W210 alarm problem

[E55BOF]

Oh B****R. The old girl ('98 E55 AMG estate) knows I'm just about to sell her, and she doesn't like the idea of a new, and possibly non-caring, owner...

It started to play up three days ago. The alarm goes off every evening, about 11:30 pm (and possibly all through the night; I haven't checked. I just leave it unlocked). All the windows, and the tailgate and bonnet, are fully closed, and the battery is fully charged.

UPDATE: it went off last night at the same sort of time, and I used the unlock button on the key fob to stop it - one press. I did not open any doors, just left it at that. I assumed it was unlocked (it was, and stayed that way, the night before, but that time I DID open the door, and turned on the ignition, to check all the windows were fully closed). I went out to the car two minutes ago, and it was locked (!), but the alarm did not go off during the night.

The disable functions for the interior motion sensor and tow-away protection seem to work fine.

I've done a search, but did not find anything quite fitting the problem. It would be nice to think it was just a moth waking up, but I suspect it will turn out to be the control unit. Can anyone tell me where this is located on the estate?

 

[ringway]

The car automatically locks after around 30 seconds if the open button is pressed and a door isn't opened to guard against inadvertent disarming.

I'd check to see that the PSE unit (under the rear seat next to the battery) isn't damp. It sits in a foam boot that will draw up any water in the rear footwell that gathers there if the drain below the front wiper is blocked. When the PSE becomes moist the circuit boards are affected and cause all sorts of odd behaviour from the alarm, electric windows...

 

[E55BOF]

Thank you for that; the confirmation of position that I needed. The foam surround is definitely not damp, but now that I know that's what I should be looking at, I'll do just that. Cleaning up the contacts can't hurt.

 

[ringway]

Thank you for that; the confirmation of position that I needed. The foam surround is definitely not damp, but now that I know that's what I should be looking at, I'll do just that. Cleaning up the contacts can't hurt.

It may well have been moist some time ago, perhaps years ago and then the drain cleaned out. The circuit board becomes mouldy.

 

[E55BOF]

As pristine as the day it was first assembled; no sign whatever of moisture (or anything else) ingress. Drat! Maybe it was a moth, after all. I'll try some insect spray next.

 

[E55BOF]

Update:

It's still doing it! I have the interior motion sensors masked off with masking tape, and (because I've been working on the bonnet latches) I've disconnected the micro-switch on the RHF latch. Still goes off, still only at night.

What next? Are any of the other (door/tailgate) switches known to play up?

My wife suggests it may be because the car is afraid of the dark....

 

[Dec]

The alarm siren is also the control unit, the ECU of the alarm system, the batteries inside eventually disintegrate and the PCB becomes corrupt causing false alarms.

If your alarm siren is the original factory fitted unit then its now 16 years old and is coming to the end of its life.
It is likely to be in a similar condition to the one pictured below.

You can disconnect it for now by unplugging it, ensure the car is unlocked before you unplug it. You can replace it for around £100 if you DIY.

Dec

[YOUTUBE HD]zChaQHHaeJU[/YOUTUBE HD]

 

[spike60]

My CLK which used a similar setup started crying wolf in the middle of the night and was always triggered by interior sensors with no obvious cause. Unless you feel you really need an alarm just unplug it and forget it, that's all I did.

 

[E55BOF]

Replaced the alarm siren, and all is quiet again in the night. Thank you all for your assistance.

E55BOF

 

[Dec]

Any chance you could do an autopsy on the siren, just for the record, you can open it up to determine the cause and manner of its demise, just cut around the siren casing with a hacksaw blade.

The batteries may appear pristine in their Green shrink - wrap plastic wrapping, slit the wrapping open to see what condition they are in.

Dec

 

[horatio]

My siren has set off and is making an awful high pitched blipblipblip. I've found the siren (under the front drivers wing) and am about to remove it.

Can the siren just be removed and left off the car? Or refit without the battery?

Can't see anyone wanting to nick the old thing

 

[Dec]

Remove the siren and tape up the 3 pin connection plug to keep it dry, it will be live all the time, don’t try reconnecting the siren in any way as it is defective and could cause other electrical problems.
You can replace the siren for about £100 from an MB dealer.
No siren = no theft insurance.

Dec

 

[Aerovestrup]

Hello. I have same problem with sporadic warning flashing lights, but without the siren screeming. Have searched the car for mice and other things, in vane. W 210 2000 model st car. Somebody please
Flemming