Glad to have found this site and I have a problem
 

I have a 1991 350SD with 285,000 miles on it and the car is in remarkable shape. My problem is with the alarm system. When I unlocked the door to get in, as soon as I opened the door the alarm went off. I was able to crank the car with no problem. The alarm went through its routine of sounding for 150 secs, off for 60 secs., and sounding again for 150 seconds. I talked with the man I bought it from and he suggested I disconnect the battery on the negative side and let is sit for a while to let it reset itself. I did that to no avail. Can anyone help me with this problem. Thanks.

In the past I have had the same problem with my '88 300E. Try locking it and then unlocking it until it resets and disarms the alarm.

Thank you for the reply. I have tried that several times, to no avail. Can this problem be linked to a weak battery? I am going to try locking and unlocking again, but if that still fails, how can I disconnect the alarm system as a temporary solution?