Aftermarket Alarm Double-Lock
BMW doors have a two stage lock system, you can either "single-lock" or "double-lock" them. "Single-locking" only disables the exterior door handles and does not lock the fuel door (may vary by year). "Double-locking" disables the exterior door handles, locks the fuel door, disables the interior door pulls and (rumor has it) disables the starter. When the doors are "double-locked" even if someone breaks a window, they cannot open the doors or trunk by pulling an interior door pull.
The OEM add-on alarm/keyless entry systems have "double-locking" built in to them. However lots of folks would rather use aftermarket alarm systems to save money or because of the added features available with non-OEM systems. The trick is wiring up the aftermarket system to "double-lock" the doors for added security. "Double-locking" is activated by sending a 12v+ pulse to both the lock and unlock wire on car's central locking system simultaneously. It's really simple once you know what to do.
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