In this guide, we take a look at how to troubleshoot BMW Universal Charging and the hands-free facility, also known as the ULF module or MULF2.
On BMW vehicles Restraint System Malfunction warning message pops up and the airbag light turns on to warn the driver the airbags are not working properly and may not deploy in case of an accident.
In this guide, we look at common problems that trigger the restraint system malfunctions and how to diagnose the problem yourself.
Common problems that trigger BMW restraint system malfunction.
- Fault in the airbag module
- Defective seat belt tensioner
- Belt force limiter
- Low battery voltage
- Defective clock spring
- Passenger occupancy sensor
- Wire harness
To diagnose the problem you will need to read the fault codes from the SRS module.
How to Diagnose BMW Restraint System
Below you will find instructions on how to read fault codes from the BMW SRS module. For this procedure, you will need a BMW scanner.
- Park your BMW and set the parking brakes.
- Locate the diagnostic port under the dashboard.
- Turn on the ignition but do not start the engine. Turn on the scanner and allow it to communicate with the vehicle.
- Select BMW from the main menu then your particular model.
- Select Control Units next Select SRS Airbag Module
- Select Read Fault Codes. Watch the following video to learn more.
Let's take a look at common problems that trigger BMW airbag light.
Airbags are a restraint device, which deploys only during a severe collision.
Depending on the model, there are front, side, and curtain airbags. The steering wheel and the passenger-side dashboard hold front airbags.
These are activated in case of a frontal impact to minimize head and chest injuries. Side airbags are installed within front seats and protect passengers during side impacts or vehicle rollovers.
Curtain airbags have a similar protective role as side airbags, but also minimize the risk of injury from broken glass. These airbags are located between the A and C pillars.
If an airbag has previously deployed it will trigger the restraint system malfunctions warning light. But a more common issue is the wire harnesses that connect to any of the airbags. Wires to the airbag can get damaged during a minor accident or previous repair work.
An SRS module controls the restraint system. It uses impact sensors to determine the severity of a collision.
When the system detects sufficient deceleration, it takes action within about 20 milliseconds. If the force of the impact is low, the control module may deploy the seatbelt pre-tensioners only and display restraint system malfunction on the dashboard.
In case the impact force is high enough, airbags will also deploy at about the same time. A signal ignites a chemical reaction, producing a rapid expansion of nitrogen gas, inflating the airbag within about 30 milliseconds.
Passenger Occupancy Sensor
Besides these features, most BMWs from 2003 onwards have a system that monitors when a passenger is on the seat and their weight.
The seat occupancy sensor is, by far, the most common failure point.
This sensor is, in fact, an electrical matt installed into the base of the passenger seat. As it carries the whole weight of the passenger, it usually fails as the years go by. It starts as an intermittent error, only to get worse with time.
Although there are emulators that can solve this problem, these make the passenger airbag active all the time. Although time-consuming and costly, the best solution is to replace the sensor.
Battery Safety Terminal (BST)
Cars with a Battery safety terminal, or BST system, can have an issue with a damaged BTS cable.
Usually, this happens during a battery change and causes a malfunction of a restraint system.
Connector / Wire harness
Another possible issue is a loose or broken seat connector.
They are found at the bottom side of both driver and passenger seats.
Foreign objects pushed under the seat can easily damage these connectors, especially if the seat is moved back and forth frequently.
Other components, such as airbags or seat belt tensioners, rarely fail on their own. However, they can get damaged or activated during assembly or disassembly if not handled properly.
Seat belts prevent sudden forward movements and ensure passengers are in the correct position for airbag deployment.
Seat belts have pre-tensioners, which tighten the seatbelt across the passenger lap and chest to prevent excessive forward movement.
Also, a load limiter relaxes the tension of the seatbelts in a controlled manner to reduce the amount of pressure on the passenger's chest.
Restraint System Warning Messages
This is a list of warning messages that may display on the dashboard.
- Driver restraint system malfunction. Continue wearing a safety belt. Consult the nearest service center.
- Passenger restraint system. Fault in the airbag, belt tensioner or belt force limiter. COntinue wearing a safety belt.
- Pass. restraint system. Fault in a passenger restraint system affecting airbag.
- Carefully drive to the closest service center. The passenger restraint system is limited. Increased risk of injury in case of a collision. Continue wearing the safety belt.