Power door locks (also known as electric door locks or central locking) allow the driver or front passenger to simultaneously lock or unlock all the doors of an automobile or truck, by pressing a button or flipping a switch. Power door locks were introduced on the luxury Scripps-Booth in 1914, but were not common on luxury cars until Packard reintroduced them in 1956. Nearly every car model today offers this feature as at least optional equipment. Early systems locked and unlocked only the car doors. Many cars today also feature systems which can unlock such things as the luggage compartment or fuel filler cap door.
It is also common on modern cars for the locks to activate automatically when the car is put into gear or reaches a certain speed. Remote and handsfree In 1980, Ford Motor Company introduced an external keypad-type keyless entry system, wherein the driver entered a numeric combination —either pre-programmed at the factory or one programmed by the owner— to unlock the car without the key. Early- to mid-1980s Nissan Maximas could also be installed with a keypad, which would also retract the windows and moonroof once the car was successfully unlocked by pushing a specific button on the keypad.
During the 1990s the Subaru Legacy could also be opened by pulling the drivers external door handle a specific number of times to enter a passcode number that would unlock the driver’s door only. Today, many cars with power door locks also have a radio frequency remote keyless system, which allows a person to press a button on a remote control key fob, the first being available on the French made Renault Fuego in 1982.  Currently, many luxury makers also allow the windows to be opened or closed by pressing and holding a button on the remote control key fob, or by inserting the ignition key and holding it in the lock or unlock osition in the external driver’s door lock.
The remote locking system confirms successful locking and unlocking through either a light or a horn signal, and usually offers an option to switch easily between these two variants. Both provide almost the same functionality, though light signals are more discreet while horn signals might create a nuisance in residential neighborhoods and other busy parking areas (e. g. short-term parking lots). Some manufacturers offer the ability to adjust the horn signal volume.
Other cars have a proximity system that is triggered if a keylike transducer (Advanced Key or handsfree) is within a certain distance of the car. How Stuff Works 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. Locking and Unlocking Here are some of the ways that you can unlock cardoors: • 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 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. 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. 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 The power-door-lock actuator is a pretty straightforward device. [pic] Inside the power-door-lock actuator This system is quite simple. A smallelectric 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 therotational motion of the motor into the linear motion needed to move the 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 neatcentrifugal clutch that is connected to the gear and engaged by the motor. [pic] Centrifugal clutch on the drive gear 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.