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How the Defroster Works

Car Defroster

A 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.

What Defrosters are Designed to Do

When the weather is cool, droplets of water will form on the vehicle’s windows. This is called condensation and it occurs because the air outside is colder than the temperature inside the car even if the vehicle has been sitting overnight. 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.

Types of Defrosters

Your front defroster operates a little differently than the one in the rear window. The front defroster has vents on the dashboard that face the windshield and front windows. The fan and blower motor that operates the heating and air conditioning will also circulate air through these vents to defrost the windows.

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. The rear defroster is electric for most vehicles. The rear window will have thin black lines running horizontally across it. 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

When you turn on the front car defroster, the coolant will reach a certain temperature to 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.

In addition to removing condensation on the outside of the vehicle, these defrosters can also remove fog that accumulates inside the vehicle. This phenomenon often occurs when the air outside is cooler and you have warm air inside.

Lines on the rear window are electrical. They heat up when the rear defrost is activated and begin removing the condensation immediately. The bonus for the electric defrost is that it begins working as soon as you turn the vehicle on and set the rear defrost button. The front defrost with the fan and blower must wait until the temperature of the vehicle warms up for it to be effective. Many new models are including electric defrost 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. It is important to 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 replacement or repair
  • 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

A professional can help diagnose and repair the defroster system so that it is working correctly again.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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