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Photo DIY - SRS Seat Sensor replacement

362880 Views 157 Replies 78 Participants Last post by  caleno69
Your SRS light has come on and stays on, Also you noticed that the airbag off light stays on as well. Your car has never been in an accident, and your bags and belts have never deployed. You have some basic mechanical aptitude and don’t want to pay a dealer 500-800 $US for this repair. Read On…
It is not necessary but best to look for an independent mechanic shop that displays the 'pie', and have the codes read, this will tell you exactly what the problem is. Most will not charge for this service, and unless you have the MB software and can read the exact pin, you can’t do it..

Lets try and address the most common questions regarding this condition.

Q; - Is this a safety issue, the car still runs and a friend of mine who has a 2000 210 has a piece of electrical tape over the light and just drives it. Do the air bags still work?

A; No, Operation of all SRS devices are unpredictable, they may not fire at all, or they may fire for no reason

Q; How many Sensors are there and where are they?

A; Many.. The SRS system has a pre-tensioner in each seat belt, several air bags, sensors throughout the car to activate same, and a child sensor in the front passenger seat.

Q; Before I tear my car apart I would like to know as much as I can, what the module looks like and yhe functionality and design information

A; The child sensor adjusts air bag and seat belt level for the front passenger, and effects the functionality of several other devices.

Q; Is a sensor of some kind in the drivers seat?

A: No, a seat belt switch, and seat belt tensioner , and air bags.

Q; If I replace the Ass Mass will the light go off?

A; BE Clear Here, when you fix the fault, The Light WILL go off by itself if all faults have been corrected.

Q; How do I do this repair myself?

A; On the next posts, you will see a series of pics, but the procedure is,,,
1. At some point, have the codes read, by an inde, (PIC 1 ) as having the seat pad SRS light on will 'MASK' other ( emergency tensioning retractor belt and bag) faults.
2. Buy a NEW sensor first, (PIC 2 ) I have not seen ANY signs of wear ( wires, scraping, breaks ) at all. The sensor 'dots' fail with use/age. Sensors cost 140 US$ at MB.
3. TIP- Take the seat out of the car, period. If you have never done one, and are not an MB tech, or leather repair expert, 5 bolts and some clips and the seat is out. Don't even try to in car cover remove. you will me$$ it up!
4. TIP- Allow an extra 2 hours to shampoo the carpet under BOTH front seats. which is what you will do when you see all the gack under the seat. My car gets 'mini detailed' weekly and it still had lots of filth under the seat(s). Record amount of coins found so far.. $16.83 US$’
5. Unplug all wires from the connector block (PIC 6-7 )
6. Snap off the bolt covers (PIC 8 )
7. Remove all four 'Torx' type bolts (PIC 9-10 )that secure seat rails in place on the car floor.
8. Pull OUT not up on seat belt bolt cover(PIC 11 ) and remove bolt.
9. Release the wire cable (PIC 12 )
10. Tip the seat top down and remove the complete car seat from the car.
11. Remove the connector block and unplug the yellow sensor (PIC 13 ) connector from it's housing under the seat.
12.Take off the plastic side covers.
13. Loosen the 2 back bolts, (PIC 14-15 ) and remove the 2 front ones, This is optional and done to make the job easier.
14. Push down on the leather, (PIC 16-17 ) and pull out the bar that holds it to the frame.
15. Twist 90 degrees on the plastic ‘butterfly’ clips, (PIC 18-19-20 ) and release the leather cover.
16. Change out the sensor pad (PIC 21-22-23 ).
17. Once you have removed the sensor pad unit replacement is simply a reverse procedure. (PIC 24-25-26 )


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A big "Thank you" to everybody who contributed to this DIY guide. It had been more than two years that my father's car had the "SRS" warning and "Airbag Off" constantly lit, but due to the high repair cost he didn't have it fixed. The dealer had confirmed that it was the seat sensor a while ago, so today I removed the seat and inspected the unit.

Although the unit and the wiring seemed intact, testing with a multimeter showed that it was a wiring fault. One of the three cables that exist inside the black insulation was broken due to constant stress, but the insulation on the outside had absolutely no signs of damage. Quite deceiving I must say. A small piece of cable and a little time with the soldering iron solved it. As others said before me, always test the wiring before buying a seat grid, the repair cost of the cables is minimal in comparison to a complete new unit.

Thanks again everyone!
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Yet another success

Thank you Matt, Greg, OL, I am soooo not worthy, enjoy.

I was not able to do step 14:

"14. Push down on the leather, (PIC 16-17 ) and pull out the bar that holds it to the frame."

It was difficult to pull out the bar. The end of the bar toward the back is easy to be seen and pulled out using a small flat end screw driver, but not the other end, unless I cut the leather open.

Can anyone provide some tips on pulling out the bar?

Are there total of three bars? tow on the side and one at the front? or one single continuous bar?
Well I crashed and burned today trying this. Your instructions were great, but I (am trying this on a 99 E320 Wagon) took the bolts off that held the seat back to the seat bottom and seat pan, and now I cant get it reassembled. Methinks Monday Im paying a trip to an upholsterers shop to get this reassembled without any further damage.
Update to this saga. I took the disassembled seat to an auto trim shop here in Cookeville Monday, and for the $51 he completely disassembled the seat, put the sensor pad in correctly and reassembled the seat. He did not however reinstall the seat. Which Im trying to do now...
Im trying to get the connectors in correctly, I'll describe the female side first, the male side connects to the seat. I have the yellow connector, which drives the SRS, a Red connector which has 4 female pins, of which only two are used, and a black connector which only has two male pins....Heres the rub. The black connector pins came out, and are now dangling loose. They are a purple wire and a brown wire.
The female side has a solid brown wire and a brown/white stripe wire that come in. The purple/brown combo are vertical. Can anyone tell me which male pin (purple or solid brown) connects to which female (solid brown/brown/white stripe?)
Ive tried both ways. Every function on the seat works fine except two.
1) The headrest relay works fine, I can hear it click, but the headrest will not go up/down on the seat.
2) The SRS light on the dashboard is still on, but the Airbag on/off light on the passengers side of the console now goes off when the vehicle is started, which previously it was lit fulltime.
Questions oh great internet guru-types.
1) Purple/Brown placement, can someone look on a working car and tell me how it goes?
The drivers side doesnt seem to use the same color wires I think, Im going to go disassemble it, but thought Id ask here too. The drivers side uses three wires in the black connector, unlike the passenger side which uses two.
2) Does this SRS thing now work, since the airbag light is off, even thought the dashboard light is still on?
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Put another one in the WIN column.

After running out of time and patience a couple of months ago, I finally had access to the car, nice weather and enough time to actually TAKE my time.

The seat came out easily, since I was familiar with that process from the first go around. I had the necessary torx heads bits to loosen the frame from the seat this time. I removed the entire seat pad by pulling the bar out of the back as well.

Twisting the white plastic clips popped the cover loose. The sensor slid right out after I removed the insert that covers the white control box and pulled the wire through the hole in the cushion.

The hardest part was getting the white clips to bite back into the wire that holds the cover on. I used a spark plug socket underneath and was able to press the wire into the clips.

The only way to get the yellow socket out that I could see was to remove the plastic plug panel from the frame via the two screws on either side. Then I popped the back off and the yellow socket slid right out.

I wish I had read posts from sjphill before I started. I would have paid attention to the wires. I was working from memory, though. Sorry.

All the rest was as described.

Thanks for the instructions. The SRS light is out.

On to the brake light switch next.
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big thanks to this thread....pulled seat and sensor and everything looked good....checked the wires and white wire had no continuity...soldered new wire and BINGO.....
I swaps the front cushions few days ago and while pulling the sensor, I broke it. I was not impreesed with that. MB quoted me 90 pounds for a new one...
I fixed it, now, I don't know how long it will hold but its working.
The sensor coil was broke in half. I tried to put it back together but there was no way possible. It was not working.
After many attempts like scrapping the plastic with a razor blade to uncover the conductor, it just shreed away as too thin.
I was able to separate the two plastic half enclosing the conductor but bugger, the one side that was free was the black one, the protective non conductive part...
At this time, I decide to put some acetone on the back black stuff ant it disolved it, revealing the meatalic conductor. This took a lot of time and thinking but finally I got it.
From there I just taped some wire to the conductor. Its not pretty but it work. You need to understand that I went so many times back and forth that at the end, pretty or not pretty, who cares.
The main thing, it work, dunno for how long though.


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Hey Guys,

I really appreciate this thread:
As I have the same prob.
ONly I am going to spring for the 700 beans to my mechanic
two reasons:
1. its complicated and time consuming, I can handle one or the other but not both at once.
2. i drive like a maniac and my wife sits in the passenger seat. i dont want to hear it for the rest of my life: "so you didnt fix it yata yata and we crashed yata yata"
you marrieds know the drill.

much obliged,

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Seat sensor Pad replacement 2001 E55

I replaced my sensor pad today and this thread was most helpful. My car has a pneumatic seat and this was not really addressed. I could not easily remove the air tubing and was afraid to force the angled connector from its attachment at the seat. I made a small cut in the tubing and slid it off leaving the connecto in place. The control box for the pneumatic seat is rivetted to the frame. Rather than drill out the rivets, I bent it out enough to access the seat cover attachement, When I elevated the leather, there were more layers of material than shown in this post in order to encase the air bladder in the seat bottom. I wound up unhooking the seat cover from the clips and sliding the old sensor out from between the padding and the cover. It would not fold back all the way as shown in the photos posted here. I was able to slide it in between & re-hook the cover. Re-assembled in reverse order. The next problem was hooking back up the air tube to the angled gray connector. It would not "slide" back on. I tried to heat it with a flame and got it to go baack on partially. It Leaked. I wound up using a hemostat (small surgical clamp) inside the opening of the tube and spread while applying a flame from a lighter. This softened the tubing and allowed it to open enough to immedialtly slide it back over the connector where it contracted and sealed (thank goodness). There may be a better way. If somone knows for sure how to disconnect the tubing it might help the next guy with avoiding the headache and possible leak. I was more afraid of damaging the connector and really being stuck. The SRS light is off and no leaks. I hope this helps some others to avoid the $700 the Stealership wanted.
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4524 hits World Wide.......

I had no idea this diy would be so popular.
I should have had my nails done for the photos.
It was a honor to pay back just a bit of the thousand$ of dollars (US) that this site has saed me.
You all are most welcome.
Reinstalling seat cover butterfly clips

Big thanks to all who have contributed to this thread; getting the seat out and dismantling to the occupancy sensor was straightforward. Replaced the sensor however I've now spent two hours trying to get the little while butterfly clips to re engage with the metal rods in the seat cover.

Any additional advice on how to do this?


re - Reinstalling seat cover butterfly clips

Wrestled the clips and got the cover back on, I wish I had a sure fire technique to share but no such luck. I stood a 4" piece of wooden coat rail on the ground and then sat the seat, cushion up, on top so the clip was pushed up. In this way I could feel where the end of the clip is pushing into the cover and wiggle the cover around until the clip engages in the rod. Somone who does this on a regular basis MUST have a better technique.

Thanks again to all who have contributed, no more SRS light.

I think I fixed it (Finally). Thanks so much for all the pics - they really helped me understand what all was involved in the process. Car seat is out, and wife took car to the movies - so I guess I'll figure out if it was just a bad cable as soon as she gets back and I get the seat all hooked up again. I bet driving a car without the passenger seat feels weird... :)

EDIT: the issue with my seat was the cable as well. Redid the cables and all is good now.
AH yes...... yet another success story

Those butterfly clips are a bit of a challange, I just snap them back with one hand pushing and twisting up on the clip, and the other pushing down on the seat cover rod. Also, I look between the seat cover and pad to insure line up.

Also, I have heard that the new sensors might need to be adjusted or version coded for the car. Any input would help.

Glad so many have found this helpfull, and thanks again to the person who edited.


Thank you very much been a wonderful expression alone is worn leather upholstery and the dismantling Can you give more details about my English is not good I am from Turkey on this issue Turkish Mercedes tanıtabilirmiyim Thanks again for the form

THE SOLUTION MAY BE A LOT SIMPLER, as already indicated by a previous poster.

MB are great cars mechanically, but electrically they are behind a bit. (not sure if they lack incentives to improve).

As indicated before a few times on this thread, one of the 3 sensor wires between the connector and the sensor box under the seat cushion can break. I had the same problem. Unfortunately I had already bought the new sensor (as the dealer told me it had failed and that it was very common. SURE).

So what most likely is going on is the following:
1. Each wire has only one thick thick and brittle strand of copper.
2. The way the tie wraps are placed under the seat to hold the cables together it seems that stress is placed on the wires as the seat is moved up and down and you know what happens on cheap single strand wires when you flex them back and forth a bit. It only takes one and only one strand crack for the circuit to open.

In my case the white wire lost continuity between the yellow connector and just as it went into the hole under the seat. Most likely the crack would occur near the place where the tie wrap that ties it to under the seat.

So here is what I suggest:

1. Once you are sure that the SRS alarm is caused by the seat sensor failure, remove the passenger front seat from the car, following the instructions at the beginning of this thread. (I disconnected the battery just for insurance)
2. Test the continuity for each of the 3 sensor wires from the connector to a place near the hole where the cable with those wires goes into the seat sensor box (not visible at this point). As I said before, if you were to be lucky to have a broken wire it will most likely be near the places where tie wraps attach the cable to under the seat.
3. If you find a broken wire then you have 2 options. Either bypass the broken wire using safe and proper procedures using multi strand wires with the same kind of insulation as the originals (note that this is not a trivial job for an amateur). Or replace the whole sensor anyways so as not to have patched wire works under the seat.
I chose to replace the sensor as I already had bought a new one. Yeah yeah I could have saved that $100, but I wanted to have all I could need when I decided to take the thing apart and the experts had already told me that the sensor was bad.

Incidentally, removing the leather cover from the cushion requires more than 1 person so that while one compresses the seat the other removes the bottom bar that attaches the leather to the base. The photo on this thread does show 2 guys doing that step and you need it so as not to damage the leather using tools to force your way. I was able to remove the leather without any tools or screw drivers with 2 people.

Do not remove the screw on the back of the seat, as indicated on the thread. The picture shows the screws all removed and the seat cushion completely off from the main assembly. I know why that was done, as connecting the clips to the wires on the leather is a tedious process , but if you follow the initial procedures using longscrew drivers to re-attach the leather wires to the clips you will have to disconnect a bunch of wires, including the seat belt clip sensor wires. I have seen many people who contributed with ideas on how to connect the clips to the wires, but they all require some trial and error. An idea just came to mind (maybe the thread starter did this way but did not provide details enough), if you were to flip up the bottom of the leather upwards you should then be able to see the wires and the clips and working from the back to the front hook them all. The leather is very tight at the bottom ends so this maybe difficult and the fishing method maybe the only one, but I find it hard to believe it was the way the seats were put together the first time.

The real tedious part is to have the seat assembly on the floor (use a carpet so that you don't scratch solid floors) and to try to work on the seat cushion while bending down. Ideally you should do this work with the seat assembly on top of a workbench or a table with some cover to protect it. That way you don't need to keep bending and can see the work better.

IMPORTANT: When routing the cables under the seat keep in mind that it moves up and down as well as forward and back so leave enough slack when using cable ties so that they don't get stretched or bent too hard, which would cause another failure very fast.

If you decide to do this work be patient , make sure you have the correct size star bits. Get someone to help removing the leather and have a beer at the end as you would have saved >400 in charges. If you take 4 hours to do this job then consider yourself a highly paid part time worker on a day off. Not bad. If you get your son to help you then make it one of those quality time father-son bonding moments we seem to have very little nowadays.

Now I am expert in fixing yet another MB problem I never had with any car in the past. Mind you I did not have many with electrically adjustable seats. LOL, but they did have MAF, Crank position sensors, etc and until I bought a MB I had problems with those before.
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I mounted the seat back in the car today and now have no more SRS light on. Also the Seatbelt OFF light is OFF now. great.
One more thing that I remembered happening when the SRS light came on before was that the driver side head rest would not move when commanded using the seat control at the door. Now that the SRS light is OFF the driver side seat rest works fine. Another of those "bugs" that happen on the MB firmware when an error code exists. Who knows if that is intentional to let them know easily what is causing the srs to fail? In any case I am not sure if that would happen in all cases, but if you guys have a similar problem check the driver side head rest and let us know what you find. Maybe we can use this as a tool to know when a passenger seat Ass sensor has failed.
What is the part number for this sensor? Is it different between models?
9,300 and counting, wow...

By all means check that wire and replace it if possible.

Just don't take it 2 MB and give them 1,000 US$.

DIY on dudes.
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