Between the keypads, keyless entry systems and conventional
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
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.
Locking and UnlockingHere
are some of the ways that you can unlock car doors:
- With a key
- By pressing the unlock button inside the car
- By using the combination lock on the outside of the door
- By pulling up the knob on the inside of the door
- With a keyless-entry remote control
- By a signal from a control center
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 ignition.
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
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.
Now, let's take a look inside an actual car door and see
how everything is hooked up.
Inside a Car Door
In this car, the
power-door-lock actuator is positioned below the latch.
A rod connects the actuator to the latch, and another rod
connects the latch to the knob that sticks up out of the top
of the door.
When the actuator moves the latch up, it connects the
outside door handle to the opening mechanism. When the latch
is down, the outside door handle is disconnected from the
mechanism so that it cannot be opened.
Inside a car
To unlock the door, the body controller supplies power to
the door-lock actuator for a timed interval. Let's take a look
inside the actuator.
Inside the Actuator
actuator is a pretty straightforward device.
This actuator can move the metal hook shown
in this photo to the left or right. When mounted in the
car, it is vertical, so the hook can move up or down. It
mimics your motions when you pull the knob up or push it
This system is quite simple. A small electric
motor turns a series of spur gears
that serve as a gear
reduction. The last gear drives a rack-and-pinion
gearset that is connected to the actuator rod. The rack
converts the rotational motion of the motor into the
linear motion needed to move the lock.
Inside the power-door-lock
One interesting thing about this mechanism is that while
the motor can turn the gears and move the latch, if you move
the latch it will not turn the motor. This is accomplished by
a neat centrifugal
clutch that is connected to the gear and engaged by the
Centrifugal clutch on the drive
When the motor spins the gear, the clutch swings out
and locks the small metal gear to the larger plastic
gear, allowing the motor to drive the door latch. If you move
the door latch yourself, all of the gears will turn except for
the plastic gear with the clutch on it.
Forcing the Lock
If you have ever locked
yourself out of your car and called the police or AAA to help
you get back
in, you know that the tool used is a thin metal strip with
a flat hook on it. From this article you can now see
how this strip works.
A simple vertical motion from either the knob on the
door or the power-lock actuator is all that's needed to turn
the lock and open the door. What the officer is doing with the
metal strip is fishing around until he or she hooks onto the
point that the knob and actuator connect to. A quick pull on
this point and the door is unlocked!
For more information on power door locks and related
topics, see the links on the next page.
Lots More Information!
Other Great Links