A rear window defroster is responsible for clearing a car's rear window of ice and fog. A number of defrosters use a grid system, which consists of electronic sensors that prompt heat production in a series of lines painted on the rear window's glass. Manufacturers apply the lines to the window using conductive paint, a silver polymer product that can conduct electricity. Scratched or damaged lines result in a broken electrical current. When this occurs, the sensors cannot prompt the lines of the grid to heat properly. Fixing faulty defroster lines is a vehicle repair you can complete at home.
Turn on the car's engine. Turn the vehicle's rear defroster on high.
Turn on the voltage tester. Place one probe of the tester at the end of the defective line and the other probe on the opposite end. Check the voltage tester's reading. Write down the current voltage.
Slowly slide one probe toward the opposite end while constantly checking the voltage reading. The reading should slowly decrease as you move the probe.
Continue sliding the probe until you reach an area where the voltmeter's reading increases to the number you had previously written down. The faulty part of the line lies between the two probes.
Mark the damaged area on the opposite side of the window using a grease pencil.
Turn off the vehicle's rear defroster and allow the window to cool.
Prepare the rear window for repair by cleaning it with isopropyl alcohol and a soft cloth or paper towel. Remove all dirt, debris, oils and hand prints from the window.
Place masking tape on the upper and lower part of the faulty defroster line to protect the surrounding glass. This also ensures the repaired line is straight.
Shake the conductive paint well. Use the brush supplied with the conductive paint to apply a thin coat of paint to the damaged line.
Allow the first coat of paint to dry for about five minutes, and then apply another thin coat of the paint.
Allow the second coat to dry for five minutes. Remove the masking tape from the glass. Wait 15 minutes before using the rear defroster.
