Defrosters clear condensation from windows and melt frost, ice and snow. They work differently depending on whether they are for the windshield or the rear window.
On most vehicles that have air conditioning, the A/C engages automatically when the defrost mode is selected to dehumidify the air. Drier air helps clear condensation from the inside of the windshield and front side windows faster. Vehicle manufacturers frequently recommend turning off the air recirculation mode to speed defogging because air inside the vehicle often has more moisture than outside air.
The front defroster is controlled through the heating and ventilation system. Windshield defrosters blow air from the heating and ventilation system onto the windshield through vents at the forward edge of the dashboard.
Most vehicles have a setting for directing all air to the windshield for maximum defrosting or splitting the air between the windshield and floor level. In addition, drivers can select the temperature setting and fan speed for the defroster.
During the winter, using the defrost mode with a hot temperature setting accomplishes two things: The hot, dry air defogs the windshield and melts frost and snow, making it easier to scrape the windshield and preventing ice from forming while driving.
The rear defroster is powered by electricity and operated by a dashboard switch. Turning it on activates a grid of wires, visible as thin lines on the inside of the glass, that heat the rear window to defog the glass and melt frost, snow and ice.
Most rear defrosters operate on a timer that automatically shuts the defroster off after 10 minutes or so. On vehicles equipped with heated outside mirrors, turning on the rear defroster also will activate the mirror heaters.
Rear defrosters start working as soon as they are turned on, but during cold weather, the front defroster needs more time to be effective because the engine needs to warm up first. Like the rest of the heating system, the front defroster requires that warm engine coolant be circulating through the heater core before it can start blowing warm air onto the windshield.
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