Hello all,

I've noticed over the past couple of weeks that my defrost for my front/main windshield and door windows doesn't work as well as it used to. It will defrost at first, but then as I drive they slowly start to fog back up. But if I'm at a stop light or idling then they clear up again. The rear defrost works fine. Does anyone have any suggestions on where I should start and what I should be looking at? Any help would be much appreciated. Thanks.

 

The rear defrost is just wires glued to the glass, which either works or doesn't work (e.g., if the wire breaks, you blow a fuse, or the switch goes bad).

The front defrost is a combination of fan speed, temperature selection, outside air vs recirculation, and A/C efficiency. Basically it requires the proper function of your HVAC system. If that is compromised in any way, the defrost won't work well or at all. I will also note that if your car is really wet inside (e.g., wet floorboard from a leak or spill, etc) the defrost will struggle.

Fan speed - if your cabin air filter is old/plugged up, you won't be able to move as much air; similarly, if the defrost vents at the base of the windshield are blocked/plugged, the defrost obviously won't work well.
Temperature - sometimes the flappers in the blend box, or the cables that control them, get messed up and you may not get the full range of temperatures; similarly, the heater box itself can go bad over time, meaning your car doesn't generate heat as it should.
Outside vs Recirculate - the flap in the blend box can get stuck in one position or the other; more generally if it's really wet outside you probably want to shift to recirculate so that the defrost works more quickly.
A/C - the A/C kicks on automatically when you turn the mode dial to defrost, as dry air is essential to the defrost function; your A/C can be compromised in several ways, including a failing A/C compressor or a loss of refrigerant from the system.

Figuring out what is what requires some testing. Is your cabin air filter clean? Does the fan operate at all speeds and blow as hard as it should? Are the defrost vents clear? Does the temperature of the air coming from the vents coincide with the temperature selection dial? Does the air come from the proper vents when you move the mode selection dial around? Does the outside vs recirculation lever move freely? When you turn on A/C, does it blow cold air?

I had a similar experience while driving to my sister's for Thanksgiving. We had basically zero defrost outside of me wiping the windshield with a towel. I tried most of the tests in the paragraph above (except trying to check the cabin air filter while driving) and noted that when I turned the defrost off and turned A/C on and directed to the dash vents, I wasn't getting any cold air. Not great. Luckily the rain stopped and the sun came out, raising the outside temperature enough that whatever defrost I had was enough for safe driving. But, it seems like something is wrong with my A/C system and I'll need to figure that out before another long drive for Christmas.

 

Normally, if you want to get rid of the condensation, you don't want to use recirculate!

 

Thanks Drew! I am actually going to pull the cabin filter and change that today. Also a new air filter should be here on Friday. The AC and heat both work great. Fan speed, temperatures, and fan pressure all seem fine. It is just acting funny in defrost mode while driving. Mainly building condensation on the inside of the door windows. Once I'm at a stop light the windows clear back up again. Really strange. Thank you so much for the list you provided. If changing the filters doesn't fix it then I will start working through the list you provided. Thanks a bunch!

 

Thank you! It's going to be cold again tomorrow. So I will switch it over to outside air and test it on the way to work tomorrow morning.

 

Sorry guys. I forgot to post my response. Defrost is now working great! I changed the air filter and the cabin filter. I realized when I changed the cabin filter that the old one was in backwards, so I made sure to install the new one correctly. I also switched the defrost air from recirculate to outside air. I think that may have been the biggest culprit. All these years I never new you were supposed to have it to outside air for defrost, but I'm sure the cabin filter being in backwards wasn't helping either. Now all my windows are crystal clear when running defrost! Thank you all for your help. I really appreciate it.

 

When you use recirculate, the humidity stays in the car...hard to defrost when full of humidity!

I almost never use recirculate. Only time I use it is in the summer when trying to get the car cool with A/C and when super hot outside.

Glad it worked out!