I just bought a used 2001 ML320.
When I try closing the doors with the remote, the doors lock, but after about 10-20 seconds, the alarm goes off.
Everything is ok if I use the normal key (not the one with the remote), to manually lock the doors.

Could anybody throw some light about what might be happening?

Thank you.

when you manually lock the door with the key blade, the car is NOT armed. It will lock the car but the system is not armed. Only locking the car with the remote will arm the system. It seems one of your door is not closed shut, check if any debris or anything is in the way. Also check the tail gate to see if it is shut good, i.e. without any play.

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98 ML320 (gone but not forgotten)

Many thanks
I will try it out and let you know how it went.

I could not find any debris, and the tail gate is closing properly. Could this be something to do with the remote itself? Might changing the batteries help?

Thanks

If you have another remote, try it and see if it makes any difference. My 2 cents.