W140 S500 Anti theft (alarm) problem
 

Lately my 94 S500 has developed an intriguing problem with the alarm/keyless system.
Suddenly the keyless entry system does not work (yes I checked the batteries and tested with both keys) and further more when I lock the doors manually, the alarm light does not flash, it is as if the system has died. When I try to use the keyless system I get no lights at all, no red, no green on the door handle.

The problem has changed, today I checked the fuses and now all the system works exept keyless entry for driver side door. I get no lights there at all, all the other keyless sensors work fine.

It must be the sensor I guess, do I have to change the complete door handle or can I simply change the sensor?

Not sure about what to do with your peculiar problem, but I can tell you for sure that if you do lock the doors manually, the alarm system will NOT come on and the check flashing light will not come on either.

You have to lock the car with the remote key sensor for the alarm to come on.

I am pretty sure that normaly you can arm the alarm by locking the doors with the key...

Well, I am not sure about U.S. spec W140s, maybe it can be done. My euro spec S 500 L clearly states on its owner's manual that for the alarm system to be activated, the doors must be locked with the remote key control.

Even worse, and I do really consider this feature completely illogical: to TURN OFF the alarm, you must do it also ONLY with the remote. If you unlock the car manually, the alarm will let you open the doors without the horn alarm going off, but it will NOT let you turn on the car (it will mantain the ignition lock up feature until you do turn it off by the remote). Hence, if you run out of battery power on the remote and do not have the second remote available, you are stucked. This is also clearly stated on the owner's manual.

I checked and the alrme does arm with only the key, and it also disarmes with only the key (on my car anyway)

I'll go to a MB dealer to see whats up with the sensor in the door...