Mercedes C Class (W203) Alarm Problems

If your Mercedes C Class alarm starts to behave strangely with random false alarms sounding, indicators flashing without any alarm sound or even chirping whilst you are driving, then the number one cause for these issues is the siren module.  The siren module is a round black plastic canister about the size of a small bean can, containing an alarm sounder, control electronics and back up alarm system batteries.  It is these internal NiMH batteries that will start to age and deteriorate, giving rise to the conditions stated above.

Mercedes C Class Siren Sounder Battery Replacement 1

The problem with the batteries is not quite as simple as it sounds.  What happens is they start to leak and spread over the PCB inside the sealed alarm unit.  The electrolyte from these cells is conductive, as it spreads, it bridges circuit tracks and components making the alarm system unstable and causing it to do peculiar things.  A replacement sounder is around £100 from a dealership and once fitted will in the majority of cases, cure any problems you may have.

Mercedes C Class Siren Sounder Battery Replacement 2

The first thing to do is locate and remove the faulty sounder.  On the C Class (W203) it is located behind the plastic protection panel under the left hand (near side UK) front wheel arch.

Simply remove the road wheel, remove the five plastic 10mm nuts and remaining fixings that secure the inner protection panel. Remove it from under the wheel arch.  You should now be able to see the siren, fitted to a steel bracket that is fixed to the bodywork.  The torx self-tapping fixings that hold the siren bracket to the vehicle are security types with the inner peg.  If you don’t have the correct security tool, just tap them round slightly on their outer circumference to loosen with a small chisel or punch, once slack, they will usually remove with firm finger pressure.  The rearmost fixing is a strange 8mm crimped nut affair, that quickly removes with pliers. Once the support bracket is free, the siren can be unplugged and totally removed from the car.

Mercedes C Class Siren Sounder Battery Replacement 3

To replace the siren simply plug in the new unit with the car unlocked and alarm de-activated, re-fix the bracket and build up the inner arch panel. Fitting the wheel and lowering to the ground to complete the job.  If you are doing a straight swap-out with a new component and you have good elevated access, it is possible to replace the siren unit without removing the road wheel – by just undoing the trailing edge fixings of the plastic arch liner and pulling them out of the way while you remove/replace the siren unit.

Replacing the batteries in the siren module / sounder unit

If you have been following this site for a while, you will have probably realised that there is a little more on the subject to come –  there is an option that could save you some money and get your alarm working correctly again, for around a £15 chance investment with some DIY time and a soldering iron.  This style of siren module/unit is fitted to many models of Mercedes vehicles spanning many years and this repair technique is applicable to all instances where this type/style of sounder is used.  If you are interested then read on…

Mercedes C Class Siren Sounder Battery Replacement 4

The first thing to be aware of is that the batteries, as they leak, do start to corrode the PCB within the siren unit, if you do catch this in time (and it really has to be quite bad to be unsuccessful) then you can repair the unit very economically.  If however the PCB has deteriorated ‘just too far’ or there is another fault in the unit, you will have probably wasted your money and time, needing to buy a replacement. If its worth the risk of chancing your luck – that is up to you!  Interestingly, if you can live without any audible alarm at all, then you can just leave the siren unit disconnected and tape up the connector. This has no detrimental effect on the rest of the alarm system or its linked components.  My logic in all this is:  If I can fix it for a few pounds then all well and good, if not then leave it disconnected as £100 dealer charge for new or buying a used unit that is probably almost as bad inside as the one I am removing, albeit at the moment still working, are not in this case acceptable options. Equally a customer may decide that the cost to repair a sounder on an older vehicle is just not worth it – so you can just leave it disconnected!

Mercedes C Class Siren Sounder Battery Replacement 5

If you decide to have a go at changing the batteries you will need to remove the sounder from its bracket, again a single security torx fixing is the order of the day. Once this is removed the sounder can be cut open.  It is ‘weather-sealed’ and the only way to get it open is to cut carefully around the seam with a junior hacksaw, rotating the sounder as you go.  Do not allow the blade to penetrate deeper than half its width, as if you cut too far inside you may damage the PCB or sounder wiring.  Once the cut is continuous around the siren unit, pull apart the two halves.  The front part will contain all the electronics with sounder, the rear is just a shell cover with an aperture for the electrical connector pins.

Mercedes C Class Siren Sounder Battery Replacement 5

Unclip the sounder plug, pink and white wires inside and unclip the PCB pulling it off its two locating pegs.  It should now be out of the plastic body and you will be able to get a full visual idea of how bad the PCB has been damaged by the battery leakage.  The one pictured is quite bad and to be honest if it was any worse than this I would not go further, so in this instance it was a borderline repair, reducing my odds of it all working out successfully due to the poor condition.

Mercedes C Class Siren Sounder Battery Replacement 7

Note how the batteries are polarised, take a photo so you can be sure you are fitting the new NiCd’s with their correct polarity as once you get cleaning, often the markings on the PCB also get removed.  Using side cutters clip off the old batteries flush with the soldered holes and throw them away.  Using a soldering iron heat the remaining part of the battery pin until the solder melts fully, then quickly bang the PCB onto a cardboard box.  This shocks the molten solder and pin from the board and should leave you with clean holes for the new battery contact pins.  Repeat on all eight holes.

Mercedes C Class Siren Sounder Battery Replacement 8

Now you have the board clear, use PCB flux cleaner spray or other electronic solvent to scrub away the electrolyte that has leaked from the batteries, use an old tooth brush and lollypop stick to scrape the most stubborn material away. You can now get an idea if the battery juices have eaten into the copper track, if it passes inspection, continue cleaning the circuit board with an abrasive pad, I used one similar to those used to clean model railway tracks.  Eventually it should look like the photograph.

Mercedes C Class Siren Sounder Battery Replacement 9

You will need to purchase a pair of replacement batteries from a Maplin store or other electronic component outlet, they are quite a common 3.6v 140 to 160 mAh NiMH cell pack, measuring approximately 24mm x17mm x 15mm  The only issue the replacement batteries have is that the component legs on one side will need slight modification to fit the PCB but apart from that they are drop in replacements. The batteries cost in the order of £7.50 each. Part Number BN-24B

Mercedes C Class Siren Sounder Battery Replacement 10

Once you are satisfied with your cleaning, bend up the single mounting leg/tab of each battery into  a ‘U’ shape as you can see in the photographs.  Fit and solder the twin-tab side of the battery to the PCB, observing the correct polarity.  Once you have this side soldered in, cut a short length of thin stripped wire or tinned copper wire if you have it.  Using a small piece of cardboard packing under the edge of the battery solder one end of the stripped wire into the PCB.  Now loop it over the ‘U’ shape you have formed on the battery tag, threading it back into the board. Pull the wire tightly and solder the remaining end to the board.  Now heat and solder bond the wire to the bent tag as it passes through its formed ‘U’ slot. Clip off any protruding wire from the back of the PCB

Mercedes C Class Siren Sounder Battery Replacement 11

Fit both batteries and check your work for solder bridges etc.  Once you are happy, mask off the siren sounder plug and three prong external connector pins.  Spray the complete electronic board with clear lacquer, both sides and allow to dry thoroughly.  Once dry, refit the electronics to the front section of the siren unit. Connect the red and pink wires to the sounder unit then run a continuious bead of silicone sealer or epoxy resin around the plastic seam of the housing.  Offer up the rear plastic housing section and press it into position.

Mercedes C Class Siren Sounder Battery Replacement 12

Clamp the assembled sounder/siren unit in a vice gently until the silicone/epoxy as hardened. Then for good measure, although really not necessary, tape around the seam joint with insulation tape making a tight secondary water resistant seal.

Mercedes C Class Siren Sounder Battery Replacement 14

You can now refit the rebuilt unit to the car and test your work, either it will work perfectly or your time has been wasted!  If successful then you have saved yourself  a wedge of money, if it does not work then you have wasted £15 and can just leave it disconnected or go out and buy a new one – the choice is yours!

Hopefully all will be well and your Mercedes will once again have a fully functional alarm.


43 thoughts on “Mercedes C Class (W203) Alarm Problems

  1. Thanks for the information , I have 2012 b class and the alarm triggered for 6 hrs on the ferry, charged the car batery no probs withe car but the alarm goes off randomly , also the small turn off switches above wind screen dont flash red anymore. I am that fed up I am going to disconnect from siren. Thanks so much for your help.

    1. Hi Brian,
      Yes the classic siren module battery problem. As most of the controls for the alarm are resident in that little box then I very much suspect that if you replaced this the windscreen indicator lamps would work correctly again. Disconnecting is the quick and dirty option – but I still consider the MB electronic key be the best theft deterrent and rather like house alarms – does anyone take any notice anyway!
      All the best

      1. Hi Steve I am going to disconnect the alarm siren of my b class 2012 and after I removed the wheel and inner guard. Is the alarm shape round because what I can see is not completely circular it has formed on the rim like an oval shape or am I looking at the wrong thing. The size is same as a small bean can but not completely round.Could the shape have change. Thanks for any help Brian

      2. Hi Brian,
        I can’t say for sure as thats a little modern for my regular work. I have had a quick look on Google images (search – Mercedes Siren Sounder) and there are a few sounders with a more rounded nose rather like a dome, this is most likely what you are describing. If in doubt, give your local dealer a call tomorrow just to double check what you are looking at is correct. Even if you just give him your VIN number and get the part number of the device, it is often written on the item in question and with this double check you will be sure.
        Hope that helps

  2. Steve I have now looked at nearside and offside under the wheel arch guards and mounted on brackets retained with a 6mm bolts are the horns which look like the alarm siren except for the shape. I
    think these may be also used for the alarm sirens. So maybe there is a signal electrically which activate the alarm. I am continuing to find the cause and will keep in touch.

    1. Hi Brian,
      The cause of sporadic activation is often due to the in car motion (Ultrasonic) sensors, try disarming them on the dash switch if not available, try taping some stiff card over them, often its temperature related and these frosty nights will just tip them over the edge if they are playing up. Keep me in the loop as interested as to what you find.
      All the best

  3. Hi Steve I have located the alarm siren and removed it.. It is located on the nearside wheel arch or should I say you can access it there. Remove inner plastic guard nearest to the door side and look up the back of the door cavity it sat at the top.

    Steve drop me an e mail and I will send you the photos this pin points exactly where it is located its my 3Rd attempt but got there in the end. Will keep in touch as I carry out services and repairs.

      1. I am going to split nothing to loose will let you know how I get on. The photos show you the location looking from the outside and obviously that’s where it’s located in the inner wing. Will keep in touch

      2. Hi Brian,
        Thanks for the information and photos.Here are the photographs you sent me to post explaining the location of the sounder on a 2012 B Class Mercedes. This will provide a great permanent record for those with this model of vehicle.

        Location of Alarm Sounder 2012 Mercedes B Class:

        Under the wheel arch:

        The Sounder:

        Inside the alarm sounder:

        The Battery:

        Many thanks for this very useful information Brian.
        All the best

  4. Hi Steve great informative piece. My c220 cdi seems to be alarmed when the sun shines so goes off occasionally.
    Located and striped unit and found to be very clean no leakage. Tested battery voltage and shows 4.15mv each 8.3 total.
    So have to assume that the problem is some wear else or another component within the unit. I intend to leave the siren off as if not working correctly they are more of a nuisance than safeguard .

    1. Hi Maurice,
      Often temperature related faults are due to the ultrasonic elements deteriorating in the interior movement sensor circuit. Careful use of a hair dryer can isolate the problem but be be care full not to damage any plastics surrounds! I am assuming the voltage you measured on the alarm battery set was in fact in ‘Volts’ and not ‘mV’ (millivolts) as you wrote. If it were millivolts, they are for sure faulty! 🙂
      Hope that helps,
      All the best

  5. Hi Steve, Thank you for your reply. Yes correct should have been volts. Regarding the interior movement sensors. Could I ask for your help again as to where these are located.
    Kind regards


    1. Hi Maurice,
      Where fitted, are resident in the centre of the roof here.
      Ultrasonic Sensor C200
      Ultrasonic Sensor Location C200
      You could always fully tape them over, temporarily to see if it makes any difference to the false triggers on a hot day. Sometimes a good ‘hoover’ out with a domestic vac will help. If it continues to play up try the tape. Its worth locating a used part and swapping it out if it starts to behave when taped over.
      All the best

    1. Hi Gavin,
      In simple terms yes – but obviously you would have no alarm function/s – though depending what version you have fitted, the hazard warning indication/flashing of side repeaters may occur to signal a ‘fault’ in the alarm system with the module removed or disconnected. Some will allow total removal of the siren module without this result, others will not, you would just have to try to be 100% sure.

      All the best

  6. HI Steve I didn’t fit a new alarm it is now 3. months and there are no problems with removing the alarm. I thought maybe there would be some consequences after I decided not to bother with the alarm but it has had no affect on anything else. Just thought you might want to know this information as you hear people say it could affect the electrics on the car well I haven’t come across any probs after removing the alarm.

  7. Hi my alarm on my 2004 C180 coupe has the same alarm symptoms ie flashing indicators but no audible alarm siren, also the car won’t lock or unlock on the electronic key fob. I have two fobs but neither will work. Could this be the siren sounder causing this fault please?

    Thanks in advance

    1. Hi Anthony,
      Yes it could well be as once the battery electrolyte spreads onto the PCB inside the sounder it begins to short out connections within – anything could happen! The only other source of problem could be the failure of one or more of the internal ultrasonic transmitter / receivers. See if turning the movement detection off on the dashboard switch allows you set/clear the alarm.
      All the best

  8. I replaced my GLK300 sirene but still, symptom remains, going on and off randomly until battery dies too – driving me and all my garage users/neighbors crazy – is there any “check” way to find what’s the fault item?

    1. Hi Denis,
      It could be that the ultrasonic sensors if fitted are playing up inside the cab area. Just as a temporary test, put insulation tape over them and see if the false alarm still persists. Check if there is a bonnet open sensor/switch on that model and that it is made correctly when the bonnet/hood is closed.
      Hope that helps,
      All the best

  9. Hi My C Class is 1997 when setting the alarm the indicators continue to flash but at a faster rate would this be the same problem on a car this old?

    1. Hi Graham,
      If this were the E class W124 model it has an older type of alarm/immobiliser circuit that uses in a great deal of cases an aftermarket fit system. As I remember on the 124 it is located in the passenger footwell under the carpet on the ramp section where you would place your feet. It was susceptible to water/damp ingress if by chance you had some water getting into the car, or simply condensation gets to work on the unit and its connectors. The unit could be disconnected altogether provided you had a bypass on the start run relay. This modification has been well documented and is quite simple to carry out. The C class of the same era (yours) had a little more complex unit that was linked to the ECU and key code reader, because of this it is not so simple to isolate faults. However the alarm sounder (one of two types) could still be causing the issue. It is documented here and it contains a little more info that may give you some further clues as to what might get you by.

      Sorry I can’t be more help, let me know how you get on.
      All the best

  10. I have a similar problem with a year 2000 C class estate. Alarm sounds randomly, and is probably draining the car battery somehow as even though the car has a recent new battery, occasionally it will not start and has to be jump started. As a quick fix is there a fuse I can remove that will disable the alarm but that will allow the car to run normally?

    1. Hi Chris,
      Its not quite as simple as that on your model, the slightly older E class done with a simple bypass but I think your model is coded to the ECU. Pull the passenger carpet and remove the kick panel underneath, check all this wiring for damp and corrosion as if there is any water getting in it finds its way here and in some instances causes a few issues.
      Hope that helps.

  11. I have this problem on a 1999 C220CDI the way I get around the issue is ‘set’ my remote key fob to only unlock the driver’s door on the first click. There are videos on Youtube that show you how to do this.
    1. Get everyone out of the car.
    2. Lock the car
    3. Unlock the car (so only the driver’s door will be unlocked)
    4. Open and close the driver’s door – you must do this step.
    5. Remove the ‘key’ from the fob and manually lock the driver’s door.
    6. You should then have a locked car but with no alarm set.

  12. I have a c250 sport TD auto 1999 and the alarm system has just startwred playing up again after being dormant all summer,a new battery on and still at it,not a good idea parking in the village early in the morning or late at night.
    My husband wanting to disable the system,said he tried once and no indicates worked not a good idea,what did he do wrong.

  13. Hi Steve

    I have a C180K 2007 model and am having problems with flashing indicators when locking. The alarm kicks in a little later or sometimes on locking. Could this be the Sounder Unit? I am a little surprised that MB in Durban haven’t come up with this possibility. They have just informed that it is not the door switches.

    1. Hi Wally,
      More than likely its the sounder that is faulty. It could be the alarms ultrasonic motion detectors if fitted , give them a hoover out and check for spiders!

  14. Hi,

    can you send me a bigger picture about the colour of wires, because my wires came out from the pin. I have no idea what is the correct position. I know just the earth cable. The main board is perfect. I have a w203, year 2002

    Thanks Attila

  15. Thank you very much for the information. I have a 2001 Sports coupe with the alarm problem. Your article has been a great help.

  16. I own a C240 2004 wagon. For the longest time my lights would flash at odd and random times. Today I just finished replacing the siren. All is working well. I now have audible notifications – had not had that for a couple of years. The new siren cost was 140 dollars. Dealership wanted 80″ dollars to do this job……

  17. I have a CLC 2010 model and alarm has started going off randomly since had rear tyres changed. Garage did go in boot to get locking wheel nut – is there a sensor in the boot area that might have been moved and therefore more ‘sensitive’ than normal? All windows, roof and doors double checked shut. Deactivating the inside motion alarm seems to work at mom but would like to get to root of prob. Any idea’s anyone?

    1. Hoover out the ultrasonic sensors in the headlining mouldings, you may have a spider making his home ! There are no sensors affected by the motion alarm in the boot area.

      All the best

  18. Thanks for the info!

    My W208 CLK 230 sat for a couple of weeks, and now im using it again everytime i lock it – the alarm comes on (no flashing lights just the alarm sounding) & will sound 3/4 times over then stop.

    Had a couple of times it went off randomly prior to the 2 week sit, so Im thinking this is the issue with mine! Well i hope it is as Ive just spent £104 on a new alarm and its non refundable at Mercedes (b*****ds!) Hopefully this will be the fix! Alarm going off everytime its locked is annoying and embarassing!

    Hoping that by removing the access panel between the bumper and arch lining will be enough access so i dont have to remove wheel and arch lining etc. Also hoping to replace fog light at the same time – hopefully that isnt to big of a job!!

    Wish me luck and thanks for the advice

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