Troubleshooting and Testing Your Rear Defroster
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.