When does the ‘front-right malfunction service required message’ come in Mercedes Benz cars?
By
Vroomouk
This article will explain the supplemental restraint system in Mercedes Benz and answer the following question: When does the front-right malfunction service required message come in Mercedes Benz cars? What are the causes and the remedy for the front right malfunction message in the instrument cluster? Should you get it checked or ignore it?
When does the ‘front-right malfunction service required’ message come in Mercedes Benz cars?
The ‘Front right malfunction service required’ message comes in the instrument cluster if there is a fault in any one of the following supplemental restraint system components:
- Driver airbag
- Front passenger airbag
- Front right seat belt tensioning retractor
- Front right knee airbag
The fault codes are stored in the fault memory of the supplemental restraint system control unit commonly known as the airbag ECU. Mercedes Benz has the fastest production vehicles and the SRS technology is a key system for the safety of the occupants.
What is the Mercedes Benz SRS system made up of?
The SRS system consists of an airbag control unit connected to a crash sensor present on all the sides of the vehicle and multiple airbags. The SRS control unit is directly mounted on the transmission tunnel of the vehicle. It has an inbuilt sensor that can detect if the vehicle is overturned and a body acceleration sensor is also connected to the SRS to detect the rate of deceleration.
The SRS control unit also controls the airbag squibs and there are pyro fuses on the main battery, pyro fuses when deployed by the SRS control unit can sever the connection of the main battery and prevent any risk of fire by damaged battery or short in the electrical circuit.
Mercedes Benz uses multiple airbag squibs, especially for the driver airbag so that it can be inflated in two stages in case of multiple impacts. Two additional pressure sensors in the doors are also connected to the SRS control unit. If there is a severe or light side-impact, based on the pressure difference in the door cavity the SRS control unit can deploy the window and side airbags in the passenger’s seat.
Mercedes Benz uses a maximum number of airbags in the production cars, therefore there mustn’t be faults in the SRS system. The SRS control unit is programmed only once and is permanently assigned to the vehicle. It is powered by a direct supply line from the battery as well as circuit 15 which is the ignition ON signal.
How does the SRS control unit monitoring function work?
The SRS control unit is constantly monitoring the internal resistance of the airbag as well as emergency tensioning retractor squibs. A squib is a miniature explosive device with an igniting circuit connected to the airbag electrical coupler.
Ideally, the specified value of the internal resistance of the airbag squibs is between 0 to 10 ohms but the actual value of the internal resistance never exceeds 3.0 ohms. As the control unit is connected to the airbag squib through dedicated electrical lines, it can monitor complete resistance in the circuit, including the internal resistance of the electrical lines and the airbag squib.
If the resistance exceeds 10 ohms, a fault code is generated based on the particular airbag squib and it is stored in the fault memory of the airbag control unit. The fault code along with the fault freeze frame data will be stored in the SRS control unit and cannot be erased even after multiple ignition cycles.
As the SRS control unit monitors the internal resistance of the squibs, it doesn’t require much power. It can continue to monitor the igniter circuits even when the vehicle is switched off. If there is any problem with the squibs or the electrical lines, the control unit will store it and display the SRS malfunction message in the instrument cluster when the vehicle is turned on next time.
What are the causes and the remedy for the front right malfunction message in the instrument cluster?
The cause of the fault message is due to an increase in the internal resistance of the igniter circuit and fault codes can be stored related to the following components:
- Fault in airbag
The airbag on the front right side is installed in the steering wheel if the car is driven from the right-hand side or for the front passenger, the airbag is installed in the dashboard. Electrical lines and coupler connect the airbag ignition squib to the SRS control unit and the specified resistance in the entire circuit should be less than or equal to 3 ohms. Fault code B104/105 is stored in the fault memory of the SRS control unit.
- Knee Airbag/ side airbag
Front right malfunction message can also come if the internal resistance of the airbag installed under the driver side lower dashboard panels is more than the specified limit. The front right side airbag is installed in the front right passenger seat and it is used to absorb the side impact. Fault codes B1529/1523 are stored in the SRS control unit.
- Front right emergency tensioning retractor
Mercedes Benz cars have a reversible emergency tensioning retractor, it is part of the SRS safety system and the main component in the PRE-safe safety system. The front right tensioning retractor is an independent control unit and communicates with the SRS control unit on a CAN bus network.
The SRS control unit does not control the front right ETR but it only manages the ETR squib in the front right seat belt. The pre-safe can detect if the vehicle is going to have an impact and brings the occupants to an optimum seating position. The SRS control unit then deploys the ETR according to the severity of the impact and keeps the occupants properly secured in the seats.
The remedy and rectification of the front right malfunction can be very challenging as faults can occur and specific conditions. When the vehicle reports to the workshop with a front right malfunction, technicians can read the fault memory of the SRS control unit and identify which airbag squib is denoted by a letter and a number combination like R12/1 is responsible for the front right malfunction.
The actual value can be checked in the actual values readout tab in the diagnosis tool. Once the technician completes the checks and determines there is no loose connection in the electrical coupler and electrical lines, it is safe to rectify the malfunction by replacing the faulty airbag or emergency tensioning retractor.
The airbags usually last more than 10 years but there can be an internal open circuit in the airbag squib. The only way to determine whether an airbag is faulty due to the internal open circuit of the squib is by measuring the internal resistance of the airbag.
There are special electrical lines with insulated test probes designed specifically to test airbag squibs. The airbag can be deployed if the multimeter is connected to the squib and there is a high voltage supply from the digital multimeter. Therefore, it is imperative to use a proper test probe and multimeter while checking the airbag squib.
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Conclusion.
The faults in the SRS systems are very rare and do not occur unless there is an issue with the electrical lines or airbag couplers. If there is a front right malfunction message in the cluster, it should be remedied as early as possible as it can affect the deployment of the airbag in the event of a crash.
