What do the front/rear demisters do?

It is important that you are able to see through the fog on your windscreen on a cold day. Demisters (also referred to as defoggers or defrosters) are used to clear the front windscreen and rear window of frost and fog. This allows you to see clearly when driving, reducing the risk of accidents in cold weather.

In order to clear your windscreen of fog/frost, the front demister requires your car to draw air into its heater core. Once it reaches the heater core, it is warmed up and directed through small vents in your car’s dashboard. This causes it to blow upwards onto the windscreen, removing fog and ice.

If you car has air conditioning, then the it will most likely also act as a dehumidifier. The  dehumidifying function built into your cars air conditioning will quickly defog your front windscreen (much faster than just using the standard car heater). Therefore is at all possible, please always try to use your car air conditioning to defog your front windscreen / windshield.

Learn more about how your car’s heating system works >

Rear demisters usually operate a bit differently to front demisters. This is because they use electrical currents to warm the glass. Instead of hot air, the rear defroster sends electrical currents through a metal grid which is attached to the glass panel. The metal generates enough heat to evaporate fog (and ice) from the rear window.


Causes of demister problems:

Front demister:

Damaged heater core – If your front demister isn’t working correctly, it may be due to problems with the heater core. The tubes in the heater core can become blocked, limiting the flow of coolant and preventing warm air from being produced.

Low coolant – Coolant reaches high temperatures, meaning that as air passes through it in the heater core, the air becomes warm and heats your vehicle. If your car has low coolant due to a leak, the heater and front demister won’t work properly. Your vehicle may also be at risk of overheating.

Bad blower motor – The blower motor forces air through your heater core, allowing it to warm up. If the blower motor fails, the demister will cease to work.

Car heater fan vents / flaps stuck – the cable that controls the heater fan vents can snap or become disconnected from the heater controls. This can prevent the car heater vents or control flaps from redirecting warm or dehumidified air onto the car front windshield / windscreen.  This means that your cars front windscreen / windshield won’t demist or defog when you need it, and you are forced to try to wipe it with your hand or a cloth.


Rear demister:

Faulty wiring –  The wiring grid can become damaged, severing the circuit and preventing the rear demister from working correctly.

Broken rear demister heater wires – Sometimes items are put on the rear parcel shelf directly below the rear window and rear window heater demister wires. If these items rub against the rear window heater demister wires, then they can break these wires which can prevent all or part of your rear demister from functioning correctly. For example, you may notice a strip along your back window that doesn’t get defogged or demisted.

It is also common for hatchback cars with a rear demister on the tail gate window to get the heated rear demister / defogger wires broken. This is because items such as shopping boxes and other things are often jammed under the rear tail gate window and the heated rear demister / defogger wires. This can cause these heated rear demister / defogger wires to breal over time, reducing its effectiveness or stopping it working altogether.

Blown fuse – Like most other car electrical components, the car rear demister is connected to a fuse. If this fuse blows, the front or heated rear demister / defogger will stop working.

Wear over time – Constant exposure to heating and cooling over time can cause grid to have cracks in the heated rear demister / defogger wires over time.


How important is a demister inspection?

If you are going to be driving in cold weather or your take wet items into the car which can cause fogging up of your cars windows then we recommend that you get your demisters fixed first. Your car’s demisters allow you to clearly see what’s in front and behind you in all sorts of weather conditions. This improves your, your passengers and other road users safety while driving.


Front / Rear Demister Inspection and New Parts Installation in Hamilton

Is your front or rear heated demister in need of fixing? If so, we can help!

At Grimmer Motors, our experienced, qualified and friendly mechanics can quickly diagnose problems in your car’s heating / demister / defogger system. We can then provide quality part replacements that you can count on. We can fix your heater core, fix heater controls, replace fuses, get your air conditioning fixed and more. This will allow for improved vision and safety when driving in cold weather.

For demister / defroster / defogger replacement and diagnostics in Hamilton, contact Grimmer Motors today!


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