / MRS control unit via the K-CAN. In the event of an accident, the FRM Footwell module switches on the interior light and the hazard warning lights automatically.
On the E92, the crash safety module sends a message regarding the front passenger seat occupation detection on the K- CAN. The belt handover on the passengers side is only activated when the front passenger seat is occupied.
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.
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.
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.
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 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.
The lighting for the luggage compartment and the glove box lighting are connected to the junction box electronics.
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.
E81, E82, E87, E89, E90, E91, E92, E93: Rain/light sensor
E70, E71: 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.
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.
Wrapping it up
To sum up, in this article we’ve seen the multitude of mechanisms controlled or influenced by the FRM Footwell module. We hope this article has helped you troubleshoot the weird symptoms that brought you here or that it helped you advance with your DIY project.
If you want to get in touch, you have a message or want to share something with us or with our readers, please leave a comment in the section below. Stay tuned for more articles on the workings of your favourite brand.