C250 CGi Alarm siren sounding but alarm not triggered!

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Borris1954

Active Member
Joined
Feb 4, 2013
Messages
414
Location
South Bucks
Car
S204 C 250 CGI BlueEFFICIENCY, R170 SLK 230, B 180d AMG Line
A siren activated this morning whilst my car was on the drive. I was working nearby in my garage and looked out to see no one near the car and no lights flashing on either my C Class or my wife's B Class and I initially thought it was a neighbours car. On closer inspection the sound was coming from my 2009 C250 Cgi yet the alarm did not appear to have been triggered. Using the remote to unlock the car did not stop the siren and neither did starting the car. Apart from the noise the car was normal. I shut the car off and locked it again with the remote. The siren continued when I unlocked the car again. I started the car and ran the engine a bit longer and after about 30 to 45 seconds the siren stopped. All now appears normal and when I connected SDS there are no fault codes stored or current associated with the alarm.

I am strongly suspecting I need a new alarm siren - it's original as far as I know so due replacement as its internal battery is probably at the end of its useful life. I am thinking that for whatever reason the siren thought it had been tampered with and only after running the engine for a while did it change its mind.

But before I spend the dosh has anyone had a similar experience and/or got any advice of other things to check? I see EPC lists a the siren connector as an available part so I will definitely take a look at that for corrosion as a possible cause.
 
At over £420 for a new unit from MB I think I need to check into this more fully before proceeding to replace...
 
A few used ones on e-bay if you want to go down the used parts route. This one is only £25 delivered.
Try a price from MB Newcastle or Edinburgh. £420 doesn't sound right.
I paid £100 for a new one about 4 years ago.

Part numbers that fit.
A204 870 57 26
A003 820 077 26
A003 382 089 26




Edit
Here is a one for £20

 
Thanks for the reply. The price I got was from MB Newcastle. But it almost certainly was for the later version probably A003 820 077 26. Interestingly I did find someone advertising a new one with the original A204 number for under £200. I have done exactly as you suggest and gone down the second hand route. It's an A003 820 077 26 with a late 2013 date stamp and includes plug and wiring tail. Price was under £20 so nothing to loose! Especially as I can return if its DOA.

When it comes I will give it a go and also open up my original to see if it's suffering battery leakage or water iningress. Maybe it will respond to fitting new batteries.

I see there is a footnote in EPC against the later part number to check/update firmware after fitting it. Can anyone confirm if this means the firmware in the rear SAM? I have already tweaked the alarm setting in the rear SAM (with Vediamo) for the Netherlands mod to enable chirps on alarm and disarm so it will be interesting to see if that still works.
 
On previous cars I owned, it was possible to force the sounder housing open, then using a soldering iron to replace the rechargeable batteries.
 
A quick update after looking at this again today. Basically the siren now appears totally dead. No "Netherland chirps" and if I open the car with the mechanical key and the alarm armed then the alarm flashes all lights as usual but no siren. I have now removed the siren and its an original A204... unit manufactured in 2006. I will open it up and have a look sometime but for now I have other issues...

With the wheel arch liner removed I thought I would replace the blown fog light bulb (both sides are currently blown). To my horror some cowboy has used RTV sealant to hold the bulb holder in place presumably because they damaged the light unit somehow getting it out to replace the bulb. I know have the bulb connected part removed but the metal part of the bulb and "glass" part is still stuck in the light. A problem for another day! On the plus side I did address an MOT advisory and replaced the anti roll bar drop link. Just need to do the other side and hope that fog light bulb comes out...
 
My replacement (used) Siren duly arrived and this afternoon I swapped it over and gave it a test. The result was somewhat disappointing in that the car behaves just as it does with the old siren. i.e. No lock/unlock chirps and no siren when I trigger the Alarm by opening the drivers door with mechanical key (when the alarm is armed). I just get the flashing lights.

Hooking up Xentry revealed that for both old and "new" siren:
  • There are no active or stored faults showing for the whole car
  • Both sirens show as having power and LIN communication
  • Both sirens "chirp" when using the actuation test on the screen below:
Actauations AOK.jpg

This leaves me just a bit baffled!
Anybody got any ideas?

There is a note on the adaptations tab that if you fit a new siren it needs to be SCN coded:

SCN Coding needed.jpg

I suppose its possible that both sirens have the same fault. One that shows no fault codes and passes the Activation test but won't chirp on lock or sound when alarm is activated. Seems unlikely to me.
Or maybe the old one has such a fault and the newer one just needs to be coded to sound (I will connect up Vediamo over the weekend and see how the coding compares to my saved coding).

All suggestions welcome at this point.

Addendum:
Interestingly when I looked at the alarm log I can see all my recent test alarm activations (these are dated) and one back in March when I first got the car and tested the Alarm but nothing for the Siren sounding as described at the start of this thread. In my mind this still makes me feel there must be an issue with the original siren.
 
"Or maybe the old one has such a fault and the newer one just needs to be coded to sound (I will connect up Vediamo over the weekend and see how the coding compares to my saved coding)."

It was exactly this. Once coded correctly in Vediamo all is fixed.

Odd that the faulty Siren doesn't throw a fault code. Anyway I will open it up sometime bit I am expecting to see the usual carnage caused by leaking internal batteries. Maybe it will respond to a good clean up and new batteries if not too far gone.
 
Where's the location of the alarm siren please? I'm having this trouble with my 2015 S212 E250. I need to rip it out before my neighbours torch the car lol
Thanks.
 
Just seen this so apologies for delay. It's inside the front left wheel arch. You need to remove the rear section of wheel arch liner and you will see it bottom left as you look at the car. Hope that helps.
 
Just seen this so apologies for delay. It's inside the front left wheel arch. You need to remove the rear section of wheel arch liner and you will see it bottom left as you look at the car. Hope that helps.
No worries I got it sorted. Only took about 30mins to rip out (in the dark 7pm at night 😂). I rang mercedes Sheffield and they're £130 but they're on major back order so it could be weeks or months so I'm risking a second hand one off eBay for now. See how it goes.

Thanks for reply anyway mate 👍🏼
 

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