Another W202 alarm question; how to disconnect / disable W202 C class alarm

[ontheball247]

Hi I know there are loads of threads out there about W202 alarms that randomly go off, but non of them seem to give a definitive answer as to how to actually just disconnect the alarm. Well, except for one that tells you to disconnect the two smaller connectors on the module behind the carpet on the right side of the boot (ie. in the right side quarter panel) and then says that didn't work, so you actually need to disconnect a module further back in the boot, which I don't have.

Now first of all, I have a 1998 S reg W202 C250 TD and the alarm seems to go off randomly while the car is parked up, and sometimes simply walking past the car triggers it. But only my hazards flash, which suggests the previous owner disconnected the siren at some point. Also, there is a little plastic bracket next to the fuse box in the boot (next to the spare wheel) which another member seems to have a little green box attached to, with wires coming out of the bottom of it. But I only have loose wires going up to the bracket from underneath it, that aren't connected to anything.

Secondly, the reason I just want to disconnect the alarm altogether is because I'm not willing to spend any more money on the car. I've just spent loads on small annoying issues and spending more would not be viable considering the age and value of the car. So I simply just want to disconnect the alarm now. What's more, it seems to be popping off a lot more now than it used to and its draining the battery more and more frequently.

Can anyone help? I'm looking for either a quick cheap fix (which I know is hard to say without checking the car), or how to disconnect the alarm altogether.

Thanks a lot.

Baz.

 

[Dec]

You have siren A 220 820 28 26, you can either disconnect it which will solve your problem or replace it with A 219 820 32 26 for about £80 from the dealer, replacing with a new one will restore your alarm too working order, don’t use a second hand siren from a scrap yard.

Check your PMs.

 

[ontheball247]

Wow. Thanks a lot. If you're sure that'll cure it, i'll consider getting the new one. But I'll try unplugging the old one first.... Or see if it's already been unplugged, since the siren doesn't actually go off, as described... I only get the flashing hazard lights. I'll post back with the progress.

Thanks again for the reply and the pm

Baz.

 

[Dec]

The siren contains the brains of the alarm so unplugging it will stop the hazards flashing.
The hazards flashing indicate that the alarm is triggered,so, as there is no sound from the siren then the siren must be faulty.

The alarm siren plug will still be live so cover it in tape to protect it from any wet until you replace the siren.
Open the siren and you may see the cause of the fault, see here as to what you can expect… Alarm / Siren Issues : Goes off for no reason/Kills Battery FIX DIY.. - MBWorld.org Forums

Dec

 

[ontheball247]

Ok...makes sense. Well I've unplugged it, and so far so good. I'll just have to tape the wire and remove the siren to check it. Thanks once again.

 

[ontheball247]

Ok so I've taped off the plug and there doesn't seem to be anything I can fix inside the old siren so I left it there till i get a new one, if i decide to do so. The alarm hasn't gone off randomly since I disconnected the siren, but when i lowered the jack a little fast (hydraulic jack), the hazards (i.e. alarm) went off....the car was locked at this time and the siren was disconnected. I'm confused. But as long as it doesn't go off randomly anymore, I don't care to be honest. lol. Thanks again anyway.

Baz.

 

[Dec]

The siren has still got back up batteries inside it, these batteries will still have some charge in them but should deplete very quickly as the siren has no power supply to charge them since you unplugged it.

The hazards shouldn’t go off with the siren disconnected, did you disconnect the car horn instated of the alarm siren by any chance?

Dec

 

[ontheball247]

Yep I'm pretty sure I disconnected the siren. I removed it and had a look at the part number, plus its the same as the pictures you gave me in your earlier response. That's strange then, since it went off the once, when i dropped the car, but it hasn't gone off since (fingers crossed) and the light on the little button on the dash still blinks when the car is locked.

Baz.

 

[Dec]

Ok so I've taped off the plug and there doesn't seem to be anything I can fix inside the old siren so I left it there till i get a new one, if i decide to do so. The alarm hasn't gone off randomly since I disconnected the siren, but when i lowered the jack a little fast (hydraulic jack), the hazards (i.e. alarm) went off....the car was locked at this time and the siren was disconnected. I'm confused. But as long as it doesn't go off randomly anymore, I don't care to be honest. lol. Thanks again anyway.

Baz.

I missed that bit, when you unplugged the siren, the car was locked and so the alarm was armed, unplugging an armed siren/alarm will trigger the alarm, s o what happened with the hazards was normal, the alarm should have been disarmed by unlocking the car first before the siren was unplugged.

No matter anyway, the Red blinking alarm light will go off when you next unlock the car, you shouldn’t see that light again until you fit a new siren.

When you fit the new siren make sure the car is unlocked with the remote, after fitting the new siren plug it in, the Red alarm light will come on steady and will not blink, unlock (the already unlocked) car with the remote and turn on the ignition.

When you lock, the siren will chirp 3 times and the hazards will flash 3 times.
When you unlock, the siren will chirp 1 time and the hazards will flash 1 time.

Let us know how it goes.

Dec

 

[ontheball247]

Well the car's been used all day now and as you said, the alarm hasn't gone off, randomly or otherwise. And all the information you've given is a lot more help than I could have asked for. So much appreciated.

Only thing is, the light in the dash still blinks when the car is locked. It's not an issue, so not to worry. But if that does seem odd to you, then I wonder if the loose wires next to the fuse box in the spare wheel well have anything to do with it? (the ones that should be connected to a little green box, as I've seen in a picture of some one else's W202's boot). Or it could be another thing due to the fact that I disconnected the siren while the alarm was live.

Either way, my problem's solved. So thanks. If/when I do decide to go for a new siren, I know what to do

 

[Dec]

I think the Red light is still blinking because you unplug the siren while the alarm was armed.

The only things I can suggest is to lock the car, plug in the siren again, unlock the car then unplug the siren… in other words you will be unplugging the siren when the car is unlocked as opposed to unplugging it when it was locked the last time, I think that might stop it but don’t know for sure.

If you do fit a new siren, do the above first with the old siren, hold off takeing it apart for now.

Mind you it is useful that the Red light is still blinking when you lock as the car still appears to have a working alarm fitted, I would leave it alone as it won’t do any damage as it is.

I wouldn’t think the wire in the boot would be related to the alarm but it should be insulated in case it is live at certain times.
The picture below shows the fuel pump relay for a petrol W202, you wont have that but is that where the wire is?

Dec

 

[ontheball247]

Ok, that makes sense. And yeah, the red light is actually helpful. So I don't mind it. And yes, the fuel pump relay would be the box I was talking about. So that's that answered as well. You're awesome mate...thanks once again.

Baz.

 

[luke06051987]

Hi i have a 1999 c240 estate and the hazards keep going off randomly even when the car is left parked unlocked and also when driving there is no siren just the hazards when it happens i have tested the siren and it works

 

[Dec]

The alarm can seem to be operating normally when tested but if the hazards are going off when the shouldn't then its best to discount and/or replace the siren, the central locking will work ok without the siren.

I take it that when you say “there is no sire” you mean there is no noise, just flashing hazards?

Dec

 

[luke06051987]

Yes no siren
the car was parked for 2 years prior to me buying it a couple of weeks ago so i imagine the internal battery is flat in the alarm but it seems strange the hazards also go of when driving

 

[luke06051987]

Also i disconnected the siren earlier and the hazards continued flashing

 

[Dec]

The shouldn’t go off when driving, unless the alarm siren is faulty.
The alarm siren is the brains of the alarm the ECU so if it goes wrong then stranges things happen. This is a very common problem on 10 year old Mercedes.
If the siren is the original factory fitted unit then it is on its last legs and will probably be in the condition of the one in the picture below. Battries and PCD destroyed, 2 years without power will just kill the batteries.

Dec

 

[luke06051987]

Also i dont know if this helps but when the hazards come on randomly when driving i have to press the hazard switch to stop them sometimes i have to press the hazard switch numerous times

 

[Dec]

Also i disconnected the siren earlier and the hazards continued flashing

Was the car unlocked when you disconnected the siren?

Dec

 

[luke06051987]

Reading through lots of your posts i was wondering whether it could be the siren or maybe the n10 relay in the boot will try disconnecting the siren with the car unlocked tomorrow and see what happens and let you know thanks for the help