Failed alarm battery causing main battery drain?

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AMGeed

MB Club Veteran
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Joined
Nov 1, 2008
Messages
18,883
Location
Bournemouth/Poole Dorset
Car
W210 E280 x2, w211 E55, W212 E63 biturbo, S204 C180K
The car was reluctant to open this morning using the remote on the chrome keyless go key. Started the car on the gearshift button and got a red message on the dash with something like "key not recognised". Waved the key by the EIS and the message disappeared.

I drove a couple of miles and parked up and locked the car with the remote. It took quite a few presses for it to finally lock.
On returning to the car, no amount of trying the unlock button would unlock the car, even inches from the handle.
So I removed the metal key and unlocked the door and opened it.
I expected to hear the alarm, but only had all the hazard lights flashing.
I started the car using the key in the ignition and drove home. On my return, I tried to lock the car with the remote and it worked first time

The spare key both unlocks and locks the car using the remote and also starts it in the EIS and on the keyless go button.

6 Months ago, I visited Malc (TV) and he did a drain test on all the fuses to try to find the draw I appear to have with the car showing the convenience functions not available after a couple of days. Thought I found the culprit when changing a door control module, but now the colder weather has returned, so is the convenience message.
I also find I have been having to change the batteries in the remote every 3/4 months after getting the key low battery warning.

So, after today's episode with the alarm not sounding after opening the door on the key after locking with the remote, could I have found the problem to my battery drain? I now suspect the alarm battery.

Would I be correct in thinking the alarm is found behind the rear o/s wheel arch cover? Also, I have a fuse diagram (printed by Malc) and cannot find a fuse for the alarm? Is there one?

One other query, why am I having to change the remote batteries on this chrome key when the older spare black plastic key still works fine after a year with no new battery?

I'd really appreciate your thoughts re the alarm before I go and order a new one.
 
When the car is locked with the remote there should be a blinking light on the dash to show the alarm is armed.

To test the alarm;
Open driver’s window.
Lock car with remote.
Wait 1 minute for alarm to set properly.
Reach in through the open window and open door/s and or bonnet.
Siren should sound for abut 30 seconds and hazards should flash for about 3 minutes.

The alarm fuse is usually referred to as the ATA… Anti Theft Alarm.

Dec

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New alarm module was £80 from MB for the CLS. I didn't even realise it had failed :eek: The guys at Comand did a diagnostic scan and reported a dead module.

When I had a new battery supplied and fitted at the dealers they did an update on the BCM (common culprit for battery issues apparently) but I still got the conveniences messages in cold weather...
 
I replaced the siren unit on my W220 (the battery is inside it) as I was having similar issues to you. So I wouldn't mind betting that's it.
 
Dec just posted what I was going to !

it could be a culprit, but may not be !

it shouldn't drain the keyfob bettery.

if you have a tracker unit in the car from previous this can also drain the battery.
 

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