Same alarm problem with my 87 mercedes 420SEL

Question edited for clarity, full maker name.
Question moved to category, albeit '2005'.

Yes, it the battery goes flat, or is disconnected, the anti-theft and alarm needs to be reset.

https://www.google.com/search?q=2003+sl500+reset+security+system

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When you open it from the outside? If this is the case, try putting your key in the drivers door and turning it, whether it is locked or not. I have seen many people who keep their key fob in their pocket have all kinds of issues. So, don't keep your key fob in your pocket. If it is doing it from the inside after driving, that's a different story. It all depends on the specific conditions, if you would like to elaborate.

I’m happy to assist further over the phone at https://www.6ya.com/expert/kim_3202917d7c587455

have a battery shop make a call and have a load test done on the battery as it sounds like a bad cell.

turn the key slightly back and forth while pulling the key out

reset the alarm/immobiliser according to the book......(every time the battery is disconnected.)

only the person who fitted it would know the combination of wiring(which color goes to what)so that disconnection was possible.you would need a fully qualified vehicle electrician who has your car for at least 2/3 days to be able to disconnect fully.and still then you may end up with a problem.........yes i know that you can unplug the horn but sometimes doing that kills the car.....

under the passenger side false floor in front footwell is the alarm module. Pull the carpet up and remove the 2 10mm bolts from the false floor. There are 2 control units on the backside of the floor. One of them is the alarm module. unplug it and you won,t have an alarm any more. good luck

Disconnect the battery lead at the starter motor.Leave off for a few hours then reconnect it.It should reset it for you.

under the rf floor board in the front you will see a false panel after removing carpet. remove false panel by loosening 10 mm plastic nuts your alarm module is 1 of the 2 modules on that false floor-it should have a 820 number in the part number-if it is not obvious call parts dept. and cross reference numbers-unplug module and you won't have worry about that annoying alarm anymore-hopefully i have been helpful

There should be a relay under the hood for the security system, if you can not find it, try jacking the front of the car up, it should have a sensor that tells it that it is being towed and should turn off, unless of course it has a bad wire that is grounding out.