Components Parts
SRS (Supplemental Restraint Systems)Air Bags & Seat belts.

Special Note Avoid accidents
When ever working on parts of the car fitted with AIR BAGS and SEAT BELT PRE-TENSIONERS you are advised to DISCONNENECT THE BATTERY BEFORE COMMENCING WORK. Failure to do so could result in accidental activation of an Air Bag & the SRS warning light remaining ON when you next turn ON the ignition. This includes Door panels and steering wheels where air bags are stowed. Should you not follow this safety rule then UNDER NO CIRCUMSTANCES TURN THE IGNITION ON WHILE SRS COMPONENTS ARE DISCONNECTED .THIS will PRODUCE THE WARNING LIGHT WHICH CAN ONLY BE DELETED BY 'STAR' Diagnostics equip.

The above diagram indicates the location of the various components associated with the SRS on right hand drive cars (UK) The components numbers are reversed for left hand drive cars (Supplemental Restraint Systems) It is not advisable to tamper with any of this equipment unless you are a competent Auto electrician/engineer. Star diagnostics equipment is also required to locate the fault and delete the fault code from the SRS management system.

There is provision of seat belt in the 'A' Class for five passengers. In the UK there are legal requirements for seat belts where fitted to be worn by both front & rear seat passengers. Drivers of vehicles would be well advised to ensure their passengers are wearing the belts provided. The centre rear seat is cramped but does provide a waist and diagonal belt However it is the only belt not provided with a pre-tensioner.

When having just purchased the car I sure other owners must have thought I hadn't read the handbook when I asked the question on the 'A' Class Owners Club Site

"What was the belt in the roof above the boot lid for?" I thought it could have been for securing the dog, of course you'd want a special Mercedes Dog collar! Which of course in keeping with a lot of other MB spares cost a fortune, 'Harrods of London' might be a better bet?
I looked around in the boot as well as the back seats for the other half of the clip and due to it being recessed into the cushions of the seat missed it, well that's my excuse anyway! Within hours of asking the question up came the answer! "It was for the third person in the back seat" So that you do not get caught out like I did, the layout for fixing can be seen in the photo. I'm just glad I didn't offer somebody a lift that required them to use it ! Most embarrassing when you can't find the fittings in your own car? Fig 1 Stowed clips, Fig 2 Diagonal belt positioned across seat. Fig 3 Small clip anchored in tandem with O/S/R/ belt coupling. Fig 4 Standard seat belt connection.

Seat Belt Anchorage Height Adjustment
One thing that I feel is worth reminded owners of is the that the seat belt anchored point on the 'B' Post is adjustable and makes the seat belt much more comfortable , simply adjust the anchorage up or down to suit your individual needs.

SRS(Supplemental Restraint Systems)
When talking about seat belts we should understand that the belts are interconnected with the SRS system, if the car is involved in an accident the front seat belts being worn will automatically tighten as the air bags deploy, this is a sophisticated system and any faults on them should be treated as urgent repair faults and generally speaking do not fall within the DIY category. The standard fitment of airbags is:- Driver, housed within the steering wheel centre, passenger bag front/dash side air bags, housed within the two front door panels. window bags are optional equipment.

If the front passenger seat is free ,the seat recognition sensor deactivates the front and side air bag, If the seat is occupied with a child seat fitted with a transponder again the airbag will not deploy, However a child seat not fitted with a transponder should not be placed in the front passenger seat as the air bags will/may ( depending on the severity of the accident) deploy in the event of an accident, causing extreme danger to the child/seat occupant.
Seat belts in the front and two rear outer seats are all fitted with pre-tensioner, belt force limiters (standard on front seats ) prevent the pressure from becoming excessive.

The paragraph below is an answer I put on the forum where an owner had a SRS (Supplemental Restraint Systems) light displaying on the instrument cluster. It is also worth noting that even if this was a DIY item the fault recorded in the ECU cannot be deleted without having 'Star diagnostics equipment available, even though the light may not remain illuminated on the instrument cluster.:-

Posted: June 11 2007, 14:18 Post subject: SRS light permanently displaying on instrument cluster information on problems associated with the SRS system (Air bags deployment and seat belt pre-tensioner activation) I feel bearing in mind the roll this equipment plays in the safety of the occupants, that you should get professional help in locating this fault. It may well be something simple but DIY repairs on this equipment unless you are in the trade and familiar with the system would be a little fool hardy. This is also the possibility that the air bags will activate if the wrong procedures are employed while looking for this fault. Testing the circuit in the front seat for faults may well fall within the DIY field but I have learned that there are apparently over 50 sensor points in the unit and so your findings like mine may be useless.

In this photo that air bag had activated following an accident, the air bag having been cut-away, The last thing you want when working on your car is a short circuit that causes any of the air-bags to activate.

The available information does however say -: Where the SRS (Supplemental Restraint Systems) indicator lamp comes on and fault codes B1867 or B1871 are stored then there are problems with the contacts at the front door separation points left and or right Action Repair the connector of the side airbag ignition wires at the two separation points. Again this can only be determined by using Star diagnostic equipment. lofty, I have your best interests in mind.

However having said that this is not a DIY service item/repair, with the battery disconnected (follow the correct procedure for Dis, & Re-connection) there is not harm done in checking the contacts at the front door separation points left and or right, this is the electrical circuit services cables that goes to Air Bags, Powered Windows, Electric mirrors, Radio/CD speakers in the front doors which are of course being hinged every time the door is opened and closed.

A common cause for SRS light remaining on, especially the one on the multi switch between the front seats is the plug connector having been removed and not reconnected following the refitting of the front passenger seat.
Also ensure that the plug section has not been dislodged from the cables, when ever I remove the front seat I tuck the mail plug and cable under the carpet to avoid such damage. Make sure the plug is correctly fitted before going to the garage.
With the ignition OFF removing and refitting the plug having lightly sprayed with 'Electro-lube', (electrical contact cleaning fluid) to clean the contacts will again do no harm. Allow the cleaner to evaporate before reconnecting permanently If you are not fully familiar with your car and not into Automobile DIY THEN please LEAVE WELL ALONE. albeit removing the seats and associated tasks should not be beyond the capability of the average owner.

The price of this component is 113.00 plus vat 30/07/12 as with all MB parts your Vin or chassis number is required when ordering.

For those who are conversant with their cars, the socket beneath the passenger seat that is connected to the detector pad in the seat cushion can be removed by simply flicking back the two tabs and removing one cable tie, this will enable you to fully test the circuit in the seat .

A number of owners have now tackled this sensor replacement with success , once repaired the problem SRS light will be gone however the part is expensive at 90.00

Thanks to the owners who have send me various photographs of this mat and repair it is one that you can do yourself and in the comfort of you front room simply remove the seat and away you go, Pay attention to getting the mat flat and also replace all seat cover securing clips in the same location as removed. One owner who had problems with the warning light showing was quoted 300€ (214.00 ) for the repair as a result he changed the sensor in the seat himself which cured his problem, the sensor cost him 106 €, (76.00 inclusive of VAT) It was also interesting to note that he plugged the new sensor into the connector before removing and stripping the seat to confirm that the problem was being caused by the sensor, a wise move as these defects can be obscure and difficult to identify. The SRS warning light was extinguished by replacing the sensor ,although the defect will still be recorded in the ECU memory until deleted. This does not have to be cleared for the unit to work correctly and even disconnecting the plug for seat removal will be noted by the memory of the car's computer system.
Well Done jobs like this can be tackled and it certainly helps to keep your running costs down. thanks for the photographs, it makes explaining the job so much easier.

Andrew a fellow owner had a slightly different problem, the SRS light was on but when and adult sat on the passenger seat the light went off but when his son did the same, being lighter in weight the light remained on.

When he stripped and checked the seat the sensor was being fouled by one of the upholtestery support wires which was causing the corner of the sensor to be bent almost double, having realised this was the cause of the problem he took care to ensure the sensor was positioned so as not to be affected by the support wire which can happen on both front corners of the seat, having carefully reassembled the seat all worked correctly, the light coming on for a few seconds when the ignition is turned on and then going off.
Cost Nil but a couple of hours labour.
Had he taken the car to Mercedes-Benz Workshops they would have almost certainly have said they had fitted a new sensor at 76 pounds, plus the labour which would have made an expensive repair. again diagnostics would have proved to be inconclusive but would have cost a minimum of 50.00 plus VAT, Yes DIY does Pay. A on the photo shows where the wire had fouled the seat sensor circuit which caused the problem
.Thank you Andrew for the photo and information it may well save other owners the cost of new sensors.
BUT owners do not attempt repair these seat sensors. Lives can be saved by the SRS (Safty retraints system)the last thing you want is for it to fail if you are unfortunate enough to have and accident.

The available information does however say -: Where the SRS indicator lamp comes on and fault codes B1867 or B1871 using ('Star' diagnostics equipment) are stored then there are problems with the contacts at the front door separation points left and or right, (these cables supply the left and right front door air bags)

Repair the connector of the side airbag ignition wires at the two separation points.

A cable repair kit is available from MB but again it is stressed that only competent Auto electricians should carry out repairs to the wiring harness leading to the air bags and other services, wires should be repaired one at a time to prevent incorrect wiring, which could cause either non function of the bag or premature deployment.

If on connecting the battery the SRS light has been extinguished then you have cleared the fault. But be aware the fault will have been logged /stored in the cars MS(management system) again this can only be determined and deleted using Star diagnostic equipment or hand held OBD2, EOBD hand held tools which allow SRS codes to be deleted. earlier models do not detect or allow deletion of SRS codes.

When Martin contacted me because his SRS light remained on all the time I suggested the passenger seat sensor as being the most likely cause. However this was in fact a major failure of the N2/2 module stowed under the centre consul which had been severely damaged by spilled coffee or alike .
Be warned standing drink containers anywhere in the area of the centre consul or you will damage not only the multi switch block but also the components stowed below which will cause you a lot heartache and cash if they are accidental spilled.

Lofty, I did as you suggested and checked the sensor pad with a multi-meter for continuity, all checked out ok

So I removed the centre console and discovered a heavy brown dried out stains all around the ETR Module and over it as well, it looked as if a cup of coffee had been spilled.

I Ordered an ETR Control Module, I have now fitted it made all connections, seats and console, switched on and the SRS light stays on for 3 seconds then goes off and stays off. So my problem is solved. Best Regards, Martin

It is again stressed that this system is a life saving system, unless you are competent to deal with such systems and are prepared to get a diagnostics test done after completion of the work then it is best left to a garage.
as mentioned earlier all fault codes including the disconnecting of the front passenger seat will be logged on the ECU, these can be deleted when confirming that the fault has been successfully dealt with by diagnostics test.

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