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Discussion Starter · #1 ·
Hi,
I owe a Passat 1997 TDi with climatronic. For some reason as soon as temperature drops below 3 degrees Celsius the climatronic stops dehumidifying and all windows become so foggy that becomes really hard to drive in those conditions. Anyone has an idea what the problem may be? Is there a way to reprogram the climatronic not to turn off below 3 degrees Celsius?

Thank you
 

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I think most cars turn off their compressor as you approach 0C / 32F It's to protect the compressor and keep it from pumping liquid refrigerant.
 

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X2 but if your windows always fog up when the compressor cuts off, why do you have so much humidity in the car at 0*C? Coolant leak? Water leak? Plugged evaporator drain?
 

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Turn on the HEAT. You can't get fog when it's hot, because the air absorbs all the water.

I never use the A/C to defog my windows. Always the heat.
 

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Turn on the HEAT. You can't get fog when it's hot, because the air absorbs all the water.

I never use the A/C to defog my windows. Always the heat.
The OP has climatronic. If you have climatronic and you push the windshield defroster button on the climatronic, the a/c will stay on until the outside ambient temp is 3*C / 37*F or less. So if the ambient interior temp is less than the set point temp on the climatronic, the air will also be heated and moisture will be pulled from the air by the evaporator core.
 

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Turn on the HEAT. You can't get fog when it's hot, because the air absorbs all the water.

I never use the A/C to defog my windows. Always the heat.
you do know you can use the heater AND the a/c at the same time, right? using the a/c will defog the windows faster because it takes the moisture out of the air.
 

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you do know you can use the heater AND the a/c at the same time, right? using the a/c will defog the windows faster because it takes the moisture out of the air.
Yes, I understand. Running the heat and the A/C at the same time literally turns the climate-control system into a dehumidifier. And...well, I should say that I HAVE used it that way before, but it must be REALLY wet inside my car before simply heating the air doesn't dehumidify it enough to defog the windows. (heating the air is like unclenching your fist around a damp sponge -- it might have been dripping wet before, but once expanded it feels nice and dry.) Besides, if it's that cold out, I'd be using the heat anyway.
 

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The OP has climatronic. If you have climatronic and you push the windshield defroster button on the climatronic, the a/c will stay on until the outside ambient temp is 3*C / 37*F or less. So if the ambient interior temp is less than the set point temp on the climatronic, the air will also be heated and moisture will be pulled from the air by the evaporator core.
I don't follow. I understand the minimum operating temp for the A/C, but I don't understand the second part. I think you're saying the Climatronic is responsible for the A/C having a minimum operating temp, but ALL A/C systems have a minimum operating temp.
 

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Yes, I understand. Running the heat and the A/C at the same time literally turns the climate-control system into a dehumidifier. And...well, I should say that I HAVE used it that way before, but it must be REALLY wet inside my car before simply heating the air doesn't dehumidify it enough to defog the windows. (heating the air is like unclenching your fist around a damp sponge -- it might have been dripping wet before, but once expanded it feels nice and dry.) Besides, if it's that cold out, I'd be using the heat anyway.
true, but you can clear a fogged windshield with COLD air so you don't have to wait for the car to heat up before you can drive safely. did you know some cars (mostly domestics AFAIK) will turn the a/c on automatically when you turn on the defroster? i even go a step further and turn the recirc on as well.
 

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true, but you can clear a fogged windshield with COLD air so you don't have to wait for the car to heat up before you can drive safely. did you know some cars (mostly domestics AFAIK) will turn the a/c on automatically when you turn on the defroster? i even go a step further and turn the recirc on as well.
The A/C won't dehumidify the air by itself. It will drain water out of the air, yes, but only after the cooled air reaches 100% humidity. All I can think is that, in your scenario, non-air-conditioned cold air would work just as well, unless there's something in your cabin air system that's heating the air-conditioned air just a little bit so it can soak up the fog.
 

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umm..taking the water out of he air is the very definition OF dehumidfying.
From Merriam-Webster:
Main Entry: de·hu·mid·i·fy
Pronunciation: "dE-hyü-'mi-d&-"fI, "dE-yü-
Function: transitive verb
: to remove moisture from (as air)
 

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Discussion Starter · #12 ·
X2 but if your windows always fog up when the compressor cuts off, why do you have so much humidity in the car at 0*C? Coolant leak? Water leak? Plugged evaporator drain?

Thank you. This may be the reason why at 0*C I'm having foggy windows. I will check on coolant or water leak. Does someone know any post with pictures how to check the coolant leak?
Also, how do I check the plugged evaporator drain?

Thank you.
 

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Usually if you have a coolant leak in the heater core, you will have an oily residue on your windows, may have a sweet smell (antifreeze smell), and you coolant level will drop. Check coolant level, if low add G12 antifreeze mixture and monitor coolant level. Hopefully, the heater core is okay because its a labor intensive process to change it.

The evaporator drain is on the right/lower side of the HVAC housing under the dash. The drain is funnel shaped plastic that drains out through the firewall. Some members have removed the glove box and were able to reach in and remove the funnel (I believe there are a few bolts/screws holding it in place) and clean it. The other way to check is from underneath the car. You have to remove the exhaust heat shield in the firewall area to access the drain. Take a coat hanger or other stiff wire and use to clear drain if clogged.

If you haven't checked already, make sure your two drains in the plenum area are clear - on under the battery and one between the brake booster and battey. Also check your sunroof drains. Open your passenger and driver's doors and the check the drains. The drains exit on the front pillars. IIRC, the rear drains exit behind the rear wheel wells.
 

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umm..taking the water out of he air is the very definition OF dehumidfying.
From Merriam-Webster:
Main Entry: de·hu·mid·i·fy
Pronunciation: "dE-hyü-'mi-d&-"fI, "dE-yü-
Function: transitive verb
: to remove moisture from (as air)
Dehumidifying the air means reducing the humidity of the air. Humidity is relative to temperature. While blowing humid air across a cold surface (like A/C coils) will cause water to condense out of the air, it only does so because the air is cooled to a point that the existing moisture exceeds the air's capacity to hold it all, and the excess drops out.

What does that mean about the remaining moisture in the air? It means that the remaining moisture is the most the cooled air can hold -- or rather, the cooled air is now at 100% humidity. Yes, it's holding less water than it used to, but it's also CAPABLE of holding less as well. So the humidity of air-conditioned air is actually HIGHER than before, at least until something else heats the air up again.

I'm sure I'll get flamed for this, but Webster's is wrong on this one, or at the very least, the definition you found was pathetically lacking in detail.
 

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i guess it's a good thing i got the definition from Merriam-Webster and not Webster.

your "theory" is also not valid because you're still stuck on the air getting colder. you can use your a/c and heat the air up while dehumidfying and that would negate your whole stance.
 

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i guess it's a good thing i got the definition from Merriam-Webster and not Webster.

your "theory" is also not valid because you're still stuck on the air getting colder. you can use your a/c and heat the air up while dehumidfying and that would negate your whole stance.
I was responding to this statement, which, if you're talking about using the A/C by itself to clear the window, is wrong:

true, but you can clear a fogged windshield with COLD air so you don't have to wait for the car to heat up....
And for the last time, YES, running the air through the A/C and THEN through the heater WILL dehumidify it. Running the air ONLY through the A/C will NOT dehumidify it.
 

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I have seen many people with foggy windows when it gets cold. Most of the time, the problem seems to be that they have Recirc selected. Essentially, it pulls the air for the HVAC system from inside the car, instead of outside the car. On the Climatronic system, it's the little half-circle arrow. If you have AUTO selected, this shouldn't be an issue, the system will do it automatically. If this is not the case, get your car scanned by a fully functional VAG-COM or something similar - could be a Climatronic issue - stuck in Recric mode.
 

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Discussion Starter · #18 ·
Usually if you have a coolant leak in the heater core, you will have an oily residue on your windows, may have a sweet smell (antifreeze smell), and you coolant level will drop. Check coolant level, if low add G12 antifreeze mixture and monitor coolant level. Hopefully, the heater core is okay because its a labor intensive process to change it.

The evaporator drain is on the right/lower side of the HVAC housing under the dash. The drain is funnel shaped plastic that drains out through the firewall. Some members have removed the glove box and were able to reach in and remove the funnel (I believe there are a few bolts/screws holding it in place) and clean it. The other way to check is from underneath the car. You have to remove the exhaust heat shield in the firewall area to access the drain. Take a coat hanger or other stiff wire and use to clear drain if clogged.

If you haven't checked already, make sure your two drains in the plenum area are clear - on under the battery and one between the brake booster and battey. Also check your sunroof drains. Open your passenger and driver's doors and the check the drains. The drains exit on the front pillars. IIRC, the rear drains exit behind the rear wheel wells.

Thanks a lot for your detailed explanation. I will check on these.
 

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All you need is to get the glass temperature warm enough to exceed the dew point of the air blowing across it. Dew point of air does not vary with temperature like relative humidity does. :thumbup:

Warm the glass above the dew point of the interior air...no moisture condensation. A/C compressor does not need to be running to accomplish this, btw.

For an ice cold car, I've found it's best to leave the HVAC fan off until warm coolant has reached the heater core.
 

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All you need is to get the glass temperature warm enough to exceed the dew point of the air blowing across it. Dew point of air does not vary with temperature like relative humidity does. :thumbup:

Warm the glass above the dew point of the interior air...no moisture condensation. A/C compressor does not need to be running to accomplish this, btw.

For an ice cold car, I've found it's best to leave the HVAC fan off until warm coolant has reached the heater core.
The dew point of air is the inverse of the humidity.

That is to say, the dew point is the temperature at which the amount of water contained by the air will exceed the air's ability to hold that water indefinitely.

If you dissolve water into air at x parts-per-million, the dew point (or frost point, if below 0C) will be a certain temperature. If you add more water, the dew point will rise. If you remove water, the dew point will drop.

So what you need for foggy windows is air that is almost completely saturated with water PLUS windows that are just a little bit cooler than the air, so it can serve as a condensor.

Running the A/C without the heater will remove water from the air, and will cool the air below the temperature of the glass, BUT the air will also be even more humid than before and will be unable to absorb the fog on the inside of the glass. FURTHERMORE, the colder air will chill the glass, possibly bringing it below the dew point of the OUTSIDE air as well. Then you'll have a fine mess, with fog on BOTH sides of the glass.

So I reiterate, using the A/C without the heat cannot possibly work as a defogger for more than a few seconds, while the air ducting is still warm enough to re-heat the chilled air ever-so-slightly. After that it makes matters worse, or at best the chilled air accomplishes the same effect that non-chilled air would.

Using the A/C with the heat is the best approach, because it dehumidifies the air. Using the heat alone is almost as good, because it expands the air so it can hold more water.
 
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