BazUK

Hi all at MB World,

RE:2004 Mercedes CLK 200 K Elegance Coupe. 43,000 miles.

This is my first post so I will do my best to be clear and concise.

SRS light only coming on when a passenger is in the front seat. No other warning lights or messages and warning light clears on restart without a passenger.

After some reading up on this issue I applied pressure to the passenger seat whilst driving. The SRS light activates and then clears when pressure is released.

The problem would seem to be related to the passenger seat sensor. This has also been confirmed by a diagnostic test needed to clear the fault in order to pass the M.O.T. last week. I'm not sure how they found the fault because the SRS does not activate unless a passenger is in the vehicle. They said the SRS light was on under the M.O.T. test but I think they checked the management system for faults with their software but who am I to know???

Anyway, I would like to know if there could be a problem with a connection or wiring under the seat and whether an issue with the wiring here would trigger the SRS warning light with pressure applied to the seat occupancy sensor?? Before removing the seat to check the sensor & connections, I would like to eliminate any other possibilities.

Any advice would be very much appreciated!

MJ50

moved to W209 forum...

sailorben

I can't offer much advice, however, the car will store codes for sensors, so even if they're not present at the time of testing, if it was faulty at one point, then a fault code will be logged, for when it is later scanned.

Rudeney

You need to get codes pulled from the SRS module to know for sure, but I would suspect a defective seat weight sensor. In fact, I think it's the only SRS error that will clear itself - all others remain on the dash until you have them reset.

BazUK

Hi Sailorben, that's what I was thinking but I would like to know if a fault logged would fail the M.O.T? I thought the fault would have to be present at time of testing. Anyway, I paid for the diagnostic and they cleared the fault they said was present rather than just logged?
I wonder if garages check the management system for logged faults when your car is having an M.O.T?

BazUK

Hi Rodney, fingers crossed somebody registered on MBWorld may have experienecd the same fault or with a bit of luck, an MB auto electrician might read my post? I know the wiring is known to break, especially where it is tied to the seat frame and there can also be problems with the connector block, but can these issues bring up the SRS warning light when pressure is applied to the seat sensor? It would seem the sensor is working, i.e. when pressure is applied to the seat, a contact is being made and then the fault is being found. I will try and find a wiring diagram for the seat sensor circuit to find out if anything else could be involved??

Rudeney

The wire to the squib that run up the seat back will trigger a permanent code that will remain until the problem is fixed AND the system is reset. Like I said, the only SRS code that I am aware of that will clear itself is with the passenger seat occupancy sensor. Get the codes scanned and then we will know for sure what is going on.

BazUK

Thanks for the advice. To replace the seat sensor the seat will have to be removed. Any idea if it's ok to disconnect all the wiring blocks without disconnecting the battery? I might add another post to understand if disconnecting the battery is essential or just good practice.

Rudeney

There is no reason to disconnect the battery to remove the seat. Just make sure the ignition is switch off and the key is removed (or at last 6 feet away if you have KeylessGo).

icethomas

I am agree with you. I have connected seatbelt emulator and the SRS has disappeared from the dashboard. That’s doesn’t change the fact that the problem still appears. Now I am trying to find solution, where the problem is? If I need to replace whole seat? Who knows

any help/ advise will be much appreciated. I will share any post if I am able to fix the issue

BazUK

Looks like you have a faulty Passenger Occupancy Seat Sensor and possibly an additional issue with the passenger seat belt. My issue resulted in the SRS warning light remaining on. Initially it would come on and after a while go off but eventually it just remained on. A diagnostic confirmed the passenger seat sensor was an issue. These are to expensive to buy new. £850 plus vat. Yes that's correct, I even double checked through another Merc dealer and they had to double check after seeing the price.

I suggest contacting your local car scrap yards and then Mercedes breakers yards s for a second hand sensor. They may supply the sensor and base of the seat as one unit, only charging for the sensor. Removing the sensor is quite fiddly and you need the special tool to remove the clips from rods that are attached to the leather trim. If not done carefully you could easily damage the sensor circuit.

If you find a yard that has a working sensor you could ask them if you could plug it in to check if it's working prior to buying it. This would mean visiting the yard but due to Covid-19 restrictions they may not allow it.

Anyway with a working sensor whether still fitted inside of the seat base or if it has been removed, you can then plug it in prior to having it fitted into your seat. This clears the SRS Warning light if the fault is with your current passenger eat sensor.

If you are supplied a full seat base just push the passenger seat fully back, remove the correct yellow connector plug under the seat. Place the seat base in the passenger well and plug it in. Fingers crossed the SRS light will clear. If you have other faults that can trigger the SRS light obviously have them fixed prior to purchasing the seat sensor.

I will attach numerous photos to show you the components of the seat, including where the seat sensor is located. If a second hand sensor solves the problem, have someone who knows what they are doing to fit it. A trim specialist might do it if you show them the part and they are happy to do it. It may be cheaper than having a Mercedes specialist garage do it.

Good Luck!


W209 & C209. Leather seat base cover. Pic 1.

W209 & C209. Seat base cover underside. 3 Vertical metal rods and clips. Attach to rods in foam. One rear horizontal metal rod.

W209 & C209. Metal horizontal rod. Attaches to 3 clips in metal seat frame.

W209 & C209. Occupancy Seat Sensor situated on top of the seat foam. Metal rods situated left right and in the middle for leather cover clips.

W209 & C209. Underside of foam. Sensor Control Box protected with thin grey foam layer. Yellow connector block and wiring. Wiring fits through hole in the metal seat frame.

W209 & C209. Metal seat frame. Note three clips to rear. Horizontal rod attached to leather cover attaches through the foam to these clips.

W209 & C209. Close up of three seat fram clips. Note centre one broken trying to remove rod. Best left attached if sensor can be removed and replaced unclipping from the leather trim.

W209 & C209. Three slots in foam rear of sensor to allow clips to attach to horizontal rod.

W209 & C209. Underneath seat frame. Connector block and wiring. follow wiring to locate correct connector in block attached under the passenger seat. Remove this connector and attach new sensor to check SRS fault clears.