W212 E250 CDI - Alarm going off for no reason

Discussion in 'Electronics' started by ledlights, Jul 10, 2017.

  1. ledlights

    ledlights New Member

    Messages:
    7
    Joined:
    Jul 10, 2017
    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
     
  2. Abcan

    Abcan Hardcore MB Enthusiast

    Messages:
    411
    Joined:
    Aug 17, 2015
    Location:
    Cardiff
    Car:
    S212 250CDI 2012
    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.
     
  3. OP
    OP
    ledlights

    ledlights New Member

    Messages:
    7
    Joined:
    Jul 10, 2017
    Location:
    Weymouth
    Car:
    W212 E250 CDI
    Thanks for your reply. Yes, I've tried switching off both the ultrasound and the towing sensor but it still goes off randomly.
     
  4. GordonTarling

    GordonTarling Hardcore MB Enthusiast

    Messages:
    1,675
    Joined:
    Jun 29, 2004
    Location:
    Grantham
    Car:
    2007 E63 AMG V8 Estate
    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.
     
  5. OP
    OP
    ledlights

    ledlights New Member

    Messages:
    7
    Joined:
    Jul 10, 2017
    Location:
    Weymouth
    Car:
    W212 E250 CDI
    Thanks for this.

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

    AMGeed MB Club Veteran

    Messages:
    12,644
    Joined:
    Nov 1, 2008
    Location:
    Bournemouth/Poole Dorset
    Car:
    2014 E63 Biturbo
    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.
     
  7. OP
    OP
    ledlights

    ledlights New Member

    Messages:
    7
    Joined:
    Jul 10, 2017
    Location:
    Weymouth
    Car:
    W212 E250 CDI
    Thanks, will do.

    Steve
     
  8. OP
    OP
    ledlights

    ledlights New Member

    Messages:
    7
    Joined:
    Jul 10, 2017
    Location:
    Weymouth
    Car:
    W212 E250 CDI
    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?
     
  9. Dec

    Dec Hardcore MB Enthusiast

    Messages:
    4,475
    Joined:
    Jul 28, 2004
    Car:
    C180,A.
    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

    [​IMG]
     
  10. OP
    OP
    ledlights

    ledlights New Member

    Messages:
    7
    Joined:
    Jul 10, 2017
    Location:
    Weymouth
    Car:
    W212 E250 CDI
    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
     
  11. OP
    OP
    ledlights

    ledlights New Member

    Messages:
    7
    Joined:
    Jul 10, 2017
    Location:
    Weymouth
    Car:
    W212 E250 CDI
    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.
     

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