I bought the car used, and didn't notice when I did my inspection that the RF portion of the key remote wasn't working. When the sales guy showed me the car, he always unlocked it using the IR receiver on the drivers side door, so it seemed to be working. I did notice the problem later on, but thought that it wasn't a big deal, and that I would just take care of it. I love the car so much that a little thing such as my key remote not working wouldn't have stopped me from buying the car...

Well, a month later I bought an antenna amplifier module (from ebay), thinking that it is the obvious culprit - since I see those issues every so often. So when I replaced it, I thought all would be good - but - nothing...

I then went to check the fuses (which is what I should have done first - I know). I noticed that the fuse for the alarm/antenna module (7.5 amps) was blown - so I try to replace it. As soon as I put a new one, it blew right away. So I tried to disconnect the antenna module, thinking I got a bad unit and it may be shorted - but after completely disconnecting the antenna module, and trying another fuse - the fuse popped again! (son of a...)

So the question is, have any of you had any experience with something similar? It's obviously a short somewhere, but I am hoping that someone here would read this and say "yes this happened to me, and here is what we found..." I will be checking the alarm horn next, but after that - I really don't know what to check next. The wiring for the alarm system is everywhere....

The central locking works flawlessy - both from the IR receiver, and from the center console - but NOT from the RF Key, obviously because the antenna amplifier has no power due the the fuse being blown.

The alarm horn does not work - the lights blink when I set it off - but no sound (i'm guessing this may indeed be the culprit)

Help!!

 

My siren died aswell, and apparently there's a battey in the siren to power it.... It died and the dealer wanted $500, I'm going to do it soon myself assuming it's a battery that can be replaced

 

Are you having the fuse issue also, and are your Keys working?

 

Well when it happened the key wouldn't work AT ALL, even starting it wouldn't work.. No fuses blown, except I did find that my locking gas cap fuse blew! Good catch. My SOS was malfunctioning too. Turned out that when I bought a new fob it was coded wrong and over 1 year later the car reads it as stolen.. New key and no problems since (7 mnths)

 

Update:

I went ahead and removed the Alarm Siren. It's located behind the driver side front wheel well (directly behind the "kompressor" badge).





After unplugging the siren, I put the fuse back - and SUCCESS!!. The fuse didn't pop! I took the siren completely out, checked my key FOB, and the RF features all work - lock, unlock, etc... I then started the car (knock on wood), and see that there are no warning messages... Drove it around, stopped, restarted, unlocked, locked.. and everything seems to be fine. So apparently, taking out the alarm will just leave your car without the audible alarm - not sure what other side effects there are - ill report on this thread if I encounter any...

Now I'm going to attempt to crack this thing open, to see how or why it was causing a short, and if it can be fixed.

 

Step by step, can you tell me how exactly you got to the siren, how you took it off, and which fuse controls it? looks like it took you 5 minutes.

 

Sure!

- Jack up the car, high enough so that the wheel is just about off the ground. This is so you have enough space to get to the siren.

- Loosen up the wheel well cover. To do this, you have to remove the screws that hold it in place. There are 5 plastic nuts that can be removed with a 10mm socket wrench - look around the wheel well, you will see these quite easily.

- Remove two 8mm screws from underneath.



- Remove the two plastic thingamajigs (not sure what theyr'e called) that hold the well cover on to the front portion of the side skirts. I used a sharp ice pick like tool to pry the center part out and long nose pliers to pull it out all the way.



- Now you're ready to pull the well cover back. When you pull it back, you will see the metal bracket holding the siren right away. Its held by 3 8mm nuts.



- Remove the 8mm nuts, and the bracket comes loose.



- Remove the wire that connects to the siren

- Remove the siren from the bracket. its using a screw type that Ive never seen before - I just used long nose pliers to turn it loose enough for me to be able to loosen it by hand.



- The Fuse in question is the 6th one down from the top (7.5). Its #8, I believe from the fuse chart, and its labelled Alarm/Antenna Module.



- I just replaced the fuse after the siren was disconnected, and voila - no pop, and my key now works..

 

So, I opened up the alarm siren to see what was going on.. and sorry to say that its not worth fixing. There is no way to open it up without destroying the shell... Although I could have "epoxied" it back together if it was fixable (or worth fixing).

I had to use a utility knife to open it up.. (there is no other way). I cut it right on the seam in the middle.



Here it is opened up..



The top looks pretty clean. If it was a battery issue, you should be able to replace it, as long as you are comfortable with a soldering iron.



The underside of the circuit board is a different story.. I was able to pry that burned out diode with my fingers.



I wonder what would have caused this thing to burn out... Its very unlikely that there were any voltage spikes. I'm pretty comfortable with simple electrical and electronics work, but this looks like a multi layer PCB, and other than that diode, I'm sure there are other bad components on this thing...

Oh well, at least we now know what the inside of the siren looks like

Now I need to find a replacement siren . Parts.com doesn't seem to have it, neither does genuinemercedesparts.com ... Anyone have a spare they want to sell?

 

if you find the P# ill try and find one, im in need of one too.

 

https://mbworld.org/forums/attachmen...0-18.01.09.jpg

I cant make it out but its there

 

A 171 820 25 26

 

^ ^ ^ ^

That is the right number.

 

whats the other side look like, the one opposite the P#?

 

can you tell me what fuse it was that you replaced? ive checked all my fuses but none seem to be blown. I have the same issue, rf doesn't work, and siren doesn't work. Everything worked fine until i tried to hard wire my radar to the wires going the the rearview mirror, and i guess i must have touched the two wires because after connecting the radars wires the wire no longer have +12

 

Wow, that was quite a while ago. Thread #7 shows pictures of the fuse box (scroll up) its on the trunk next to the navi dvd.

Does your alarm work? The alarm was on the same circuit as the rf. Not sure about the mirror.

 

checked all the fuses in the trunk, and I went back and rechecked the 7.5 fuse to make sure, it is fine. is it possible I blew a module? the alarm will only work via IR sensors and when the alarm goes , the lights flash but no siren.

 

Wow, there wont be an easy answer here... you could start by hunting down where the 12v+ started to become an open circuit. Id start to check the fuse to see if that has 12v+, then check the antenna amplifier to see if it has juice.. I assume it should always have power so that you can unlock via rf. If it has power coming in, that might be your problem.

If you determine your problem is the antenna amplifier, I can sell you mine - the one I bought but never used.. its just gathering dust here.

 

Brother...God Bless you for this...solved my problem

Thank you!