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Photo DIY - SRS Seat Sensor replacement

362997 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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Man… Talk about unpredictable and defying all conventional logic and reasoning!!!

I reasoned that if there were a break in any one of the wires, it would have occurred where the wires experience the most flexure. In my case that would have been somewhere after the wires disappear into the hole of the seat cushion. Just by pressing down on the seat bottom, I could see that that was the only place where any visible movement or flexure was occurring. Elsewhere, the wires were tied down and would essentially remain unmoving when anyone adjusted the seat or sat down on it.

I almost gave up right there, because I didn’t want to remove the leather seat cover entirely to get at the wires near the sensor itself. Nevertheless (against my logical intuition), I decided to test the continuity of the wires anyway, because of the experiences reported by others in this thread. Outside of the cushion, the three wires are encased in a black molded-on sheath. I carefully cut open this sheath, as near to the bottom of the cushion as I could get, and tested continuity of each wire between there and the yellow receptacle (don’t call it a plug). I simply clamped a sharp needle into some locking forceps, attached my tester to that, and poked each wire. It took almost no pressure to penetrate the thin insulation on each wire with the needle and make contact.

Damned if I didn’t find a break in the white wire! Then it became a matter of locating the break. Again, I made small cuts in the black sheathing and tested the exposed wires with the needle, using logic to guess where the break might be. I ended up making radial cuts at 6 different places along the length of the sheathing before I narrowed it down, because the break in the white wire wasn’t anywhere near a tie, a sharp bend, or anywhere else where you might have guessed it would be. I am thinking now that the break occurred, not because of normal movement of the seat, but simply because of small vibrations. I think the manufacturer used a bad batch of wire, where at some places the copper strands were mostly already broken and just barely hanging together by a thread to begin with, or that wires got pinched during the assembly process.

So, let this be a lesson. Set aside all logic and reasoning and just go by the experiences of others and myself in this thread. This has to be one of the weirdest problems I have ever encountered. I don’t think MB can be blamed for using solid-core wire. In fact, the conductors inside my white wire were clearly multi-strand. But I do think MB can be blamed for not investigating why so many sensors fail and for shoddy quality control, particularly because this problem seems to transcend many different models and years. A simple stress test would reveal when a bad batch of wire comes in to the manufacturing facility. Nowadays, there are also electronic tests that can detect and pinpoint where faults or weaknesses occur along long lengths of insulated wire in bulk shipments. And a simple review of assembly procedures would reveal if handling there were causing the breaks.

My white wire clearly showed telltale signs of having been pinched and nearly severed before it was encased in the black sheath. I think I was just plain lucky that it was in an accessible place. Sure, it still took several hours to find and repair it, but it saved me hundreds. Kudos to this forum.
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My car failed its mot yesterday on this sensor. Seems the 2013 mot rules now cover this in the UK.
Going to tackle it today.
I will look back when I get a second through this email but can anyone tell me if I need to reset with star after fixed or does it fix itself once repaired?
Many thanks!
Once the problem is fixed, it resets automatically and immediately when you turn the ignition on. At least it did on my car.
This is a nightmare repair. The only people I have allowed to sit in the passenger seat are those who don't weigh very much. The GF weighs less than 120 and the tech at the dealer I request is maybe 150. So far, mine works fine. I might out in left field on this but it has worked so far.
Sometimes reset has been needed, my personal thought is that it depends, which wire is broken and how the fault is generated (open/short circuit etc.).
Well this turned out not as hard as i thought.
Bit tricky taking it apart but once done I found the cable from the sensor was totally cut through! Very strange?
A quick solder and it was done. Maybe 4 hours taking it slow end to end.
Thanks for the tips and mine self reset itself thankfully.
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The fact that the break in one of the wires always seems to occur inside the black outer sheathing and (at least in my case) looks like it was physically cut or pinched before the wires were encased, makes me think this is sabotage by a disgruntled factory worker. I wonder how many of these sabotaged seat sensors ever even left the factory. For all we know, the worker might have been caught long ago and we were all just unlucky enough to get cars in which the wire was not cut completely and passed inspection.
Mine had been cut right through the whole cable close to the underside of the seat padding. A place impossible to be affected by rubbing o the metal work. Why and by whom is the question. Strange indeed.
That's exactly where mine was cut too. It turned out that the break was less than 1/2 inch from where I made the first cut in the black sheathing to expose the wires, but I didn't discover that until I had cut the sheathing in 5 other places. Locating the break, without cutting open and mangling the entire harness, was the most difficult part.
Mine was right through the cable - surgically clean - I would struggle to cut a cable as clean with wire cutters. I have no explanation of how it could come to be like that.
2005 c230 Kompressor:

I've got the SRS light and took it to the dealer. Codes came back saying that the child safety sensor in the passenger seat and the steering column harness were faulty. They want to charge me $1800 to repair them.

I believe I can do it myself and then have the codes reset, but I don't know the part numbers. Does anyone happen to know them or where I can find them?

2005 c230 Kompressor:

I've got the SRS light and took it to the dealer. Codes came back saying that the child safety sensor in the passenger seat and the steering column harness were faulty. They want to charge me $1800 to repair them.

I believe I can do it myself and then have the codes reset, but I don't know the part numbers. Does anyone happen to know them or where I can find them?

I don't know the part number but the dealership would (I know thanks buddy - but it is a dealer only part I am sure). However in my case when I looked into it it the sensor pad was around $500. I would take the time first and check the wires - in my case they looked fine but when I did a simple test to see if the was a fault in the section of wire from before and after where the wiring gets pinched - a wire that looked fine was actually broken inside the sheathing. You can test this will a volt/ohm meter you get from Radio Shack - or an electrical store. This was in my E320 but in my C230 I have had that code go off when someone had been in the passenger seat and did not clip in their seat belt.

It looks a bit daunting to fix yourself - but really it is not that bad. If you are a bit handy with a soldering gun and have a few hours you can do it yourself - just go slow and I always use my phone to take pictures when I disassemble so that when things go back in I remember how it went.

Good luck
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excellent write up!
SRS light on


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.
When you test the continuity in those wires, where do you place the probes? Are both ends of the wire available, or do you just probe sections of the wire until you find a break?
I did this job today. The cable for the sensor looked fine, but when I cut the tie wrap and tugged on the cable, it broke off in my hand.

I wound up removing the bottom seat cushion entirely to get the sensor out. An old audio patch cable gave me the three wires I needed. Got it all back together, went to run the seat back to the rear position, and discovered I should have run the car a bit in between bouts of working with the door open and the key in position 2 in the won't start. I get to jump it in the morning when my roommate gets home from work, whee. (And I just read the sticky about making sure to not have the key in the ignition when hooking up the jumper cables.)

Once I do that, then I can actually put the seat back where it normally goes and see if all that actually fixed the sensor...
Grrrr. Got it started with no problems, took it out for a drive...and with my roommate in the seat, airbag off light still lit. Guess it's time for a new sensor.
Grrrr. Got it started with no problems, took it out for a drive...and with my roommate in the seat, airbag off light still lit. Guess it's time for a new sensor.
I found out my airbag control module was bad. $550.00 from the dealer. Found on Ebay for $100. After install, had to take it to dealer for coding $130. The module is under the ash tray, 10 minutes to replace. Make sure both negative cables have been disconnected. Under the hood and trunk.
I found out my airbag control module was bad.
How did you find that out?
How did you find that out?
I took it to a place here in Toledo Ohio, European Autowerkes. They put it on their scanner and were unable to communicate with the airbag module. They did not charge me to do that. They have a customer for life. Mercedes Benz doesn't let private garages do any coding.
Grrrr. Got it started with no problems, took it out for a drive...and with my roommate in the seat, airbag off light still lit. Guess it's time for a new sensor.
Actually, it wasn't.

I replaced my clockspring to fix a steering wheel button problem, and then the #10 fuse under the hood because that had popped (probably due to the clockspring being intermittent).

Today was the first time my roommate sat in the passenger seat since I did that. No airbag off light!

Moral: If you've done this fix and the airbag light on the console (or the SRS light on the cluster, I suspect; mine's burned out) still comes on, check the fuses...
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