Supplemental Restraint System (SRS) (Mercedes-Benz S-Class W140)

The devices and components that make up the supplemental safety system (SRS) include:

• a) Two front (frontal) airbags are standard equipment. The driver's airbag module is located in the steering wheel hub. The front passenger airbag is located in a special cavity on the right side of the vehicle's instrument panel. The locations of both airbags are marked with warning signs "SRS AIRBAG";
• b) Two front seat side airbags are standard. The side airbag modules are mounted in the outer side parts of the seat backs;
• c) Head airbags (curtain airbags) (additional equipment), located along the side edges of the ceiling panel and providing additional protection for the heads of the driver and all passengers in the event of side impacts and vehicle rollovers;
• d) Directional overload sensors that are triggered in the event of a severe frontal/side impact;
• e) Electronic self-diagnosis system, which, when the ignition is on, provides continuous testing of the overload sensors, control unit, airbag gas generators and connecting wiring of the corresponding circuits;
• f) Automatic emergency seat belt tensioners for front seats - standard equipment;
• g) SRS malfunction indicator lamp mounted in the instrument panel;
• h) A spare autonomous power source designed to duplicate the main power supply system in the event of its failure during a vehicle collision.

A special SRS warning lamp is installed in the vehicle's instrument panel, designed to warn the driver about failures in the additional safety system detected by the self-diagnostic system.



Depending on the vehicle's configuration, the warning lamp may indicate possible malfunctions of the front/side airbag modules or automatic emergency seat belt tensioners.

Operating principle of frontal airbags

Warning! Never transport young children in the front seat of a vehicle equipped with a passenger airbag! This also applies to installing a special child seat in the front seat.

Warning: In minor collisions and rollovers, the front airbags should not deploy.

The front airbag modules are mounted in the steering wheel hub and in the right-hand side of the instrument panel, directly above the main storage compartment.

Directional overload sensors react to the excess of a certain limit value, which may occur during a strong frontal collision. At the signal from the sensors, the control unit turns on the airbag gas generators, causing their immediate filling.





During a collision, the seat belt keeps the lower part of the occupant's body from moving. Front airbags help prevent injury to the driver and front passenger when their heads and chests hit the steering wheel/dashboard.

Since the signals for both airbags to operate come from the same sensors, both airbags operate simultaneously. However, it is possible that only one of the airbags will be activated. This becomes possible when the magnitude of the directional overloads that arose during the collision was at the limit of the sensitivity range of the sensors. In this case, the seat belts provide sufficiently reliable protection for the driver and front passenger, while the effect of the airbag operation will be minimal.

Immediately after the airbags are triggered and perform their functions, the gas is released from them, which allows the driver to maintain visibility and freely manipulate all vehicle controls.

The entire duration of the process from the moment the overload sensors send a signal to the release of gas from the airbags takes a fraction of a second. The airbags are triggered so quickly that the human ear is simply unable to perceive the pop of the gas generators when filling the airbags.



After a traffic accident in which the airbags were deployed, the interior of the car looks slightly smoky. In fact, this is a suspension of fine powder, which is usually sprinkled on the folded airbag shells. People with respiratory diseases may experience some discomfort from the gaseous products contained in the air, emitted by the airbag gas generators.

Operating principle of side and head airbags

The side airbag modules are mounted in the front door panels of the vehicle.

The locations of both side airbags are usually marked with inscriptions "SIDE AIRBAG". The head airbag gas generators are located along the side edges of the ceiling panel.



In the event of a severe side impact, the directional overload sensors record the excess of a certain threshold value by the resulting loads and send an information signal to the control unit, which generates a command to trigger the gas generator of the corresponding side airbag.



Moreover, if the impact occurs from the passenger seat, the passenger side airbag will deploy even if there is no passenger in the front seat.

To ensure that the airbags provide the most effective protection, the driver and passenger sitting in the front seat should sit up straight and be wearing seat belts.

SRS Malfunction Indicator Lamp Operation Procedure

When the ignition is turned on (the ignition key is turned to position 2) the control lamp lights up for a short time and then immediately goes out, which confirms that all system components are functioning properly.

If the warning light comes on at any other time, the vehicle owner should contact a Mercedes-Benz service station for help as soon as possible. Such situations include the following:

• a) The indicator lamp does not come on when the ignition key is turned to position 2;
• b) The indicator lamp remains on after the engine has been started;
• c) The indicator lamp is activated and lights up in a constant or flashing mode while the vehicle is moving.

If the above symptoms of malfunction are present, the additional safety system may fail in the event of a vehicle accident.

Warning! Ignoring the SRS warning light signals can lead to the most serious consequences if the airbags fail during a traffic accident!

Seat belts with automatic emergency tensioners

The tensioners are activated by strong frontal overloads that occur during a frontal collision and provide instantaneous selection of slack in the belts, as a result of which the latter's webbing tightly encircles the body of the occupant, significantly increasing the level of protection for the driver and front passenger.





The tensioner (A) is mounted in the assembly of the belt return mechanism. The procedure for using such seat belts is similar to the procedure for using conventional belts equipped with inertial return devices. The tensioner operation mechanism is absolutely transparent to the user and does not require any additional skills or knowledge from the latter.

The tensioners are activated only under significant frontal overloads. The activation of the modules is accompanied by a loud bang and a small amount of smoke.

Warning! The smoke released when the tensioners are triggered is safe for human health, but when inhaled directly it can cause irritation of the mucous membranes.

It should be noted that failures of tensioners to operate during a frontal collision in most cases are explained not by a malfunction of the system, but by the fact that the overloads that occur during the impact do not reach the threshold value at which the modules are activated.

As with airbags, the emergency tensioner modules are designed for a single deployment and after that are subject to replacement together with the belts - contact the Mercedes-Benz service stations. All claims regarding failures of SRS components are also accepted there.

Maintenance of SRS components

Airbags and automatic emergency seat belt tensioners require virtually no regular inspection or maintenance. However, the owner of a suitably equipped vehicle should keep the following points in mind:

• a) After the airbags have been deployed, they must be replaced together with the system control unit. Do not attempt to dismantle the airbags on your own. These operations must be performed only in a car service workshop;
• b) If the SRS warning lamp comes on and indicates a fault in the system, you should immediately contact your nearest Mercedes-Benz service station for a full diagnosis and the necessary repair of the system. Otherwise, the airbags may not deploy in an accident and may not perform their protective function;
• c) Only specially trained personnel are allowed to work on the additional safety system components. It is prohibited to independently remove the airbags and emergency seat belt tensioners from the vehicle. In the event of SRS failure or after an emergency deployment of the airbags and seat belt tensioners, you must contact a Mercedes-Benz service station to repair or replace the system components;
• d) Do not open modules or modify the wiring of the airbag systems and automatic emergency seat belt tensioners - such intervention may lead to inadvertent deployment of the airbags and tensioners and cause injury to the operator or nearby people;
• e) Do not replace the front seats without first consulting a Mercedes-Benz service station. Careless or incorrect replacement or removal of the covers may result in the failure of the side airbags to deploy in the event of an accident.

When transferring the vehicle to another person, it is mandatory to inform the latter about all installed SRS elements, indicating the relevant points in the vehicle operating instructions.

On all models equipped with SRS, special warning signs are installed in places where the active elements of the system are located.