W212 E250 CDI - Alarm going off for no reason

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ledlights

New Member
Joined
Jul 10, 2017
Messages
7
Location
Weymouth
Car
W212 E250 CDI
Hi All,

Recently, my W212 alarm has been going off occasionally whilst parked and locked. In an attempt to stop the neighbours getting annoyed I've been leaving the car unlocked which seems to have solved the problem but a couple of times, after taking my daughter to school, when I've locked the car again the alarm has sounded as the electric mirrors wind themselves in.

I've checked the battery voltage, which seems okay at 12.4V and I can't find an auxiliary battery behind the dash on either side (the car is a manual RHD). I've also vacuumed out around the ultrasound sensors. Does anyone have any ideas?

Thanks,

Steve
 
Have you tried turning the internal sensors off and locking it? You might be able to rule those out if it still goes off with them off.
 
Thanks for your reply. Yes, I've tried switching off both the ultrasound and the towing sensor but it still goes off randomly.
 
Might be time for a new alarm siren then. They have a battery which deteriorates with age and may cause problems such as you're experiencing.
 
Thanks for this.

Do you know where would be the best place to buy one from?
 
Thanks for this.

Do you know where would be the best place to buy one from?

Order it from MB Grangemouth (01324 409697) or MB Newcastle (0191 447 3687)

Both have decent discounts on parts and very fast delivery.

Have either your Reg # or preferably your Vin# handy so they can confirm correct part.
 
Okay, I removed the siren and split it open to find a small amount of corrosion on the circuit board caused by both a battery leak and a tiny water leak caused by ingress around the piezo siren. I dried the unit, replaced the battery, cleaned up the board and resealed the siren with epoxy before boxing it back up. I managed a full 24h without a false alarm but tonight it's gone off 3 times.

I can't believe it's the siren unit as I would have thought it wouldn't have gone for a full day and night before sounding. Also, I'm pretty sure it's not the tow-away sensor or the ultrasonic sensor as the alarm still sounds with them switched off. Does anyone have any other ideas?
 
Like AMGeed said, you need a NEWE siren from an MB dealer, about £100 if you fit yourself.
You can’t really fix them, too many components that would be damaged or faulty on the PCB.
Look at the picture, full size, to see what gets damaged.
What year is the car?

Dec

2m2j4g5.jpg
 
Thanks for the message. My car is a late 2009 W212 but the board was not damaged to any real extent, just some staining on the pcb (Nothing like the photo above). I can change the siren out for a new one but I wanted to make sure that there wasn't something else that commonly fails that could be causing the problem instead.

Thanks,

Steve
 
In case it helps someone else.

I replaced the unit as suggested- an easy job with the wheel and liner out and the problem was fixed.
 
Okay, I removed the siren and split it open to find a small amount of corrosion on the circuit board caused by both a battery leak and a tiny water leak caused by ingress around the piezo siren. I dried the unit, replaced the battery, cleaned up the board and resealed the siren with epoxy before boxing it back up. I managed a full 24h without a false alarm but tonight it's gone off 3 times.

I can't believe it's the siren unit as I would have thought it wouldn't have gone for a full day and night before sounding. Also, I'm pretty sure it's not the tow-away sensor or the ultrasonic sensor as the alarm still sounds with them switched off. Does anyone have any other ideas?
Siren location please?

Thanks
 

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