Automotive defrost systems absolutely essential in cold weather, but they are also essential when the combination of humidity and temperature causes the windows to fog up. When your defroster stops working, reduced visibility can lead to very dangerous driving conditions.
There are actually two different types of car defrosters, so a different process is required to track down and fix this type of problem depending on whether your front or rear defroster has stopped working.
What causes a car de-icer to stop working?
Because there are two types of car defrosters, the reason you stopped working will depend on the type you’re dealing with.
The car’s front de-icers typically use air from the heating, ventilation and air conditioning (HVAC) system to melt ice and clear foggy windows, while rear de-icers typically use a grill of hot wires attached to the window pane.
There are exceptions, but these types of defrosters are found in most vehicles.
Reasons for the front defroster to stop working:
- Damaged or stuck controls : the buttons or dial you use to switch between hot and cold, as well as the switches that blow air out of it, can get stuck or broken. Some of them even use gears or cables that can pinch or pinch.
- Problems with the vent and air intake : If you can hear the fan motor running but no air comes out of the defroster vents, the vents may be clogged or the fresh air intake may be blocked.
- Coolant problems : If your defroster blows only cold air, the engine may have a low coolant level. The thermostat may also be stuck, or the heater core may be clogged.
- Fan motor problems : If you don’t hear anything when you turn on the heater, air conditioner, or defroster, the fan motor may be defective. You may also have a bad switch or fuse.
Reasons for the rear deicer to stop working:
- Broken defroster mesh: The rear defrosters rely on a very fine mesh of wires attached to the window pane. If the wires are physically damaged, the defroster will not work.
- Mesh worn out: if your car is old, the grate may be too worn to work properly.
- Broken defroster connections: if the connections where the power is connected to the mains are broken, the defroster will not work.
- Faulty heater switch or fuse: if there is no power to the mains at all, suspect a faulty switch or fuse.
Front windshield defroster fixes
When you turn on the front windshield defroster, what really happens is that the HVAC mixer door moves to direct air out of the vents. In some cases, turning on the defroster may also automatically activate the air conditioner.
When the front defroster stops working, it’s usually a faulty switch or mixing door if air is escaping from other vents, or a faulty fan motor if no air is escaping from the vents. If air comes out of the vents but is cold even though you turned on the heat and turned off the air conditioner, you have a problem with the cooling system.
The cost and complexity of such repairs varies by vehicle, as some heater switches, fans, and mixed doors are easily accessible, while others require you to remove the entire dashboard assembly.
Keep in mind that if your heat doesn’t work, it doesn’t necessarily mean your front defroster is off as well. While blowing cold air from your air conditioner onto your windshield won’t melt the ice, it will actually lower the relative humidity inside your car, making it great for cleaning windows on a cold, rainy day.
How to fix windshield defroster:
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With the engine off and cold, check the coolant level. If the coolant is low, fill it up. At this point, the defroster may start working again, but you have a coolant leak problem that needs to be fixed. If your front windshield is sticky and you can’t wipe it off, the heater core may be leaking.
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Check your heating, ventilation and air conditioning (HVAC) controls. If you have button or dial controls and they don’t move smoothly, you might have bad controls or something might be tied inside the dash.
If you have vacuum control, there may be a break in the vacuum lines.
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Check if the fan motor can be heard. If you hear this but no air comes out of the vents, check the fresh air supply. If it is connected, clear it. If it doesn’t, then your mix door may be stuck, or the vents themselves may be plugged from the inside.
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Check the power on the fan motor. If you can’t hear the fan motor running, check the power. You can fix this by replacing the fuse, but it’s more likely a bad fan, a bad switch, or a bad ballast resistor.
Rear window defroster fixes
Unlike front windshield defrosters, rear windshield defrosters are actually specialized devices that can and do break. They consist of relatively simple wire grilles that are powered by the vehicle’s electrical system when you press the heater switch.
When electricity flows through the grid, the wires heat up, causing the ice to melt and the condensation or mist to dissipate.
The most common cause of rear defroster failure is an open circuit or short in the defroster grille. The easiest way to check this is to use a voltmeter or test light to test for power and ground, and use an ohmmeter to test for continuity along each grid line.
Another common point of failure, especially in hatchbacks, station wagons, and some SUVs, is the actual spade contacts where power and ground are connected. Of course, the switch can always fail.
When a rear window defroster fails, repairs are usually either expensive or time consuming. Cheap repair kits can sometimes take care of breaks, aftermarket replacement grilles are also available, but sometimes it’s necessary to completely replace the rear window.
How to repair a car rear window defroster:
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Check the defroster grid. If you can see where the grille is broken or worn, then your rear defroster is not working for that reason. Some meshes can be repaired, but you may need to replace the rear window.
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Check spade connectors. Most deicer grids use connectors to provide power and ground, and are sometimes disconnected from the mains. If the shovel isn’t broken on the window pane, try gently plugging it in. If you can plug it back in, your defroster should start working.
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Check for power at the spade connectors. If there is no power or ground on the wires connecting the connectors to the space, it means there is a problem with the wiring or the switch. Follow the wires to the source to see if there is a broken wire or a bad switch, relay, or fuse.