W204 Alarm Going Off

[richybm84]

Hi,
I have recently purchased a 2012 C180 Estate to replace my wife's car but having an issue with the alarm. It has been going off sporadically, I checked the battery voltage (which was just under 12v) and replaced the battery. The alarm hadn't gone off since then (10th November) until two days ago.
I have run a scanner on the car which is one of the Bluetooth ODBII scanners which showed no faults.
Is there a way to check what caused the alarm to go off as I understand that the Bluetooth scanners don't read all modules?

I'm thinking it might be a door, boot or bonnet sensor that could be setting it off but would like confirmation before replacing bits.

Any help is very much appreciated

Rich

 

[whitenemesis]

MB STAR would be the best scanner for all the car systems.

You could try manually operating the door and bonnet pins to see if any are a bit sticky?

 

[Petrolheed]

Try the suggestion above, if there's still a problem it COULD be the alarm siren starting to fail. I believe it's possible to replace the internal siren batteries, and there's plenty info on the net in this regard, but I've always opted for a new alarm siren.

 

[richybm84]

Thanks for your replys!
I will check the pins and spray some switch cleaner on them just to be sure later and hopefully that's all it is.
If it is the alarm siren would that show up on a diagnostic scan?
I've been quoted £42 + vat for a 30 minute diagnostic for a local independent Mercedes garage so might have to do that as I don't know anyone with the software to scan it for me.
If only VagCom worked on Mercedes, I'd be alright then! Lol

Cheers
Rich

 

[Petrolheed]

I've found that Star diagnostics doesn't always pinpoint a failing siren. You could remove the siren, carefully cut it open and check for internal battery leakage on the circuit board. The Star scan price quoted seems reasonable, and ample time time to run a "quick test" which would show all faults present, "current" and "stored", and further tests/activation's should be possible with the siren too. It's usual to erase all faults, where possible, after a quick test, be sure to get a print out or note them down beforehand, ignoring those related to low voltage faults. I hope this helps a little, I'm short on time to explain further at the moment.

 

[richybm84]

That's great, thanks. I'll have a search on the forum for info on the siren location etc and see what I find.
If no joy I'll book it in for diagnostics.

Cheers
Rich

 

[190]

You have fitted a new battery which temporarily cured the problem but what voltage is it now ?

 

[richybm84]

When the alarm went off the night before last I got in and checked the voltage of the battery through the dash display and it was reading over 12v (I can't remember exactly how much over 12v it was though).

Cheers
Rich

 

[Martygt550]

Id be thinking if the new battery cured it then it has to be the charging system (altenator) put a multimeter across the terminals and see what it reads cold,then do the same when the motors running,is it pushing enough to charge the battery and run the car? Do the same test with as much load on the system that you can get,full lights heated seats/windows ect and see what it's reading,if it's poor check the belt condition/tension then the tensioner then the roller/clutch mech that drives the belt,if all that's good it could/will be your altenator,

 

[richybm84]

I'll test it directly from the battery tonight.
The dash display reads 12v without the engine running and 14v with it running which seems right to me but I guess the dash reading could be wrong?

Cheers
Rich

 

[Martygt550]

You could drive it until the headlight beams are visible IE up close to a wall/garage door and fully load the electronic system at idle and see if the lights dim or get brighter when you Rev it,that could be a sign the regulator in the altenator is faulty

 

[richybm84]

Thanks, I'll give that a try later.

Cheers
Rich

 

[190]

I'll test it directly from the battery tonight.
The dash display reads 12v without the engine running and 14v with it running which seems right to me but I guess the dash reading could be wrong?
Cheers
Rich

A true battery voltage can be obtained by measuring at the battery after a period of rest since the engine was last run. 4 hours should do it or overnight. Put the bonnet up before the period of rest if you can because as soon as you open a door the stupid car will draw several amps and spoil the "no load" reading. When I do that on my W204 I get 12.7 volts. The whole thing is complicated these days as the intelligent charging system deliberately under charges the battery at little. 12.0 volts or the low 12's would still indicate a battery that is nearly flat probably do to a charging problem. Measuring in the car engine off doesn't work for various reasons not least that the battery is under a load.

 

[richybm84]

I'll see if I can try that at some point. So I just park up and open the bonnet and leave the car unlocked for 4 hours then test the voltage?

Cheers
Rich

 

[190]

I'll see if I can try that at some point. So I just park up and open the bonnet and leave the car unlocked for 4 hours then test the voltage?

Cheers
Rich

Yes, provided that doesn't pose a security problem. Mine is parked in a garage. The rest period is required to dissipate what is called surface charge which would otherwise cause a high reading. If it's not convenient there is another way. Turn on the head lights for 3 minutes and then rest the battery for 10 mins before testing. It's a less perfect way in my opinion but it will give a much more accurate reading than doing nothing.