2004 C240 Anti-Theft Alarm Problem
 

Help! My 2004 C240 4Matic started having problem with its anti-theft system. It seems the problem is intermittent - works sometimes but goes crazy some other times. Sometimes I can set the alarm on (3-flashing lights) and turn it off (1 flashing light) with the blinkers. Other times I could not. The panic switch on the remote works on and off also. The alarm also goes off while the car is running/traveling and could not turn it off (it is supposed to be off when the key is in the ignition let alone when the car is actually running). The alarm just keeps going off to my embarassment. I checked all my door switches also, including the trunk by keeping open a door/trunk and turning on the system. When any door or trunk is kept open, I don't get the 3 flashing lights when I tried setting the alarm on. I can only get the 3 flashing lights when all the doors & trunk are closed - an indication that my door and trunk switches are working fine. I can only suspect problem with my anti-theft control module, perhaps it is about to go bad. Could I be correct?? Or is there something I'm missing to check? Can anyone offer some pointers/checks that I could do? BTW, where is the anti-theft control module located in the car? Appreciate all the help I can get from our MB experts/techs.

Many times the alarm issue is just the alarm siren.

About $160 for the part.

OF course the only real way to diagnosis is by connecting to a MB scanner & read codes from the rear SAM.

2004 C240 Anti-Theft Alarm Problem
 

Thank you, MB Doc! You could be right! The alarm siren maybe going bad. It goes off and on intermittently. One thing I've noticed when I tried the 'Panic' switch on my remote, the blinker lights were flashing but there is no sound coming from the alarm siren. I can also lock/secure the doors/trunk with the remote without any problem but when I opened a door (with the window down) there was no sound from the siren (sometimes), but once in a while it's sounding - like I said intermittently. This is probably the same reason why the alarm siren is going on/off while I was driving. The anti-theft system should be off when the key is in the ignition. I will definitely start with a new alarm siren and see if this will correct the problem I'm having.

You guys and all the MB Master Techs are definitely a saving grace to all of us out here!!!! Keep up the good work.

2004 C240 Anti-Theft Alarm Problem
 

MB Doc, where is the location of the anti-theft siren? Since my siren no longer works, I cannot tell where to look ofr it.

Under the RF fender inner liner

Look at this

2004 C240 Anti-Theft Alarm Problem
 

Found it, Thank You! Like MB Doc said, the horn was behind the right wheel well plastic liner. I was able to replace the offending anti-theft alarm (ATA) horn. By the way, the new part number provided by MB is A219 820 32 26. The old PN was 171 820 25 26. The new part is a little different in design, therefore, you'll need a new 8mm nut to secure it to the horn bracket. Tools needed to do the job includes T-40 & T-30 torks with the 'hole' in the middle, 10mm socket and 13mm wrench for the new nut. The job was straight-forward, took about 30 minutes. Now, my anti-theft alarm is working well again. By the way, the MB guy who sold me the part said the old horns go bad so often on these cars.

unplug and plug it back in
 

I saw everyone is jumping at replacing the part. Try unpluging and plugging back in (clean if necessary). I did that and everything is working great.