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Frosty the Windows

dave_the_nerd

dave_the_nerd

Lifer
So, I get in the car in the morning and there's some frost on the outside of the windows. Normal for Minnesota.

I turn on the defroster, and it works, and the frost goes away, and it's clear for a while. But my morning commute is about 40 minutes long. By 20 minutes in, I'm starting to get frost on the INSIDES of the windows instead. The defroster is running, but the air temp isn't super-warm, (I swear I remember it being, like, actually hot, and I didn't have this problem in previous years - I've had the car since 2010, so it and I have seen some shit together).

Unless I crank the fan speed up to 4/4, I get completely frosted-over side windows, and the front windshield is pretty bad too. (With it up full blast, I get enough clear area on the side windows that I can see my rear-view mirrors, but checking blind-spots is still a no-go.) Rear defroster (heating element in window) works fine.

Looking around, I don't see any other cars with this problem, so it's not climate, air, humidity, etc.

Is it possible I just am a super-humid Atlantean? Or is there maybe something wrong with my vent/heat/defroster system? (Coolant level is ok, engine temp normal, and there's no sweet coolant smell, so I don't think I have a cracked heater core...)

Any thoughts would be appreciated. I'm probably going to pick up a dehumidifier tomorrow and then call my mechanic. This is getting kinda crappy.
 
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Your AC should be running when in defrost mode and the air should be hot. You may have a blend door issue with your Heating / AC System preventing the air from getting heated properly.
 
LTC8K6 said:
Is the air hot?
Click to expand...

Warm, not hot.

Is the A/C compressor running when you are in defrost mode?
Click to expand...

The A/C light is off. If I press the a/c light, it turns on. (I know that recirculating the air will cause exactly the problem I'm seeing.)

I've never had to do that before, but I'll give it a whirl.

pcgeek11 said:
You may have a blend door issue with your Heating / AC System preventing the air from getting heated properly.
Click to expand...

Blend door? I'll google that tomorrow. 2:40AM. Brain... fried. Thanks.
 
Drove to work with a/c compressor on this morning. It was quite a bit better, although not perfect, so thank you for that recommendation.

But the air temp is still a lot less warm than I remember, and nothing short of full-blast will actually defrost the side windows. :-(

If it were summer, I might take the dash apart, clean out the fan, check the heater core... but I want this done quick. Hi ho, hi ho, it's off to the mechanic we goooo...
 
Just a heads up, you do not have to turn your A/C on when in defrost mode as it turns on by itself...you should hear the periodic "clicking" of the A/C clutch engaging/disengaging.

I would imagine your A/C compressor is shot.

The reason you are fogging up is because of the humidity in the car, the A/C compressor kicks in to remove the humidity when in defrost mode.

Or as mentioned above, your blend door might be stuck in re-circulation mode making it difficult to remove all the humidity.

What you can do to help is crack your window open a bit while driving until you can get the issue sorted out.
 
Crack your window when this happens, it'll give the moisture a way to escape. Your windows will clear up within a few minutes.

Turning off recirculate helps too.
 
DaTT said:
Just a heads up, you do not have to turn your A/C on when in defrost mode as it turns on by itself...you should hear the periodic "clicking" of the A/C clutch engaging/disengaging.

I would imagine your A/C compressor is shot.

The reason you are fogging up is because of the humidity in the car, the A/C compressor kicks in to remove the humidity when in defrost mode.

Or as mentioned above, your blend door might be stuck in re-circulation mode making it difficult to remove all the humidity.

What you can do to help is crack your window open a bit while driving until you can get the issue sorted out.
Click to expand...

I would think that varies by car though. I've had cars that do exactly what you're describing (turn on A/C automatically in defrost mode) but it actually indicates this is happening by showing the AC is also on. On other cars, I didn't see the A/C indicator but could toggle it on/off myself. In that case I don't think it's automatic.
 
Had this happen to a friend. Turned out she had a pinhole in her heater core throwing a very small stream of coolant into the airstream.

Low coolant would be another sign of a leak.
 
Billb2 said:
ChecK the coolant level in the engine. Low coolant = no heat.
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Was going to say this. Had a car years ago that must have had a small head gasket leak. The coolant reservoir was full so I didn't suspect low coolant. Finally, I took off the radiator cap and was amazed how low the coolant level was. What I was experiencing was huge swings in temp. The reservoir was full because the hose from the radiator to the reservoir was all twisted up where it ran hidden behind the battery.

NutBucket said:
Is your engine reaching normal operating temp? Maybe the thermostat is stuck open?
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This could absolutely be the problem and an easy one to diagnose if the car has a gauge.
 
gorcorps said:
I would think that varies by car though. I've had cars that do exactly what you're describing (turn on A/C automatically in defrost mode) but it actually indicates this is happening by showing the AC is also on. On other cars, I didn't see the A/C indicator but could toggle it on/off myself. In that case I don't think it's automatic.
Click to expand...

It is automatic. The only way to properly defog is to remove humidity. Any car that has A/C will turn on automatically with defrost. My car for instance does not indicate with the A/C light that the compressor is running, but the sound of it engaging and disengaging are proof it is running.
 
My old beater Camry will turn on the compressor if the dial is set to defrost, but not if it is set to def and floor. Have to hit the button then. I have owned several cars in the past that did not automatically engage A/C for defrost at all. Most of the fleet pickups I have driven for the last twenty years where I work, you have to hit the button. It is odd, as I have trouble imagining when you would want to warm the windshield but not dry it.
 
On many Hondas, there's a secret button combination on the climate control that can change the behavior of this.
 
On a lot of cars, when they start getting old, either the Blend Door (which determines heater temp) does not move all the way or maybe an air direction door has not moved all the way. Also possible is a slightly plugged heater core. That can be fixed with a full cooling system flush and a new thermostat and coolant.
 
DaTT said:
It is automatic. The only way to properly defog is to remove humidity. Any car that has A/C will turn on automatically with defrost. My car for instance does not indicate with the A/C light that the compressor is running, but the sound of it engaging and disengaging are proof it is running.
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You are correct, but a lot of vehicles with thermometers disable the A/C compressor when the outside temperature is low enough the evaporator core will freeze. My car disables the compressor on defrost under 38F.

With the forecast of 16F in Minneapolis, the OP's car may have the compressor disabled as well.
 
XabanakFanatik said:
You are correct, but a lot of vehicles with thermometers disable the A/C compressor when the outside temperature is low enough the evaporator core will freeze. My car disables the compressor on defrost under 38F.

With the forecast of 16F in Minneapolis, the OP's car may have the compressor disabled as well.
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All vehicle AC Systems monitor the evaporator core and turn off the compressor at a set temperature on defrost mode or AC mode.
 
A functioning A/C system is the only thing that will remove moisture from the air in the cabin. Defrost mode should engage the A/C as mentioned, but once in a while this part of the control can fail, and manually engaging the A/C can be helpful. Ensuring the system is charged and properly functioning is essential as well, though it may seem counterintuitive to do so in the dead of winter. Another thing that can cause excessive fogging is water leaking into the interior, coolant leaking into the interior via the heater core, and a clogged or frozen shut evaporator drain.

There may be some confusion about A/C cutoff temp. In defrost mode, the heater core will be blowing hot air through the system, so a properly functioning system will still cycle the compressor and allow water vapor to condense on the evaporator.
 
crashtech said:
A functioning A/C system is the only thing that will remove moisture from the air in the cabin. Defrost mode should engage the A/C as mentioned, but once in a while this part of the control can fail, and manually engaging the A/C can be helpful. Ensuring the system is charged and properly functioning is essential as well, though it may seem counterintuitive to do so in the dead of winter. Another thing that can cause excessive fogging is water leaking into the interior, coolant leaking into the interior via the heater core, and a clogged or frozen shut evaporator drain.

There may be some confusion about A/C cutoff temp. In defrost mode, the heater core will be blowing hot air through the system, so a properly functioning system will still cycle the compressor and allow water vapor to condense on the evaporator.
Click to expand...

From my vehicle's service manual,

The following conditions must be met in order to activate the A/C compressor:

• Battery voltage is between 9-18 V
• Engine coolant temperature is less than 124°C (255°F)
• Engine speed is greater than 600 RPM
• Engine speed is less than 5 500 RPM
• A/C high side pressure is between 269-2 929 kPa (39-425 PSI)
• Throttle position is less than 100%
• Evaporator temperature is greater than 3°C (38°F)
• ECM does not detect immoderate torque load
• ECM does not detect insufficient idle quality
• The ambient temperature is above 1°C (34°F)
Click to expand...

This seems to indicate the compressor will never operate if ambient temperature is below 34F.

OP's weather is 16F, if I was driving there I wouldn't get any compressor assisted defrosting.
 
Nope.

Don't confuse ambient temps with evaporator temps, there is a heater core in there also.
 
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XabanakFanatik said:
From my vehicle's service manual,



This seems to indicate the compressor will never operate if ambient temperature is below 34F.

OP's weather is 16F, if I was driving there I wouldn't get any compressor assisted defrosting.
Click to expand...
No compressor generally equals ice on the inside of your windows, from your own breath, when it's below freezing.

You pump out an amazing amount of water vapor all by yourself.

I have always found defrosting to be very poor in cars with marginal or no a/c.
 
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