How Power Windows Work

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­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 lock.

In this article, 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.

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.

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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 windows.

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 other doors.

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A simple power-window circuit

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, power-mirror, power-lock 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.

Interesting Features

  • 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 common. 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.


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