How to fix rear window defroster wire

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Índice Show

Window Prep
Keep Inside the Lines
Tab A Into Slot B

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If you think about it, your rear defroster is a pretty amazing accessory to your vehicle. For decades, drivers were forced to drive without any view out of their rear windows if the weather caused their windows to fog. Sure, there was the warm air blowing from the defrost vents at the base of the windshield, but this had little, if any, effect on the back window at the far end of the cabin. If you had kids, you could pass a cloth to the back seat and give orders to keep the rear window clear. Car seats were optional in those days.

Then along came that curious grid of wiring across your back window. A button on the dash activated the system, and right before your eyes, you saw the fog disappear. Ice would melt, snow stayed away. It was a small miracle. Some cars actually tried versions of this on the windshield, but it proved a bad idea. But the defroster became standard issue for rear windows, and the world was a better place. That is until something put a little nick or scratch in one of those delicate lines across the glass. Then, the system stopped working. It was widely accepted in the car business that a car more than 5 years old would likely have an inoperative rear defroster.

The days of having to live with a non-functioning grid of defrost lines are over. The latest defroster repair kits are amazing, addressing almost every possible problem that can arise in your system. The first step is always troubleshooting. You need to figure out why your defroster isn't working. The complete repair kits come with diagnostic tools like a special test light, too. Before you start repairing, you should check your fuse to be sure it's not a simple blown fuse problem. The two most common problems with rear defrosters are:

A break in the conductive lines.
The deterioration of the spades that connect the grid to your car's wiring.

Both of these can be repaired, and we will teach you how.

Tape off the section of the rear defroster to be repaired.

If you've tested your rear defroster using the best testing methods and have located a break in your defroster lines, proceed with your repair. The first step is to tape off the repair area. There is blue painter's type tape supplied with your kit, but if you need extra you can use the standard hardware store blue tape. As you tape around the damaged section, keep the untaped area as slim as possible. This is where you will be repainting your defrost lines. If the space is huge, your line will be huge. It will still function, just look bad.

With the area carefully taped off, repaint the line. Let the paint dry thoroughly (I'd wait 24 hours) and then give it a try.

Mix a batch of conductive epoxy to reattach the spade plug.

You may think that a broken wiring stud (or spade) connector on your rear defrost grid means that you will never have a rear defogger again. But the new kits have repair parts you can use to repair this and anything else wrong with your system. If you've tested your rear window grid and found a bad spade connector or stud, the first step is to get the old one off (if it's still hanging on) and clean the area under it. This can be done with the emory cloth included in your repair kit. Next mix up a little of the conductive epoxy and use it to reattach a new stud to the glass. Be sure you get it in the same spot as the one you removed. Allow it to dry for 24 hours and give your defrost a try.

Tip: It's also a good idea to have a circuit tester handy to be sure you have a proper ground for the system.

Minivans are sooo handy. You can cram most of the Free World in the back. Like, for instance, that 8-ft. aluminum ladder you just bought. You didn't even have to tie the back door shut--there was just enough room to close the hatch with the ladder resting on top of a pile of stuff. And it rode across town without incident. Except that it shifted backward a couple of inches, into the window and the electrical spade lug that carries current to the defroster grid, neatly popping it free of the glass. To add to the injury, the grid is scratched in three places, clear down to the bare glass, causing an open circuit that will leave a 6-in.-wide swath of frost across the window come wintertime, right at eyeball level.

These electrically conductive, heated grids on the rear window are literally painted onto the glass. They're reasonably tough, but it's possible to damage them by letting cargo rub against the glass, or by scraping off a window sticker or the tinting film. Even a credit card can damage the grid, so limit anything that touches the glass to a clean rag and some window cleaner.

Bad news: A new rear window can be more than $400, including installation. Good news: You can fix it yourself for less than 10 bucks. Really. Start by going to the auto parts store and picking up a rear-window defroster-grid repair kit and/or a defroster tab repair kit, depending on the damage you need to fix.

Window Prep

Begin the repair process by using your soft cloth and window cleaner to thoroughly remove all dirt, dust, dog drool, ice cream smears and greasy handprints from the rear window. Clean it inside and out because you need to be able to see through the glass to discern the breaches in the grid. More im-portant, you need to give the conductive paint or adhesive a clean surface to stick to. Where's That Scratch?

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We removed this Escort tailgate from the car to make the photography easier. You can pinpoint hard-to-see scratches in the grid by chasing the voltage drop with a multimeter.

You'd think it would be easy to find a scratch deep enough to interrupt the flow of current through the grid, but experience says otherwise. So, drag out your trusty voltmeter. Actually, even a 12-volt test light will work. Turn on the defroster grid. Measuring from one side of the grid to another, you should see battery (system) voltage a little over 12 volts. You marked which line or lines in the grid didn't work on that last cold or rainy morning, right?

Put the probes of the meter on the damaged grid line at both ends, right where the grid connects with the wide bus bars. Move one probe--it doesn't matter which--to the center of the line. If you now read 6 volts, the scratch is not in between the two probes. If you read the same system voltage, 12 volts or so, the break is in between the old probe location and the new one. Just move the probes until you straddle the location that has full system voltage spanning it. Now it's time to turn off the defroster grid and the ignition switch--defroster grids are high-current items. Leaving the grid powered up for more than a few minutes will drain the battery.

If you have a very sensitive ohmmeter, you can find scratches this way by measuring the resistance along the grid, without powering up the system. I've done this on windows that were off the vehicle, like when I checked a grid on a junkyard liftgate of questionable provenance before I lugged it home.

Keep Inside the Lines

Okay, got all the breaks in the grid lines identified? Use a grease pencil on the outside of the glass to mark them. Now clean the area you need to patch with lacquer thinner, mineral spirits, rubbing alcohol or cheap vodka to remove any last oily fingerprints or residue. The kit we bought had a peel-and-stick mask to be applied to the glass. For a neater job, just use some masking tape and make the repair only as wide as the grid. Don't touch the surface or you'll leave fingerprints. Shake the small vial of conductive paint thoroughly, and simply brush the paint over the scratch. Wait a minute and add a second coat. Wait a minute longer, and remove the masking. You can use the defroster within 15 minutes or so.

Tab A Into Slot B

The procedure for repairing broken-off electrical connectors is almost as straightforward. Similarly, clean the area with lacquer thinner or alcohol to remove any oily residue. We took a small file to remove a couple of burrs, so the tab would lay flat on the glass. Our kit came with a vial of accelerator to complement a small pouch of conductive adhesive. Snap the glass vial of accelerator to release it into the applicator swab, and wet the surface of the glass and the back of the tab. Allow it to dry for 5 minutes. Knead the adhesive pouch for a minute or so, and snip off a corner. Put a couple of drops on the tab, and lightly press it to the glass for 60 seconds. Don't glue your fingers to the tab or the glass--and don't ask how we know this is a bad idea.

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Allow this adhesive to cure at room temperature for 24 hours before trying to reattach the wiring harness or using the defroster.