Well my alarm started going off on its own today. The alarm didn't even have to be armed. About 8 times, 10 minutes apart, just sitting in my driveway. So I pulled in inner fender cover and unplugged the siren.

Here is a picture of the siren mounted in a metal shield. The red wire is going to it. That is the connector I unplugged.

 

OK now who can tell me where the battery for the alarm is located?

 

I believe the internal battery for the alarm is mounted in the alarm housing. I have never seen it; so, I can't give specific details.

While you have the liner down, why don't you remove the alarm and follow the wires inside the alarm.

 

I just joined third forum so this may be outdated info, but I just replaced my alarm siren which is the part you are referring to and I opened my old component and the batteries are located inside. Not sure if you can buy just the battery pack because the siren is a sealed unit. The replacement part was $200.

 

Hi,

if your alarm siren is marked with this code (203 820 38 26), like in mine W208, the battery is inside the unit.

This unit can be opened by removing the two screws on the back and by patiently open it (it's possible that it was sealed with silicon, like mine).

Once opened, you can easly access the battery.
The battery is a 7,2V 140mAh composed by 6 Varta V150H elements in series.
I was unable to find the exact replacement with cable and connector, but it's quite simple to adapt a "standard" 6/V150H battery. You have just to solder the original cable on it, respecting the polarity.

Check if the circuit board was damaged by the battery acid. I had to wash the entire printed circuit board and rework some soldering in my unit.

Bye,
Edoardo.

 

I am about to change the alarm sounder on my W203. Does anyone know what size of Torx is required to remove the alarm. Am I right in thinking it is a security Torx?

Also - Should I disconnect the main car battery prior to installation of the alarm sounder? I have heard horror stories about things going wrong when the battery is disconnected.

Thanks
Stewart

 

I just took mine apart this morning ...

* The bolt is T40 ... I drilled out the security pin so I could use a normal torx bit ... the metal for this screw is soft so it took only a few minutes to drill out the pin.

* I cut the housing apart using a Dremel with plastic cutting wheel ... kind of a mess ... a tabletop bandsaw would have done much better. Just cut around the opening and go in only 1/8" or else you will cut into the components inside.

* As you can see from the pics, the PC board is kind of a mess. I'm not sure it's worth the hassle at this point of spending $30-40 on new batteries, trying to clean that PC board, and then gluing the case back together so that it's intact and waterproof. But I at least thought I would pass these pics along in case someone else wants to attempt it.


T40 bolt .. I drilled out the security pin.

 

Okay, this is a couple year old thread, but I just recently had this problem too. I used a hot plastic knife tool to open the one from my 2008 CLK350 convertible. I ordered the batteries on ebay, but while waiting for the batteries to get here, I accidentally knocked the bucket with my parts on the floor. The board went accross the floor. I knew that messed something up. Sure enough, after getting it put back together with the new batteries, it would chirp, chirp, chirp, chirp when I locked it with the key FOB. I looked that up and found it would be caused by water getting inside the alarm. So when it dropped on the floor, it damaged something to make it act up. So, I purchased a used one on ebay, figuring I will just put the new batteries on that one, figuring the batteries on the one I just got would be old and ready to do the same thing. So I ended up using the dremel tool to open it. That worked out nicely, but then I noticed the newer one doesn't even have the batteries in it. So I guess that means the newer cars don't have the battery backup, just in case someone disconnects the battery, to try to steal the car. It does have the spots to solder the batteries in. So I will put the new batteries on the new verson. Let me know if that's wrong, if it isn't recommended please. I don't see why it would be a problem.

 

Well, did soldering batteries to this board work for you?

 

Soldering the new batteries on this newer one worked perfectly.

 

what year vehicle/type did this new alarm come from? I'm in the process of replacing the batteries in my control board is horribly corroded.

 

So my issues was blowing #8 fuse and not being able to lock/unlock the car.
The folks in the tech forum pointed me in the right direction as to it being the alarm module as
www.charm.li W203 manual had nothing about it. I actually DID see a fault for short at alarm module but no idea how old it was
and cleared all errors on the car, since a lot has happened since I ever cleared everything last.
I didn't know the 2 were related, locks, fuse 8 and alarm.
https://mbworld.org/forums/mercedes-...king-help.html
So the module is out and no more blown fuses.

Those crappy Varta batteries are $23 each on ebay.
I don't have it apart yet to check the dimensions as I dont have a dremel or a hot knife,
but wondering if something like like this could be used instead.
https://www.batteriesplus.com/productdetails/cot10028

Just jumper to the 2nd location, has more than enough current for both locations.
OR if the board isn't fried, just toss 'em and fuggetaboutit. OR
just leave the alarm out.


OK, well I was able to get the module open without much trouble at all.
Mine has 2 tiny torx screws holding together, and then it only took a little effort
starting with a tiny screwdriver and moving to a larger one to pry it open.
What I found inside is completely different than whats shown in previous posts.

So it appears the 2 Varta batteries are SERIES not parallel as it is 7.2V and 250mah.
Anyway, so far not finding this battery anywhere and the date code on the board is 2501 as in 25th week of 2001. Part number is 2038203826,
Battery is 2 stacks of 3 button batteries in each. Strange they went to 2 batteries, and they didn't hold up as well as these.
23 years and no major leak, although it appears some stuff wicked up one wire and
the pin came out with the connector when I removed the battery.
If I decide to even bother with it all, as the car is already buttoned up,
I think I would either run a wire from the other side of the board if I can find a similar battery or,
just skip the battery.


MADE IN ITALY! I always told my service advisor during my MANY warranty visits.
"Mercedes builds the best Italian cars in all of Europe!"
I even had a Magnetti Marelli starter! But they sourced it from Denso so in the end, it was not from Italy.

As I said, some stuff wicked up through the wires which is quite visible here as well as the missing pin.
Well too bad I buttoned up the car already. I don't feel like repeating the process today just to see if it works without the
battery. Plus I don't have anymore 7.5 amp fuses if it doesn't. I guess I'll wait until I have a reason to be removing the right front wheel again.

 

I was going to try something like that, but it wouldn't fit. So I just got the proper ones and soldered them in. The next one I did, didn't even have a battery in the alarm. So I ordered the battery packs and soldered them in. So, when the regular car battery died and I opened the trunk to hook up a jump pack to unlock it, since the door key doesn't turn enough to unlock it, the alarm went off and wouldn't turn off for a bit. I really need to remember to turn the amp off when I park it. I need to connect the blue wire to a power supply that is only on when the key is on. And I need to replace the door locking mechanism that doesn't turn enough to unlock the door.

 

Prior to moving to Oregon coast, I livied in a big citywith lot of shady people about.
No one ever tried to steal or broke into my car. Knock on wood.
I was VERY lucky. Car break ins happened nightly where I lived, shattered windows etc.
But it's pretty quiet here, so not sure if it's even worth bothering to put the module back,
and definately not worth putting new batteries in that might leak AGAIN IMHO.

There's a reason they deleted them later.
What are the odds someone will disconnect the car battery and
then break into the car? They'd set off the alarm opening the hood straight away.
BUT I said, the next time the wheel is off I'l see what happens.