How to Troubleshoot a Rear Window Defogger

by Dan Ferrell

Most rear window defoggers are part of simple circuits that are easy to troubleshoot. The grid on the back window consists of a resistance wire that heats up as current flows through it, defogging or melting the ice from the glass. Although manufactures may use grid resistance variations, the circuit configuration remains pretty much straightforward. We will go trough the steps necessary to locate the circuit malfunction, from the actual grid to the fuse or circuit breaker. All you need is a very basic understanding of electricity or the desire to learn basic troubleshooting procedures.

Step 1

Check the defogger circuit fuse and make sure it is in good condition. If necessary, replace it.

Step 2

Turn the ignition switch on but do not start the engine.

Step 3

Turn the defogger switch on.

Step 4

Open the driver’s door and look at the dome light inside your car. The light should barely illuminate. If the light is not dim, go to Step 9.

Step 5

Inspect the defogger grid on the back of the window. Start the engine and turn on the defogger.

Step 6

Wait for a couple of minutes to allow the grid to warm up. Trace the grid with your fingers and feel for warmth along the wire. Take a note when the wire feels cold. That’s where the defogger grid is broken. You might be able to see the wire gaps.

Step 7

Use a test light if you can’t see the broken grid segments. Start at the positive connection of the grid. Clip the test light to a good ground on your vehicle and touch the connections of the grid with the pick of the test light. If the test light comes on, you have found the positive side. Follow the grid wire by touching the line with the test light at small intervals. The point where the test light does not illuminate and the previous point is where the broken part of the grid is located.

Step 8

Repair the broken grid segments.

Step 9

Check the connection at the grid and make sure there is continuity between both sides of the connection with a multimeter.

Step 10

Turn on the ignition switch but do not start the engine. Use a test light to check for voltage at the grid connection. If there is no voltage, check the wire between the grid connection and the relay for continuity.

Step 11

Check the relay for incoming voltage and outgoing voltage to the defogger grid using a test light. If there is incoming voltage but not outgoing voltage, check the relay for proper operation. Most defogger relays use an integrated timer that stays on for approximately 10 minutes. If the relay is not working properly or does not stay on for the time required, replace it.

Step 12

Check for circuit continuity from the wire coming from the fuse panel to the defogger switch and check the switch for continuity using a multimeter. Also, check for continuity at the wire going from the switch to the indicator lamp and to the relay. Make the necessary repairs.

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