How the Defroster Works

Whether it’s on a cold winter day, or it’s humid outside and the front or rear windows are fogged, having a reliable working car defroster is vital to maintain visibility. A fully functional vehicle defroster is a valuable component for your car, especially on those cold winter days when you have frost or ice on your windshield. While older models only have defrosters on the front windshield, many newer models have them on the rear window as well to improve visibility for drivers.

The actual components used to activate the front and rear window defrosters differ depending on what year, make, and model of vehicle you own. In general, the information below will provide an outline as to how these systems work.

What is the Job of a Window Defroster?

There are two different types of window defrosters: front windshield defrosters and rear window defrosters. The front windshield defroster is designed to blow large volume of air on the windshield with the goal of dissipating condensation that has built up on the inside of a windshield. When the weather is cool, droplets of water can form on a vehicle’s windows. Condensation on the inside of the windshield occurs because the air outside is colder than the temperature inside the car. When the temperatures become even colder, the condensation turns to frost or ice, which must be scraped away by hand or thawed by the defroster.

How Front and Rear Window Defrosters Work

In simple terms, the front defroster works by circulation of air, while the rear defroster is electrically charged. The front defroster has vents on the dashboard that face the windshield and front windows. The fan and blower motor that operate the heating and air conditioning will also circulate air through these vents to defrost the windows.

The process of operating the front defroster is unique to your vehicle. In general, to activate the front defroster, all you have to do is make sure the vents are open, turn on the fan and turn the setting to defrost and set the temperature to the desired setting. In most cases, warmer air blowing on the inside of the window will expedite this, but on the first engine start of the day, it will take time to create heat.

The rear defroster is electric for most vehicles. The rear window will have thin lines running across the window. These lines are electrical fibers embedded in the glass that heat up as they are activated. This defroster has its own button that you access when you want to defrost the rear window. You will notice the condensation or ice dissipating first along the lines until the entire window is clear.

How the Defrosters are Activated

Front defrosters work best when the air blowing onto the window is warm. However, it takes time for heat to build up in the engine and activate the heater core. When the coolant reaches a certain temperature, it will open the thermostat. Hot water will move through the core of the heater while the blower fan pushes the warm air through the vents of the defroster to warm up the windows. The condensation or ice will begin to dissipate as the window reaches the desired temperature. If the heater is not working, the front defroster will have difficulty working correctly.

The rear window defroster is electrically powered. The lines on the rear window are electrical. They heat up when the rear defroster is activated and begin removing the condensation immediately. The bonus for an electric defroster is that it begins working as soon as you turn the vehicle on and push the rear defroster button. Many new models include electric defrosters along the edges of the front windshield to enhance the defrost system and remove condensation faster.

Heated side view exterior mirrors also use electrical defrosters to remove condensation so that you can view the area surrounding your car. The difference is that you don’t see visible lines as with the rear window defroster. Note that these heaters provide a low amount of heat and won’t burn you if you touch the window while they are activated.

Common Issues with Defrosters

You often won’t notice a problem with a defroster until you need it and it fails to work. This can happen for a variety of reasons:

  • Buttons or knobs that are stuck or have stopped working – may need replaced or repaired
  • Blown fuse – when a circuit is overloaded, it may blow the fuse that connects to the defroster, the fuse can be tested and replaced by a professional
  • Missing terminal edges on the window – this may be caused by tinted windows that have started cracking or tint that has come off
  • Lack of antifreeze – when the antifreeze level is too low, the car may not heat properly or allow the defroster to work
  • Frayed wires – wires that are disconnected or frayed will prevent the defroster from working
  • Clogged vent – when a vent is clogged with dust and debris, the air cannot flow through to warm up the windshield

If your front or rear window defroster is not working, it is a good idea to contact a professional mobile mechanic to come to your location and complete a car defroster is not working inspection. This will allow them to pinpoint what is broken or not working so the right repairs can be completed quickly.


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The most popular service booked by readers of this article is Defroster is not working Inspection. Once the problem has been diagnosed, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. YourMechanic’s technicians bring the dealership to you by performing this job at your home or office 7-days a week between 7AM-9PM. We currently cover over 2,000 cities and have 100k+ 5-star reviews... LEARN MORE

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David

13 years of experience
393 reviews
David
13 years of experience
Mitsubishi Eclipse L4-2.4L - Defroster is not working Inspection - Marietta, Georgia
David K. was knowledgable, prepared, and very professional. I highly recommend his service.

Peter

36 years of experience
346 reviews
Peter
36 years of experience
Toyota Avalon V6-3.0L - Defroster is not working - San Diego, California
Only disappointment is YourMechaninc was not able to perform the repairs and Peter did not recommend one or two local mechanics who could perform the work he diagnosed.

Corey

17 years of experience
332 reviews
Corey
17 years of experience
Honda Accord V6-3.0L - Defroster is not working - Charlotte, North Carolina
I just want to thank Corey,for his timely attention to analysing my cars defrost system,not only did he give me some great pointers he was very kind,professional and knowledgeable, I recommend this young man very highly.

Brian

13 years of experience
4 reviews
Brian
13 years of experience
Honda Civic L4-1.8L - Defroster is not working - San Pablo, California
Brian was extremely helpful today. He answered all of my questions and even showed me all of the parts on my car that needed my attention. Really nice guy and easy to talk to. Thanks, Brian!

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