We just purchased a 1987 300TD Turbo and it has the factory alarm system. The 2 times we test drove this car, we had no problem, but shortly after getting it home, the first time I locked it, then unlocked it, opened the door, the alarm went off . I used the key with the red dot on it which is supposed to disarm the system when you unlock the doors. What I have done for now is disconnect the alarm horn to spare the neighbors, but the lights flash for the duration of the alarm. I have locked and unlock the doors to reset the sys., but no luck. Somewhere I heard that if the car battery is old, this alarm problem can happen. The battery in this car is 4 years old, but reads a solid 12 volts. ANY ideas would be appreciated.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member