Car windows don’t like severe temperature differentials. This comes to an element when the outside is very cold, and your body, plus the air conditioning, keeps the interior warm and moist.
The temperature differential makes the moisture in the cabin air condense on the windows upon contact (since the glass is colder than the interior). The impact is most profound and more problematic on the windshield and the rear window.
Cars have two main systems to combat this problem.
- Heater vents that use heat from the heater core and the AC blower to blast hot air over the windshield and get rid of the condensation
- A heater grid on the rear window keep the glass warm enough to prevent condensation
Since the two systems work independently and differently, troubleshooting them takes a different set of steps.
Troubleshooting a Failed Windshield Defroster
The windshield is very important. You look out through it when driving. That’s why keeping your windshield wipers in the best condition is in your best interest.
With the water and snow out of the way, you might still find yourself in trouble if the defroster isn’t working. Here are the top things that could make your front defrosters stop working.
Your Car Hasn’t Heated Up Yet.
Your car’s heater gets hot air by running regular intake air through a heat exchanger that draws some heat from the car’s cooling system. This means that it relies on a hot engine to heat your car.
Consequently, you won’t get any heat if you just started the car. The defrosted air will blast cold air over the windshield. While sometimes this might be enough to get rid of light fogging, you will need the heat to clear your windshield.
If you are in a blizzard or heavy storm, it is wise to let your car warm up before driving off. This is not only good for your engine but also ensures you have all the air conditioning you need – and most importantly, front window defrosting.
Broken or Stuck Controls
While this is rare, your AC controls could be faulty and fail to activate the defroster in demand. You can confirm this when you don’t:
- Feel air blasting from the vent below the windshield
- Your engine is warm, and the air blasting out is cold
- You can’t control any other temperature settings in the car
You could also be activating your defroster the wrong way. If it’s a new car, check with your manual for the right defrosting procedure. If you follow this to the latter and the defroster isn’t kicking in, troubleshoot further to rule out any other problems before having a pro check your AC controls.
A Failed AC Blower Motor
The AC blower motor circulates air over your heat exchanger element. It draws the heated or cooled air into the car through vents hence that nice breeze you feel when the car turns on.
While they are designed to last for long, the blower motor could fail after a couple of years – especially if you run AC a lot.
A telltale sign of a failed AC blower motor is intermittent or no airflow at all. This problem is hard to miss since your car won’t heat up or cool down no matter what AC setting you select.
You can hear the blower motor whirring when it kicks in. If you can’t hear that normal AC noise, then your blower has an issue.
Blocked Vents
Everything else in your AC might be working, but the air isn’t moving smoothly because you have a blockage somewhere in the system. While the filter ensures debris doesn’t get into the system, a car parked for very long could have vents blocked.
Animals could take over your car. Rodents are known to build nests in very crazy places.
If you drive your car frequently, a blockage shouldn’t be an issue unless you or a mechanic forgot to unplug a vent or accidentally dropped something down a vent when working on the car.
ProTip: Changing your cabin filter frequently could improve your ACs efficiency
Coolant Problems
Your AC system uses a heater core to exchange heat with the car’s coolant. This means you might lack hot air if:
- You have low coolant levels, and the heater core isn’t getting enough of it to warm up efficiently intake air
- You have a stuck thermostat that is preventing your coolant from warming up to the right operating temperature
- Your heater core is damaged or blocked and isn’t exchanging heat well. It could also be punctured, leading to a coolant leak.
Heater cores rarely block or go bad. Only check it if your car never blows hot air, even on air conditioning mode.
Troubleshooting a Rear Window Defroster That Isn’t Working
Unlike the front defroster that uses air conditioning air, the rear defroster uses resistive heater wires.
Ever wondered why your rear window seems to have a grid of fine wires? Well. That is the rear defroster heater grid.
When you turn on the rear defroster, a current is applied to the wires, heating them. They then heat the rear window hence evaporating off the fog and keeping the window clear.
Some of the things that could go wrong here include;
An Old or Broken Defroster Grid
An old rear defroster grid can be too worn out to heat the rear window sufficiently. It might even be broken at a place where it limits the current from flowing through the remaining sections of the grid at all.
Most modern cars have the grid embedded into the glass. Replacing or repairing it if it’s broken is next to impossible.
Your best bet would be replacing the entire glass.
Also, your rear defroster won’t work if you have major cracks that cut across the grid. Chances are they have broken and discontinued contact in the defroster.
Damaged Defroster Terminals
Another common problem that could put rear defrosters in an old car out of commission is dirty or corroded spade terminals. These deliver power to the defroster grid.
Tug them off carefully and test if they have power with the defroster on. After this, clean them up well and retest the rear defroster.
If you are lucky, this should bring your rear defrosters back into commission.
A Blown Fuse, Relay, or Damaged Wiring
Lately, if you can’t see any damage to the defroster grid, you can check your fusebox and confirm if the respective fuse and relay are working fine.
Consult your manual for the fusebox schematic to make identifying the fuse and relay easier.
Most cars don’t run the defroster grid constantly as it draws a lot of currents. They have a timer that switches it on and off repeatedly to maintain temperature. Also, check if the delay timer is working well and not damaged.
Bottom Line
The front defroster might be the most important in your car. Luckily, if your AC blower is working fine, you can still keep the fog off by running your air conditioner. The natural process of changing air temperature takes moisture off. Your AC could keep the fog off if the temperature differentials and relative humidity is right.