defogging problems in winter

[foggingup2003]

we have a 2003 nissan quest, when two adults and four children get in the car the interior foggs up and it is difficult to clear up. Van has been to dealer already, they found no problems, used ac with heat, read manual with their suggestion, still didn't help. this problems only happens in the cold weather. any help or suggestions would be greatly appreciated. Anybody incounter this problem also?

[johnebgoode]

Hi, I've heard of your problem before and i discussed this with a mechanic that was quite knowledgable about the topic. It seems that this is a sign that your a/c is not functioning up to par. There has to be enough cold air mixed with the hot air to stop the fogging. It evens out the temperature of your windshield. The dealer just doesn't want to admit the problem because they are going to have to replace your a/c! Either that or he is not educated on the matter. Check it out with a/c expert.
GOOD LUCK
JOHN

[foggingup2003]

Originally Posted by johnebgoode

Hi, I've heard of your problem before and i discussed this with a mechanic that was quite knowledgable about the topic. It seems that this is a sign that your a/c is not functioning up to par. There has to be enough cold air mixed with the hot air to stop the fogging. It evens out the temperature of your windshield. The dealer just doesn't want to admit the problem because they are going to have to replace your a/c! Either that or he is not educated on the matter. Check it out with a/c expert.
GOOD LUCK
JOHN

thank you for your response. Is there a defininate way to determine if the ac is working properly? It does put out cold air when it is on. I do have a temperature gauge to check the temperature when it is on.

[johnebgoode]

Originally Posted by foggingup2003

thank you for your response. Is there a defininate way to determine if the ac is working properly? It does put out cold air when it is on. I do have a temperature gauge to check the temperature when it is on.

As I said you will have to have an expert have a look at it. I'm just giving you a lead on the problem. I do know that you are not an isolated incident. It seems to be a problem in the newer vehicles.
I don't know if it's so much the temperature that your a/c is putting out, I think it has more to do with the mixing of the cold and hot air, and from what I was told it is usually the fault of the a/c.
Hey....finding a good mechanic is impossible!!
JOHN

[BMW_4.4i]

All you may have to do is add some "Rain-X" brand "Fog-X" on the windows. This stuff does wonders for fog, and costs next to nothing. Hope I could help you.

[wwest]

Originally Posted by foggingup2003

we have a 2003 nissan quest, when two adults and four children get in the car the interior foggs up and it is difficult to clear up. Van has been to dealer already, they found no problems, used ac with heat, read manual with their suggestion, still didn't help. this problems only happens in the cold weather. any help or suggestions would be greatly appreciated. Anybody incounter this problem also?

Given that your vehicle is of Asian/Japanese origin the climate control system is most likely of NipponDenso, Denso USA, design. With the change-over to the new less efficient non-freon refrigerant several design changes were deemed necessary.

First, the A/C cooling evaporator, heat exchanger, was made a lot more dense and complex. The more chilled surface area for the air to flow over, the higher will be the efficiency of the heat transfer.

The second issue is more directly pertinent to your problem. The slower the air flows over the evaporator's cooling surfaces the more it will be cooled and the more it is cooled raises the likihood that it will be simultaneously DEHUMIDIFIED.

Another benefit of the lowering of the incoming airflow rate is better MPG, something I'm sure the automotive manufaturers haven't over-looked. Almost all modern day passenger vehicles are very well sealed against atmospheric outflow for these reasons.

The way NipponDenso has chosed to keep the evaporator airflow as low as possible is by restricting the cabin atmosphere OUTFLOW. The result of that is that during the cooler months it will be really easy for the passenger cabin to become extremely humid, so humid that the windows and windshield interior surfaces, cooled continously by exposure to COLD outside atmosphere, begin to fog over.

My suggestions...

If it is possible to completely disable the A/C during the winter months then do so. My 01 RX300 has C-best options which allowed the dealer to "unlink" the automatic operation of the A/C in both "normal" and defrost/defog/demist modes. ON a previous 92 LS400 I fooled the system into thinking the OAT was below 32F by adding a resistor in series with the OAT sensor. I have also, on occassion, simply disconnected the A/C compressor clutch electrical circuit. My 01 Porsche has a switch added within the glove box that does that.

If you park the vehicle inside it will help to leave a window or windows open so the cabin can more completely dry out. During A/C operation the ~10,000 square inches (92 LS) of evaporator surface area will become saturated, clogged, with mositure and while much of that will drain away via the provided drain a lot will remain and then evaporate into the A/C plenum and thus into the passenger cabin at the first start of the system blower.

My most common occurance of intermittent and spontaneous windshield fogging was about 5 miles into my morning trip to the office during the cooler months. That instance was cured, mostly, by leaving the windows slightly open during the night in the garage.

Another instance is that the A/C compressor will oftentimes shut down spontaneously, without any indication to you, if the OAT declines to near or below freezing. A few minutes after that happens the condensed moisture remaining on the evaporator will begin to evaporate into the airflow entering the passenger cabin.

Be VERY CAREFULL, this can be a very HAZARDOUS situation.

If you are traveling in a colder climate and begin to see wisps of condensation on the interior surface of the windshield DO NOT ACTIVATE the defrost/demist/defog function before you turn the system heat all the way to MAX!

Those wisps of condensation might be indicative of the A/C compressor having previously shut down due to low OAT and now if you activate the defrost/defog/demist function the COLD windshield will become suddenly opaque from condensation due to the moisture bomb building up with the A/C plenum.

The best procedure to use if you sense that the humidity is to high within the passenger cabin is to lower the rear windows slightly and quickly purge the humid atmosphere from the cabin.

While I remain a strong advocate of the use of A/C to dehumdify the incoming cabin airflow and thereby help to remove windshield condensation and prevent it's formation, I don't understand why, other than the slight discomfort one might feel, the defrost/defog/demist airflow isn't also HEATED.

Wwith Outside Air Temperatures, OAT's, below about 55F it is impossible to predict whether or not the the A/C can be efficient enough to be of any help. The A/C's dehumidifcation efficiency is a function of the relative humdity of the incoming airflow. The lower the temperature of the incoming airflow the lower is the probability of efficient dehumdification by the A/C until at about 35F it is practically non-existent.