Motor vehicle door lock and method for selectively operating a motor vehicle door lock with or without a security device

A motor vehicle door lock comprising:

• a locking mechanism comprising a rotary latch and pawl,
• an actuating lever chain,
• a locking unit,
• a base module which surrounds the largest part of the locking mechanism, the actuating lever chain and the locking unit;
• an optional security device that comprises an actuating pin that interacts with a coupling element of the actuating lever chain in its installed state, wherein the optional security device is designed as an anti-theft device, wherein the optional security device selectively sets the actuating lever chain and/or the locking unit in a non-functioning/functioning state, wherein, depending on the position of the optional security device, the coupling element couples or uncouples the actuating lever chain and the locking unit, wherein the optional security device is a retrofitted module adapted to be selectively connected to the locking mechanism, and wherein the optional security device contains a securing element, a security motor impinging on the security element, wherein the security element and the security motor are covered and supported by the security module housing and wherein the security element is a circular disk segment that can be pivoted by a worm gear coupled to the security motor.