What to Do If Your Rear Defroster Doesn't Work

The rear defroster is a strange little system, but quite ingenious. It's been no secret for a long time that if you run current through a circuit with a little resistance built in, you'll get heat. But utilizing this to eliminate fog and frost from your car's windows only surfaced in the past few decades, give or take a couple. These days, with the touch of a button all those little lines on your back window (and the similar radio antenna on your front windshield) heat up to melt away fog and frost. When they are working well, these systems are great. When they aren't working, less than great. There are so many tiny issues that can make these defrosters fail that I would guess a large percentage of them are inoperative. The good news is once you figure out what's wrong with it, you can fix it yourself.

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Troubleshooting and Testing Your Rear Defroster

Rear defroster tab disconnected
A broken or disconnected tab can keep your rear defroster from working. Check your connections. photo by Matt Wright, 2012

The good news is it's easy to figure out what's wrong with your defroster. Like I said, the defroster system is one long circuit that heats up as electricity passes through it. (Ok, technically some systems are a few long circuits with multiple attachment points, but that doesn't affect how you troubleshoot or repair them!) Those little lines are actually made of conductive paint that is applied directly to the glass. This makes the defroster very compact and durable. It also means that any chip or scratch in that conductive paint can render the system inoperative.

Visual Inspection: Sometimes there is a very obvious break in the painted circuit or some other problem that can be easily ferreted out with a visual inspection. First check the connection tabs located toward the left and right sides of the painted grid. Sometimes these connections are soldered in place. You can spot a failed solder connection because there will be a loose, dangling wire that obviously should be connected to the grid, but no way to reattach it. If your soldered connection has come loose, it can be repaired with a special kit that contains soldering paste (basically an epoxy glue that is full of metal so it will conduct electricity). Ask your parts store for this type of kit. If you have a dangling wire that has a connector on the end, chances are it has just worked its way loose from the other part of that connector located on the painted grid. If you're lucky enough to have this type of problem, just reconnect the wire and see if you're back up and running.
The next step in your visual inspection is to look at the small lines of the grid itself. Sometimes a break in the gridwork was caused by something inside the car and has left a noticeable sratch or missing section in the paint. Follow the entire grid with your eyes to see if you can find such a break. If you don't see anything wrong with the naked eye, it's time to get out the test equipment. On the next page I'll discuss how you can test your rear defroster using a simple testing device available for a few dollars from the auto parts store. If you're not sure where the connections to your defroster circuit are, it might be a good idea to consult your repair manual.

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Using a Test Light to Troubleshoot Your Rear Defroster

Rear defrost test lamp
A test lamp like this one will help you find a break in your defroster circuit. photo by Matt Wright, 2012

Once you've completed a visual inspection of your defroster grid (and if you came up empty in terms of solutions), you can begin to test the system more scientifically. You can use a standard test light for this procedure, however, some 12-volt test lights require more electricity to light up brightly than the rear defroster provides. For this reason, it's a good idea to buy an inexpensive tester specifically for rear defrost systems. Their test lamp will illuminate with the slightest bit of power running through it, and this is useful in troubleshooting the defroster lines.

Overall Power Test: The first thing you need to find out is whether your defroster grid is geting power at all. Sometimes old wiring or a blown fuse can cause a complete disconnect. To test this, you can use either the defroster test lamp or an automotive circuit tester. Disconnect both wires from the either side of the defroster grid. Touch or clip one end of your tester to each of these wires - if the light comes on, you have power. If it doesn't, you should check your fuse and replace a bad or questionable fuse if you're not sure. *Note: Be sure you have the rear defroster switch on and the key turned to the ON position when you test.

Grid Power Test: Before you start to worry about all of those little painted lines, you need to see if there is even power getting to your defroster grid. Disconnect the wire that is closest to the passenger side of the car, then attach the test lamp clip to the wire (not the little metal tab attached to the glass). Next touch the other test lamp wire to the tab on the other side of the circuit (don't disconnect the wire on this one). If it lights up, there is power actually getting to the grid.

Grid Breakage Test: If you've verified that there is power getting to the grid itself, your bad defroster is probably due to a break in the painted circuit someplace. This is especially likely if your defroster seems to work on only part of your rear window. Attach the clip end of the test lamp to the driver's side metal tab or some exposed metal part of that wire. Next, begin touching the other end of your test lamp to the painted circuit. Frost Fighter, a well known brand of defroster repair kit, suggests wrapping a little aluminum foil around the end of the wire to be sure you don't scratch the painted circuit. Touch the grid every three inches or so in order to discover the location of the break. It's normal for the light to be brighter in some spots, as long as it lights up. If you find a break in the circuit, you can systematically move the test wire back and forth, closer together, until you know exactly where it stopped working. Once you know exactly where the break is, you can make a proper repair!