Automotive power windows use a switched motor to operate a mechanical window regulator. This regulator moves the pane of glass into the door. If the windows don't work, any of several issues is likely to blame, including no electricity, bad switches, bad motor, damaged window regulator, or even a broken wire. The average backyard mechanic can diagnose the cause of power window failure in about 20 minutes.
Open the fuse panel and check the fuses for disrupted elements. Most automobile manufacturers place the fuse panel at the driver's side kick panel, or between the dashboard and the door. German cars typically have fuses in the engine compartment, in front of the driver's side firewall. Pull the fuse out with your fingers, or with the tool that came with the car and is located in the panel. Look through the fuse's transparent case for a broken filament. Replace any blown fuses and try the window again.
Test the wiring harnesses from the fuse panel to the switches, and from the switches to the window regulator motor. If the circuits test fine, then proceed to the next step. If a wiring harness is shorted or damaged, it will need to be replaced by a professional mechanic. Most modern wiring harnesses are interconnected and woven throughout the body of the vehicle. They take many hours to replace. It may be possible to locate the short with a visual inspection, then splice the burnt wire and wrap it with a liberal amount of electrical tape. Most shorts occur at pinch areas, where the wiring is crimped by a mechanism such as a door.
Remove and test the switches on the driver's side door panel. Pry them up with a screwdriver and pull them free of the panel's opening. Disconnect the switch assembly from the wiring harness by pulling the adaptor plug from the assembly. Test the switches individually by touching both sides of one switch with the circuit tester. Each switch will have two terminals, and if the tester is positive on both terminals while the window is activated, then the switch is good. Replace the switch assembly if any of the switches fails the test. Most power window switches come in assembled modules and cannot be replaced individually.
Remove and test (if possible) the power window motor. Turn the armrest bolts and interior door handle screws counterclockwise, then pull the door panel free from the pop rivets. Behind the panel are a window regulator, the window glass, and the electric driver motor. The motor is usually located in the lower right section of the door. Unplug the motor from the wiring harness and unbolt it from the window regulator by turning the bolts counterclockwise. Pull the motor straight out, away from the gears. Replace the motor with a new unit by pressing it into position and securing the mount bolts in a clockwise direction. Plug the motor back into the wiring harness by pushing the adaptor plugs together. Replace the door panel by pressing it to the pop rivets, giving a tap to seat each one, then turning the armrest bolts and interior door handle screws clockwise until they are snug. Test the window.
Inspect the window regulator's joints for rust or stiff movement, if the electrical system is operating correctly. These joints can corrode or break, causing the window to stop working. With the door panel off, the window regulator can be removed and replaced by sliding the glass away from the track mounts, then turning all mount bolts counterclockwise and manipulating the regulator out of the door.
Spray the window regulator with lithium grease for smoother operation.
Use extreme caution when working around a vehicle's live electrical system.