Alarm issue

[iammdf1978]

Hi all

I have recently purchased a w204 estate which seems to have an alarm issue where it just goes off randomly,
So I have taken it back to the dealer for him to have an auto electrician look over it. He's had it now for 2 days as there has been no reoccurrence

My question is he suggests that if it when the alarm goes off it's stores that info and can be read back after the fact for diagnosis, is this true ??

Regards and thanks in advance

Mark

 

[Dec]

What year is the car?

Dec

 

[richard]

Yes, alarm trigger causes are stored. A bit troublesome to find them unless you know where you are looking (I seem to recall somewhere in the front SAM)

R

 

[Druk]

Could this be the perennial duff PCB in the siren fault?

 

[richard]

Given it happens in one location (owners' home) and not in another (nice dealer workshop), i'd say it is environmental - i.e directly or indirectly temperature related. Which could be a PCB, it could be the interior motion sensors, it could be a spider !

So the trigger history is important. It seem to recall that it is quite easy to clear the history, so I suspect its been cleared (can't remember if clearing all faults does that), and not happened again so tech can't determine why !

R

 

[richard]

PS, when it goes off, if the indicators are NOT flashing, its highly likely the alarm siren (or possibly a flat battery) - as that means the siren is triggering but can't tell car to flash lights

R

 

[iammdf1978]

Could this be the perennial duff PCB in the siren fault?

Well I bought a replacement Bosch unit for just that reason

 

[iammdf1978]

PS, when it goes off, if the indicators are NOT flashing, its highly likely the alarm siren (or possibly a flat battery) - as that means the siren is triggering but can't tell car to flash lights

R

The lights do flash yes

 

[iammdf1978]

What year is the car?

Dec

59 plate

 

[iammdf1978]

Given it happens in one location (owners' home) and not in another (nice dealer workshop), i'd say it is environmental - i.e directly or indirectly temperature related. Which could be a PCB, it could be the interior motion sensors, it could be a spider !



So the trigger history is important. It seem to recall that it is quite easy to clear the history, so I suspect its been cleared (can't remember if clearing all faults does that), and not happened again so tech can't determine why !



R

It's does happen in other places not just my gaff

 

[Dec]

Modern Mercedes alarm sirens house the brains of the alarm system; it is common for the siren to give trouble when it is around 10 years old.
See below link for an illustration of what happens to the Printed Circuit Board inside the siren over time, if your siren is in decay then it’s not going to give a straight answer to any diagnostic equipment that is hooked up to it, so, it’s likely that is why the dealer can’t find the fault.

Link… https://www.google.com/search?q=mer...hVBIMAKHQaKB0sQ_AUIBigB#imgrc=FB2KpQ4smJ4JCM:

When your alarm goes off, the hazards lights should flash for 3 minutes and the siren should sound for 30 seconds after which the alarm will rearm itself, if either of the above didn’t happen then the siren is faulty.

Dec

 

[iammdf1978]

The dealer suggested the auto electrician reduced the sensitivity of the alarm

 

[iammdf1978]

Modern Mercedes alarm sirens house the brains of the alarm system; it is common for the siren to give trouble when it is around 10 years old.

See below link for an illustration of what happens to the Printed Circuit Board inside the siren over time, if your siren is in decay then it’s not going to give a straight answer to any diagnostic equipment that is hooked up to it, so, it’s likely that is why the dealer can’t find the fault.



Link… https://www.google.com/search?q=mer...hVBIMAKHQaKB0sQ_AUIBigB#imgrc=FB2KpQ4smJ4JCM:



When your alarm goes off, the hazards lights should flash for 3 minutes and the siren should sound for 30 seconds after which the alarm will rearm itself, if either of the above didn’t happen then the siren is faulty.



Dec

The above does happen, which suggests not a faulty siren?

 

[Dec]

The dealer suggested the auto electrician reduced the sensitivity of the alarm

I don't believe it.
Never heard of that.
You have no choice but to accept what the say, and see what happens.

Dec

 

[Dec]

The above does happen, which suggests not a faulty siren?

Ok, but not absolutely proof that’s its not faulty.

Dec

 

[iammdf1978]

So the siren was replaced albeit with an eBay purchase and the issue still persists

 

[Codger49]

I would never buy a used siren unit from Ebay, very high chance they have expired.

 

[Dec]

Agree with Codger, you have no idea for sure how old a used siren is.
The all expire when the are at around 10 years old, if the are deprived of a 12v supply for a long period of time then the never fully recover, your car is new to you and it may have been resting with a flat battery for a few months, that, me thinks, is why its acting up now.

A new siren from an MB dealer is about £100, if you are handy with a spanner you can fit it yourself in under an hour.

Dec

 

[iammdf1978]

Yeah I had kinda thought that myself and have already priced new unit from mb and preliminarily booked a diag at mb too new siren 130 inc vat and diag is 100

Which should I go for ? Probably the siren first right?

 

[AMGeed]

Have you got an indy with Star nearby? Diagnosis shouldn't cost as much as £100

I just bought a new alarm siren for my W211 today from MB Newcastle. £99 60 delivered.
Give them a call for a price. I bet its less than the £130 you have been quoted unless that is a fitted price. (Highly unlikely knowing MB labour rates!)