Have you ever wondered what kind of mechanism makes your
car windows go up and down? How about the power windows with
the automatic-up feature that raises the windows by
themselves, but stops raising them if there is an obstruction?
Or maybe you've seen the Volkswagen TV commercial
where the guy opens the windows by turning the key in the door
In this edition of HowStuffWorks,
you'll learn about what's going on in all of these
power-window features and more!
The Lifting Mechanism Let's start with the
lifting mechanism. This cool device is the heart of a
power-window system. Click
here to see a short movie of the lifting mechanism.
The window lift on most cars uses a really neat
linkage to lift the window glass while keeping it
level. A small electric
motor is attached to a worm gear
and several other spur gears
to create a large gear reduction, giving it enough torque to
lift the window.
An important feature of power windows is that they
cannot be forced open -- the worm gear in the drive
mechanism takes care of this. Many worm gears have a
self-locking feature because of the angle of contact between
the worm and the gear. The
worm can spin the gear, but the gear cannot spin the worm --
friction between the teeth causes the gears to bind.
Animation of window lifting mechanism at work, with
inset of motor and gear reduction
The linkage has a long arm, which attaches to a bar that
holds the bottom of the window. The end of the arm can slide
in a groove in the bar as the window rises. On the other end
of the bar is a large plate that has gear teeth cut into it,
and the motor turns a gear that engages these teeth.
The same linkage is often used on cars with manual windows,
but instead of a motor turning the gear, the crank handle
turns it. In the next section we'll learn about some of the
neat features some power windows have, including the child
lockout and automatic-up.
The Wiring and Switches Car doors are wired
in many different ways, depending on which features are
incorporated. We'll go through the wiring on a basic system --
one that allows the driver to control all four windows on the
car and can lockout the controls on the other three individual
A Basic System On this
system, the power is fed to the driver's door through a 20-amp
circuit breaker. The power comes into the window-switch
control panel on the door and is distributed to a contact in
the center of each of the four window switches. Two contacts,
one on either side of the power contact, are connected to the
vehicle ground and to the motor. The power also runs through
the lockout switch to a similar window switch on each of the
A simple power-window
When the driver presses one of the switches, one of the two
side contacts is disconnected from the ground and connected to
the center power contact, while the other one remains
grounded. This provides power to the window motor. If the
switch is pressed the other way, then power runs through the
motor in the opposite direction.
An Advanced System On
some cars, the power windows work in a completely different
way. Instead of the power for the motor going through the
switches directly, the switches are connected to one of the
many electronic modules in the car (the average car contains
25). Some cars have one in the driver's door, as well as a
central module called the body controller.
Cars that have lots of controls on the door are more likely
to have a setup like this. Some cars have the power-window,
and even power-seat controls all on the door. This would be
too many wires to try
to run out of the door.
Instead of trying to do that, the driver's door module
monitors all of the switches. For instance, if the driver
presses his window switch, the door module closes a relay that
provides power to the window motor. If the driver presses the
switch to adjust the passenger-side mirror, the driver's door
module sends a packet of data onto the communication bus of
the car. This packet tells the body controller to energize one
of the power-mirror motors.
Automatic Up/Down - The automatic-down feature is
fairly common on cars with power windows. You tap and
release the down switch and the window goes all the way
down. This feature uses a circuit that monitors the amount
of time you hold the switch down. If the switch is down for
less than about half a second, the window will go all the
way down until it hits the limit switch. If you hold the
switch down for longer than that, the window will stop when
you release the button. Automatic-up windows are less
The problem with automatic-up windows is that if anything
gets in the way of the window, such as a child, the window
has to stop moving before it hurts the child. One way that
carmakers control the force on the window is by designing a
circuit that monitors the motor speed. If the speed slows,
the circuit reverses the power to the motor so the window
goes back down.
Window Control From Outside - On the Volkswagen
in the TV commercial, the windows can be lowered by
inserting the key in the driver's door, turning and holding
it. This feature is controlled by the driver's door module,
which monitors a switch in the door lock. If the key is held
turned for more than a set amount of time, the driver's door
module lowers the windows.
Courtesy Power-On - Some cars maintain the power
to the window circuit after you turn your car off, which
saves you from having to stick your key back in the ignition
if you forget to roll your window up. The power-window
circuit will have a relay on the wire that provides the
power. On some cars, the body controller keeps this relay
closed for an extra minute or so. On other cars, it stays
closed until you open a door.
For more information on power windows and related topics,
check out the links on the next page!