What does SRS malfunction mean in Mercedes Benz?

This blog will answer the following questions: What is an SRS malfunction in Mercedes Benz? What are the components of the SRS system? How to diagnose a fault in the SRS system? Should you drive a car with the SRS malfunction message in the instrument cluster?

What does  SRS malfunction mean in Mercedes Benz?

An SRS malfunction means that there is a fault in one of the SRS components installed in the vehicle and the airbag may not deploy in the event of a collision. SRS stands for supplemental restraint systems and the malfunction message or the airbag light does not reset on its own. Therefore, a fault in the SRS system must be rectified immediately.

The SRS control unit is the brain of the complete system. It is installed on the transmission tunnel of the vehicle. The location allows the control unit to detect vehicle rollover, all the electrical harnesses of the SRS components are colored in yellow and yellow white. The wires are integrated into the main vehicle harness.

The airbags are installed on all sides of the vehicle and depending on the direction of the collision, the SRS control unit can deploy specific airbags. For example, font airbags will not be deployed in case of a side collision or a rear collision.

Only the driver airbag has two fuses and it can be re-deployed if there are multiple collisions detected. Once deployed the airbags cannot be reused, there is an explosive charge of sodium azide in the airbag squib which explodes and fills the airbag with nitrogen. 

When the vehicle is turned on for the first time, the SRS control unit runs a self-diagnosis to ensure all the systems are working correctly. If there is a fault in the SRS system then the airbag warning light in the instrument cluster will stay on. An SRS malfunction message will also be displayed in the multifunction display in the instrument cluster.

What are the components of the SRS system?

The standard components of the SRS system are given below, there can be optional extra SRS systems like pedestrian protection or PRE SAFE plus which needs additional components.

• Airbags

The minimum number of airbags required in a vehicle is 6 but premium car manufacturers like Mercedes Benz, Audi, and BMW use more than 6 airbags in the vehicle. There are side airbags in the passenger seats and curtain airbags for side collision. 

Mercedes Benz recently also introduced a Frontal airbag for the rear passengers in the newly launched S Class.

• Crash sensors

Crash sensors are present on the front and the rear side of the vehicle. All the sensors have separate electrical lines to the SRS control unit. Additional systems like pedestrian protection use special sensors in the front bumper to detect an impact with the pedestrian

• Door pressure sensors

Pressure sensors are located in each door, if there is a side impact the pressure between the door changes due to deformation. The SRS control unit detects the change in pressure and deploys side airbags accordingly.

• Pyrofuses

They are located on the main battery and the pre-fuse box of the vehicle. The pyro element separates the electrical connection of the vehicle to the main battery when the crash signal is received from the SRS control unit.

How to diagnose a fault in the SRS system?

The procedure to diagnose a fault in the SRS control unit is listed below:

• Perform an initial quick test or read out the fault codes using a diagnosis OBDII scanner.
• Locate the SRS component according to the fault code stored in the SRS control unit.
• Check the actual value for the component, if it is an airbag the internal resistance should be between 1 to 10 ohms.
• Disconnect the main battery before performing any electrical measurement on the SRS components.
• Do not disconnect and remove the SRS control unit with the main battery still connected as it can trigger the deployment of the airbags.
• If the electrical lines and airbag squib is okay then priced with replacement of the faulty component.

The old and new SRS components like airbags and belt tensioning retractors must be handled with care. There is a fuse inside which can set off an explosive charge if the component is damaged. A special wiring harness is used to measure the resistivity of the airbag squib. 

All the SRS component’s electrical couplers are designed with a self-shorting pin. Both the terminals are connected to each other when the electrical coupler is removed. Therefore, if a multimeter is used to check the component, the probe must be isolated.

Use the correct digital multimeter to measure the resistance of the airbag, if the voltage given by the multimeter is high it can trigger the airbag squib and the bag will explode. The SRS control unit is constantly monitoring the internal resistance of the components even when the ignition is turned off. 

Components like pyro fuses are not reusable so if a fault code is present in the SRS control unit related to the pyro fuse, it must be replaced. After the vehicle experiences a crash event the engine control unit is disabled and it will not start the engine until all the SRS faults are rectified.

Should you drive a car with the SRS malfunction message in the instrument cluster?

No, driving the car with an SRS malfunction message or warning light in the instrument cluster must be avoided. The SRS system is responsible for the deployment of the airbags which are a passive safety system of the vehicle.

If the vehicle is in an accident and there is a fault in the SRS control unit then the airbags can fail to deploy. Similarly, when the SRS control unit detects a crash it uses data from various sensors present in the vehicle to determine the rate of slowing down and the G-forces experienced by the vehicle in a crash.

This data is stored in the fault freeze frame data which can be accessed later to determine the severity of the impact. However, if there is a fault in the SRS control unit related to any one of the sensors connected to the control unit then the complete data can be rendered useless.

Conclusion

This blog explained the meaning of the SRS malfunction in Mercedes Benz. The procedure to fix the SRS malfunction is easy but due to the explosive nature of the SRS components, precautions must be taken while replacing or checking the components like the airbag squibs. Otherwise, injury to the person working on the SRS system can occur.