Thank you for your question.
Central Locking Systems are a broad term for security in vehicles.
Some only lock the doors, some the trunk, hood and gas cap as well.
Some disable the ability to start the vehicle.
For example, a discussion page on BMW systems has the following
information on a trouble shooting page:
"...The system allows the entire car to be locked from either front
door key lock, the trunk key lock, or by depressing the driver's door
lock button inside the car. It also allows the car to be double-locked
(from the driver's side key lock) so an intruder cannot unlock the car
just by breaking a window or using a tool to lift one of the door lock
buttons. If the car is locked - but not double-locked - it can be
unlocked via the key lock from either front door or the trunk, or by
lifting either front door lock button from the inside. The doors
automatically unlock via an inertia switch that operates on impact
greater than 5g. Door lock buttons on 4 door models have no control
over central locking, although they can lock, and under certain
circumstances unlock that particular door. Never double-lock a car
with occupants inside!..."
This is a very advanced system. It double locks, as it mentions. Not
all Central Locking systems employ this feature. It also unlocks upon
impact for added safety in the event of an accident.
They continue with a list of components:
"...The system consists of the following components:
Key lock cylinders in each front door (and the trunk), which operate
the door latches that prevent the door handles from operating, and
which in turn have mechanical linkages to the door lock button and to
the door lock motor unit mounted inside the door cavity. (There is
also a trunk lid lock motor unit that locks the trunk latch button,
and a gas filler lock motor.) Most door lock motor units have two
motors: the door lock motor, which operates the door latch locking
mechanism and door lock button, and the double-lock motor, which
"pins" the mechanical linkages and the door latch so they cannot be
manually unlocked. The driver's door and trunk lack the double-lock
motor; the trunk doesn't need one, since there's no "lock button"
there, and the driver's door is double-locked by the mechanical
constraint of the 90 degree position of the key lock cylinder. The
door lock motor unit also contains the lock and unlock switches which
send "lock" and "unlock" signals to the Central Lock Control Unit
(CLCU). The CLCU contains some electronics and the LOCK and UNLOCK
relays, each of which has a set of normally-open and normally-closed
contacts with a common connection, and which operate all door motors.
The CLCU is located behind the speaker in the left front kick panel.
In addition, there are microswitches mounted on the front door key
lock cylinders. On the driver's door, this is the "unlock inhibit"
microswitch, which closes only when the key lock cylinder is rotated
to the 90 degree position, and initiates double-locking. A similarly
positioned microswitch in the front passenger's door is operated at
the 45 degree key position, but is used for unlocking only..."
This is a very informative article and includes troubleshooting
Some Audi systems feature an interconnection with the airbag system so
that it will unlock should an airbag deploy. Some newer Audi systems
also require that an encoded key is physically inside the vehicle for
the car to start.
So as you can see, not all Central Locking Systems are created equal.
They can be a simple as automatic door locks with or without a remote
sensor, or as advanced as the BMW and Audi mentioned above.
Whenever I have questions of this type of nature, I trot over to How
Stuff Works where I am seldom let down with their explanations. In
this case, it seems that reviewing a few articles will provide a good
understanding of Central Locking. Power door locks, for example:
How Power Door Locks Work by Karim Nice
"Between the keypads, keyless entry systems and conventional locks,
some cars today have four or five different ways to unlock the doors.
How do cars keep track of all those different methods, and what
exactly happens when the doors unlock?
The mechanism that unlocks your car doors is actually quite
interesting. It has to be very reliable because it is going to unlock
your doors tens of thousands of times over the life of your car.
In this edition of HowStuffWorks, we'll learn just what's inside your
door that makes it unlock. We'll take apart the actuator that does the
work, and then we'll learn how the lock can be forced open. But first,
let's see how the car keeps all its signals straight..."
This article continues with the following chapters:
Locking and Unlocking
Inside a Car Door
Inside the Actuator
Forcing the Lock
Lots More Information!
Here are some excerpts of interest, though I think you will enjoy the
"...In some cars that have power door locks, the lock/unlock switch
actually sends power to the actuators that unlock the door. But in
more complicated systems that have several ways to lock and unlock the
doors, the body controller decides when to do the unlocking.
The body controller is a computer in your car. It takes care of a lot
of the little things that make your car friendlier -- for instance, it
makes sure the interior lights stay on until you start the car, and it
beeps at you if you leave your headlights on or leave the keys in the
In the case of power door locks, the body controller monitors all of
the possible sources of an "unlock" or "lock" signal. It monitors a
door-mounted touchpad and unlocks the doors when the correct code is
entered. It monitors a radio frequency and unlocks the doors when it
receives the correct digital code from the radio transmitter in your
key fob, and also monitors the switches inside the car. When it
receives a signal from any of these sources, it provides power to the
actuator that unlocks or locks the doors..."
You will also find links to other related articles at this site:
How Car Computers Work by Karim Nice
How do the automobile ignition keys with chips embedded in them work?
How to Install Power Door Locks
The Original Patent on Power Door Locks
And other interesting links.
How Stuff Works also has a page on Remote Keyless Entries
How Remote Entry Works by Marshall Brain
"If you have one of those "remote entry" devices for your car on your
keychain, then most likely there have been two questions floating in
the back of your head since you first used it:
What the heck does this thing do when I push the buttons? How does it
unlock the door from 20 feet away?
How secure is it? Can I open someone else's car with it, or can other
people get into my car with theirs?..."
And Car Alarms
How Car Alarms Work by Tom Harris
"The first documented case of car theft was in 1896, only a decade
after gas-powered cars were first introduced. From that early era to
today, cars have been a natural target for thieves: They are valuable,
reasonably easy to resell and they have a built-in getaway system.
Some studies claim that a car gets broken into every 20 seconds in the
United States alone.
In light of this startling statistic, it's not surprising that
millions of Americans have invested in expensive alarm systems. Today,
it seems like every other car is equipped with sophisticated
electronic sensors, blaring sirens and remote-activation systems.
These cars are high-security fortresses on wheels!
In this edition of HowStuffWorks, we'll look at modern car alarms to
find out what they do and how they do it. It's amazing how elaborate
modern car alarms are, but it's even more remarkable that car thieves
still find a way to get past them..."
You also might be interested in viewing the following page where you
can see the components required in adding security and central locking
from an aftermarket manufacturer:
"central locking system" +definition OR function
I trust my research has provided you with a good bit of information
about central locking systems and answers your question. If a link
above should fail to work or anything require further explanation or
research, please do post a Request for Clarification prior to rating
the answer and closing the question and I will be pleased to assist