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Photo DIY - SRS Seat Sensor replacement

363067 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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Many thanks to MRBSMITH for the excellent guidance on how to replace the seat sensor. It kept me from going down a number of blind alleys. I would like to add some details and comments that I noted for the next time.
Step 1. I had the codes read by the local dealer (code B1315), and of course, received a quote of $220 for the sensor and $348 to install it.
Step 2. Great idea to gather everything first. From MB of Cincinnati I ordered a sensor (210-821-21-51 for a MY2000) for $144 plus shipping. I would add an E12 external Torx ® socket and a few tie wraps, black if you’re fussy.
Step 4. My yield was only a quarter ($0.25) and my wife’s favorite crochet hook (priceless).
Step 5. Defer this until after steps 6 and 7, at least for the power seat connections. You may need to move the seat in order to access the bolts and the seat belt attachment.
Step 10. Place protection on the carpet in front of the seat.
Step 11. Remove the connector laterally as shown in the picture, not axially.
Step 12. Unsnap and remove the fabric covered spacer piece. The outside cover is fastened with a screw at the back and is hooked at the front. The inside one is hooked at both ends. Bend it to release the back one first and then slide it forward to free the front one.
Step 13. Add “On both sides” at the beginning. Also loosen the rear bolts considerably in order to allow the seat frame to pivot upward.
Step14. If you have no assistant, lean on the seat with your chest, leaving your hands free to pull out the bar. Pivot the seat frame upward and clip tie wraps as needed. The top of the cushion frame fits in a groove in the rubberized fiber cushion. The cushion needs to be separated from the frame in order to gain access to all of the butterfly clips, and to ease the unwrapping of the leather cover over the cushion. Note that part of the cover assembly is a white fabric that is sewn to the leather and lies between it and the rubberized fiber.
Step 17. Well, there is a tricky part here, and that is attaching the wires in the fabric sleeves on the underside of the cover to the butterfly clips. I first rotated the clips back to their original orientation, then folded the cover back on itself from one side. Rather than use a long screwdriver I used a shorter one, and ground a notch in the blade tip. I was able to grab the wire in the sleeve with the notch in the screwdriver and push it into the groove in the cushion just adjacent to the clip. A satisfying “snap” can be heard, letting you know that you’re hooked up. It helps to hold the back of the clip with one hand and to work from the rear clips to the front ones. Pull the white fabric taut over the cushion and then the leather. Insert the frame top back into the groove in the cushion, etc. Before assembly of the fabric covered spacer to the seat, clean it with a lint roller and then upholstery cleaner. Also, clean at least the sides of the leather cover before reinstalling the seat. The SRS light is now out!
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