I've got a curious lock issue. My key fob buttons don't work due to a bad circuit board, so I'm forced to use the metal key for all locking and unlocking. *Locking* the driver's door with the metal key correctly locks the whole car and arms the alarm, but *unlocking* with the metal key is a different story. After I *unlock*, opening the door triggers the alarm. Re-locking with the metal key clears the alarm as expected -- and then immediately *unlocking again* correctly disables the alarm and allows me to open my door, trunk, etc, without waking the neighbors.
So I'm guessing there must be a switch of some sort attached to the tumbler in the door and operated as the key is twisted... but I can't find that switch in any of the alarm or central locking wiring diagrams!
What am I missing?
I doubt that there is a tumbler sensor. I believe the alarm is triggered by the pin that pops out when the door is open to turn on the dome light. (The dome light and the alarm system are both controlled by the PSE pump in the trunk.) The pin is just forward of the latch on the "car" side of the door well.
Option 1: Try taping the pin in so the car doesn't realize that the door is open. If you're able to open the door without the alarm going off, you should be golden since turning the ignition on with the key deactivates the alarm so hopefully it won't sound.
Option 2: Possibly, a change in pressure in the vacuum line caused by turning the key in the tumbler is registered by the PSE, triggering the alarm. You may want to disconnect the vacuum line. Here's an article showing how to access that line in the door:
Pelican Technical Article - Mercedes SLK 230 - Door Lock and Latch Replacement
Option 3 (and worst option in my opinion): The PSE pump is controlled by Fuse 9 in the main fuse box in the engine bay (see attached). However, weird things happen when you start removing fuses in this model. The car's documents state that this fuse also covers the car's horn so check it out.
All said, getting a new key would be the best, but priciest option. However, a quote from MB is free!