DIY: Repairing Your Rear Window Defroster

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Ciulla, Vincent. "DIY: Repairing Your Rear Window Defroster." ThoughtCo, Nov. 24, 2003, Ciulla, Vincent. (2003, November 24). DIY: Repairing Your Rear Window Defroster. Retrieved from Ciulla, Vincent. "DIY: Repairing Your Rear Window Defroster." ThoughtCo. (accessed October 17, 2017).
DIY: Repairing Your Rear Window Defroster

   It snowed last night. I mean it SNOWED. Your car is buried under seven inches of snow. You open the door, start the engine and turn on the rear window defroster. It takes a little while to clear the snow off your car and by that time the engine has warmed up enough to clear the windshield and started to clear the side windows. You look at the rear window and see that there are only a couple of clear strips and the rest of the window is still clouded over and impossible to see through.

   Fixing small breaks in a defroster grid is fairly easy to do. You can go to NAPA, Auto Zone or any of the big auto parts store and get a rear window defroster repair kit. The alternative: climb in the back seat and scrape the rear window with your small window scraper. And that is not really a good idea.


   The defroster grid is silk-screened on, literally painted on to the glass. If you ever painted window trim you know it doesn't take much to scrape paint off glass. The defroster grid is easy to damage and the grids that are scratched will not work because continuity is broken. Any hard object such as furniture, boxes and toys can scratch the defroster grid. A simple credit card is enough to damage the grid. This can be prevented by not jamming stuff into your SUV or van and having it shift into the rear window. The only thing that should touch the window glass and defroster grid is a soft cloth and some glass cleaner.

If you must scrub, do it gently and in the direction of the grid, not across it.

   Replacing the rear window to repair a defroster gird is the last thing you want to do. A curved style of rear window on most cars can be several hundred dollars and for a SUV or mini-van, over $1000.00. Include installation and it's a lot more.

Don't bother to submit the bill to your insurance company either; they'll just say, "Sorry, window glass is not covered."

   Doesn't Work At All

   Let's say the rear window defroster doesn't work at all. If you have been hanging around here any length of time you know the first thing to check is the fuse. Rear window defrosters draw a ton of current so if you have a fuse that's too low, it won't last long. If the fuse looks good, check it with a test light or voltmeter. You should have battery voltage on both sides.

   If the fuse and voltage tests are both good, the problem is in the wiring or in the defroster grid itself. Look at the sides of the grid. There is a terminal on each side and sometimes they fall or get knocked off. If one or both have come of and are just hanging there are two ways to repair them. You can glue them back on or you can solder them back on.

   If you know how to solder and have a 200-watt soldering gun or iron, you can solder the terminals back on to the defroster grid. A third hand is very handy in doing this so get the spouse to come out and help you for a while.

   Most times there is a metal backing strip below the silk screening and attached to the glass.

Clean the area well, alcohol works best, and use 60-40 rosin core solder. DO NOT USE ACID CORE SOLDER!! It will eat away all the metal parts over time and leave you with nothing. Make sure your soldering gun or iron is hot before you solder the terminals. Work as quickly as you can to keep the glass from getting too hot and possibly cracking.

   If you don't think you have the soldering skills necessary or are just afraid of cracking a very expensive window, there is an alternative. Some dealers and larger auto parts stores sell a special electrically conductive epoxy to glue the defroster terminals back on. If it's cold out, you'll need to work in a garage that is heated and the whole car warmed up to above 65° F.

   To do this with the epoxy you need to clean the area, again, alcohol works best.

Using tape, mask off the area so you don't get epoxy where you don't want it. Read the package directions and mix up a small amount of the hardener and epoxy. You don't need a lot for this. Now put a little epoxy on the defroster grid and a little on the bottom of the terminal. Place it in position and use a toothpick to hold it in place until the epoxy sets up. This usually takes about 10 or 15 minutes or so. You can dampen the end of your finger with water and move the epoxy around to make it look better if you want. If you do this you have to do it in the first minute or two or forget it.

   Copyright © 2000 - 2003 Vincent T. Ciulla All Rights Reserved

DIY: Repairing Your Rear Window Defroster

   After the epoxy has set up, don't even think about connecting the wire to it. You did disconnect the wire, didn't you? It takes about 24 hours at above 65° for the epoxy to cure to its full strength. Now this repair will not be as strong as the original so be very careful putting the wire back on and be extra careful when you clean the inside rear window.

   Uh oh...

   The fuse is good and the terminals are attached to the glass correctly and it still doesn't work. If this is the case then we have to look for a bad rear window defroster switch, rear window defroster timer or relay. You'll need a wiring diagram and a little experience in tracking this down. There is an article entitled Electrical Troubleshooting elsewhere in the Troubleshooting section that will be a big help here.

   In general here's what you can do. As always these are general procedures and you should always use the specific manual for your car for exact procedures. Remove the rear window defroster switch and using your wiring diagram, determine which wire feeds into the switch and which wire feeds out to the defroster grid. If the switch only has two wires, this will be easy. Jump the two wires and if the grid starts working, you know you have a bad switch. If you have no power going to the switch, and the fuse is good, then there is a break in the wire between the fuse box and the switch.

   If the switch is good, then we need to look at the relay and timer. Some cars combine the relay and timer into one unit so keep this in mind when checking your car. Now the rear window defroster relay has two sides, the coil side that activates the switch and the switch side that provides the electrical connection.

Generally the coil side has two light wires and the switch side has two heavier wires. Most manufacturers bury this relay in the dash or some have the good sense to put it in a relay box for easy access. I hope yours is easy to get to.

   If the coil side of the relay has continuity and there is power going to it, the relay and power supply is good. If there is no power then you have a break in the wire from the switch to the relay. If you jumper the two heavier wires, and check the wiring diagram to be sure they are for the rear window defroster, and the defroster works, you have a bad relay. If not, and you have power coming in, the relay is bad. If there is no power going to this side of the relay, there is a break in the wire from the power supply to the relay.

   If there is another, exactly the same, relay in the relay box, you can swap the relays and see if the rear window defroster now works. If it does, go over to the dealer and get another relay.

   If the rear window defroster switch, timer and relay are good, then we have a break in the wire going to the rear window. Now keep in mind that the rear window defroster is on a ten or 15-minute timer so you have that much time to find where the break is.

If the timer shuts off the power while you are checking the wiring, you'll be chasing wild geese.

   Rear window defrosters draw a lot of current, up to 20 amps depending on the car. Most cars put the rear window defroster on a 10 or 15-minute timer so it shuts itself off. This is for a couple of reasons. First off the rear window will overheat if left on too long. If you accidentally turn it on in the summer and go on a long trip the heat of the sun and the heat from the grid could cause the window to shatter and, at the very least, destroy the rubber window seal.

   The other reason is to lighten the electrical load. In the winter when you have the headlights on, the heater on high, windshield wipers going and the rear window defroster on, you will be placing a high demand on the electrical system.

Possibly more than the alternator can supply. So by shutting off the rear window defroster after a period of time, you will lighten the electrical load.

   Copyright © 2000 - 2003 Vincent T. Ciulla All Rights Reserved

DIY: Repairing Your Rear Window Defroster

   Easy Repairs

   Okay, so everything is working but you have a few lines of the grid not working. The lines don't work because there are breaks in them. So, we have to find these breaks. If you have a SUV or mini-van, the grid is easy to check. If you have a car you'll need to crawl into the back seat in a position the human body was not designed to be in.

And if you want to see something really funny, watch a 250 pound, 6'3" man try to fit in the back seat of a small import.

   Anyway, if you look from the inside out, you may see the breaks in the lines against the sky. Just start at one end and work your way to the other looking very carefully. If you can't see any breaks you will need to get out your test light or voltmeter. You need to keep in mind that when you check the defroster grid the voltage will decrease as you get closer to the negative side. In a line that has a break, the voltage will be the same until you get to the break and then disappear.

   So with this in mind, lets start checking. First get a couple of pieces of aluminum foil. Wrap one end around the probes of your voltmeter or test light and leave the other end flat. This will let you check the grid without risk of further damage. I prefer to use a test light; I think it's easier.

If you use a voltmeter the idea is the same. I clip the test light to a good ground and pressing the aluminum foil gently on the grid I watch the light. As you slide it from the positive side of the rear window defroster grid to the negative side, the light should grow dimmer as you go. If it stays bright then you are on a line with a break.

As soon as the light goes out, you found the break. I have someone draw a circle on the outside of the glass with a marker so I don't lose this break while looking for others.

   Once you find the breaks, repairing them is simple. You can buy a rear window defroster repair kit at most auto parts stores. Clean the area with alcohol. Don't use window cleaner since it might leave wax or silicone behind. Also, don't use paper towels with color or designs on it. The kit will have a self-stick template and a small bottle of conductive paint and brush. If the template is not the right size if the break is longer than the template, you can use masking tape to mask the lines.

   Once the area is clean and masked off, shake, the bottle well, and simply paint across the line stroking along the line and not against it. Do all the breaks and then come back in a few minutes and give each break a second coat. Let it sit for 24 hours to allow the conductive paint to thoroughly dry and set.

   Once you have all the breaks repaired, go back and check the rest of the lines you repaired since there may be other breaks a little further down. There's no rule that says only one break per line.

   And that's it.

Rear window defroster systems are pretty reliable and most times the worst thing that happens is a broken line, which you see, is very easy to repair.

   Copyright © 2000 - 2003 Vincent T. Ciulla All Rights Reserved