This article will give you some useful tips on how to fix a rear window defroster.
This is a pretty rare car problem but when it happens it seriously reduces the rear visibility which is not only annoying but dangerous.
So, if you press the defroster button and those lines of clarity don’t appear but rather fog and ice stay in place, you should know what to do.
Besides this article, you have a tutorial at the end of the video which you can also watch on our YouTube channel.
MAIN PARTS OF A REAR WINDOW DEFROSTER SYSTEM
In order to better understand what may be causing the problem we’ll first mention the main parts of the whole system.
We’re showing a general example but the basic principle applies pretty much to any car out there as the whole system is similar.
The differences are in the location of the fuse box, relays, type of switch, and so on.
One more thing for why we’re mentioning this is that most people immediately suspect the defroster itself but often time other parts of the system may cause the problem.
So the main parts are:
Rear window defroster
This is the part that actually emits heat. The lines you see better known as rear window defroster strips are in fact heat conductors which heat up once an electric current goes through them.
They’re connected to the installation through two contacts on each side of the heater
The fuse is in charge of protecting the electric installation from a short circuit or overload.
Rear window defroster relay
The main purpose of the relay is to provide the necessary strong electrical current to the defroster and to protect the switch at the same time.
Rear window defrost button
The switch has the purpose of turning the heater on and off.
Connectors and wires
In between all of these parts, there are various connectors and wiring.
WHAT TOOLS WILL YOU NEED?
Diagnosing a defroster problem isn’t that demanding regarding tools.
All you’ll need is a simple electric tester or a multimeter.
For the repair, you may additionally need some spare parts, tools, or repair kits but we’ll be talking more about that later in the article.
HOW TO FIND A REAR WINDOW DEFROSTER PROBLEM?
When talking about how to fix a rear window defroster, perhaps the most important thing is to set a proper diagnosis of the problem.
To do this, you first have to see if there is electric input to the defroster.
This gives you a starting point and a partial answer to whether the defroster is the problem or other parts of the system.
To do this you have to use the tester or multimeter. In this case, we’re going to use the multimeter since it’s more convenient to connect.
Whatever you choose, the basic principle is the same. The only difference is that the tester lights up while the multimeter shows a voltage reading.
So, connect the multimeter to the contacts on the defroster. You’ll find one on each side of it. Try to connect it without removing the defroster connectors as they oftentimes tend to be brittle and break off.
If the multimeter shows a voltage reading or the tester lights up, then there is an electrical input and this means that the defroster itself is the problem.
But if there isn’t an electric input (the multimeter shows 0 volts or the tester doesn’t light up), then search for the problem on the other parts of the system that we’ve previously mentioned.
All in all, you must do this to make a good stepping stone for solving the problem.
HOW TO FIX A REAR WINDOW DEFROSTER?
1. PROBLEM WITH THE DEFROSTER
When mentioning how to fix a rear window defroster, know that the most common problem with the defroster is a broken or damaged line (conductor) within the heater.
Another cause of problems is broken-off contacts.
This happens mostly due to physical damage like scraping, wiping, or accidentally breaking off.
The repair is fairly simple. If the heater isn’t molded into the glass, you can buy repair kits for fixing the conductor lines. It’s pretty simple to do and is explained on every repair kit. The basic principle is to use a special coating and reconnect the broken wire.
If the contact is broken it’s a bit more complicated but the principle of repair is the same (you in some way glue or weld the contact back into place).
For heaters that are molded into the glass, the only repair in most cases is to replace the whole rear window.
2. PROBLEM WITH THE FUSE
If there isn’t any electric input to the defroster the fuse is the first place to look.
Use the tester, find the fuse for the rear defroster, and check if it’s OK. If it’s burnt out, replace it with a fuse of the same amperage and try if the defroster works.
If it burns out again, then search for the problem elsewhere, in most cases search for a short circuit.
For more information on this topic and how to replace fuses on your car, click here to read the article and watch the video.
3. PROBLEM WITH THE REAR WINDOW DEFROSTER RELAY
If the fuse is OK, the relay is the next thing to check.
It’s a pretty rare cause of problems and if it happens it’s mostly due to material fatigue.
Once you’ve found it on your car (it will oftentimes be hidden under various panels) check out if it works.
The fastest way to check it is to listen to the relay when you turn on the defroster.
If it clicks then it probably works.
If it doesn’t click, try to take it out and check it with the tester or multimeter if there’s electricity input to it.
When there’s no electric input, check the fuse and the rest of the electric installation.
But If there is input, then you should probably replace the relay. It would be best if you have a spare one at hand to try it out and be certain. Just pull the old one out and insert the new one.
4. PROBLEM WITH REAR WINDOW DEFROSTER BUTTON
Just like the relay, this button (switch) is a rare cause of problems. You should check it if there’s no electric input to the relay.
If you want to test it, best pull it out of the console and check with the tester. See if it gets electrical input and if it lets electricity through the switch.
If it doesn’t, best replace the switch.
5. WIRING AND CONNECTORS
If the fuse, relay, and switch are OK but you still don’t have an electric input, then check the connectors and wiring.
The most common causes of problems on this part are broken wires and bad connections.
To solve this problem, it would be best to have some prior knowledge about the wiring on your car and where it exactly goes and where are some of the main connectors located.
Hot spots for problems like these are places where the wiring is frequently strained or bent. For instance, in this case, the first place to look would be the rubber grommet tube through which the wires pass.
Also, look at places where there might be moisture or corrosion, like near the floor panels.
This problem is in most cases easily solved with some contact spray, WD 40, or by replacing broken wires.
A much bigger task in this case is to find where the problem actually is.
Once more for the end on how to fix a rear window defroster: first, make a good diagnosis of the problem.
Connect the tester or multimeter to the contacts of the defroster and see if there’s an electric input.
If there is, inspect the defroster and its contacts for any kind of damage.
On the other hand, if there isn’t an electric input, check the fuse, rear window defroster relay, button (switch), and the connections and wiring.
Know that the most common causes of problems are physical damage to the defroster strips and a burnt-out fuse.
Whatever the problem may be, in most cases it’s fairly simple to repair as long as, once more, you diagnose the problem properly.
Written by: Sibin Spasojevic
Former car technician, life-long car and DIY enthusiast, author for Despairrepair.com
HOW TO FIX A REAR WINDOW DEFROSTER? YOUTUBE VIDEO