New batteries and now intermittent alarm overnight

[alpacajackie]

Hi
I have a SL350 2007. It has been off road for last year. Had to jump start to get going. Went to garage for both new batteries, service and MOT.
Everything fine when got it back home. Decided to give it a 30 mile run. Only thing that I did different was to put something in the boot. Overnight I had the alarm go off every hour that could be stopped using fob. Looked on various forums and states probably volotage drop. Could something have happened when I opened the boot. I did notice that there was a slight delay in trying to get the boot to click open manually. ( not on the fob)

 

[grahamcol]

Welcome. The Mercedes alarm sirens are well known for causing issues when they age. Have a look elsewhere on here and you'll see. Re the jump start, l'm very nervous about this with Mercs and l hope you followed the correct procedure as detailed in your owner's manual. Personally I'd always try to avoid jump starting any Mercedes. You may be best off getting the car to a Mercedes independent (not main dealer) for a proper diagnostic which should confirm the cause of the issue. Hopefully just the siren.

 

[Bay Leaf]

Try disconnecting the battery for 10/15 mins ( make sure you have the radio code if needed ) With some cars and I've done it on my Jag, you can disconnect both battery leads and hold them together to clear any stored faults in the system. Before you do the latter make sure this is OK to do with your model as I have not done the procedure on my SL350 But try the disconnect first.

 

[mioba]

If your rear door didnt open properly, could be the catch is stuck giving a false signal to the alarm thinking the door is open. give the catches a clean and lube
The alarm sirens / control unit last 10-15 years. Easy to replace. Get one from your dealers

 

[markdenning]

Had similar happen to me last night on my 2008 e280..parked up on the drive after work..abt 15/20 mins later wife says has ur alarm gone off the hazard lights r flashing..went out unlocked it and locked up abck up,round abt 10pm alarm starts to go off for abt 5/6mins but no lights are flashing lights..so left it unlocked overnightive had a new bosh battery fitted before winter..checked the voltage via the car display it shows all is well..so ive taken the advice from previous replys and clean with wd40 the boot lock and drivers door lock..cars unlocked at work now so ill try to lock it tonight wen i get homehope all off u r keeping ok


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[MJJ]

The Mercedes alarm sirens are well known for causing issues when they age.

Indeed, they contain a rechargeable battery so that the alarm will go off should a thief try to cut the main 12v supply. A rechargeable battery from 2007 is beyond dead. Whilst this should make no significant difference bar a lesser protection in the event of main supply cut, Mercedes do seem to have poorly designed the failure mode handling of this piece of their cars.

On my 2009 S211 E-Class, it even caused a battery drain in the car - exhausting the main 12v battery in just a few days. The failing siren battery kept the overhead control panel (the one with the interior button to disable alarm sensors) module awake, which means the whole vehicle CAN network stayed awake, which meant the 12v battery drained down to 7v in just a few days. 12v batteries never recover once discharged like that.

Can I suggest you get a multi-meter on your main battery and just double check you don't have the same - I imagine it would be rather galling to have to replace those new 12v batteries again so soon if you do have a battery drain. Just check that the 12v battery is still reading 12.3v-ish once the car has been left standing for a couple of days.

If you wanted to replace the siren - battery is not replaceable alone unless you are happy to dismantle/solder/repair - it is £140 ish from Mercedes and is an hour's work - almost certainly fitted behind one of the front wheel arch liners on the inner wing.

Martin.