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The BMW 518 central locking relay switch is located beneath the dashboard. The central locking relay switch will be on the drivers side of the dashboard.
The central locking relay is attached to the ECU, fitted behind the right (EU) back passenger side panel.
negative triggering relay.
drivers side door
The central locking is controlled by an electronic device called the central locking control unit that is bascially a relay. There are normally 4 wires that go to each door. Two of the wires connect to the lock mechanism and tell the central locking control unit whether the doors are locked or not. The other two wires connect to the actuator which can either take the form of a motor or an electro magnet. The Central locking control unit connects these wires in one direction to lock and the opposite direction to unlock the doors. Most modern cars also have a remote sensor that detects the signal from a remote key fob. This send a radio or IR signal to a detector which then connects to the central locking control unit to tell the car to lock or unlock. This uses a coded signal that is unique to that particular key fob.
They are at the back behind the ashtray, look in the gap at the top of the fuses.
There are 4 central locking relays on this car. In one of the engine bay fuse boxes, the one whcih contains the multi fuction module. There are six slots above this, 4 of them contain the central locking relays. Good luck....
inside of the driver side door
in the fuse box under the bonnet its a 15amp blue.
If the car has an alarm system or remote locking (Key fob) the locking control box is behind the side trim in the (UK) passenger footwell, if there is no alarm there is no relay box the system works off the door being locked and it sends a lock command to the other door
drop down your fuse box and it is located on the top, next to a green relay box
If your locking system is not functioning ,and usually the power windows and alarm are all linked together, powered by one small fuse witch is located just above the fuse / relay panel very well hidden and hard to find , it will not be shown in the directory panel ., hope this helps ,thanks Louis