FWIW, The ONLY time I use recirc air is when it is over 100 degrees outside. I had the dealer set my recirc mode to manual. And NO, I would not like to open the windows while it is raining, and get my passengers more wet, to try to accomplish what the A/C did in a few seconds.

It doesn't bother me if you choose to disable your A/C during the winter, and are aware of the drawbacks. I would just hate for someone else to follow this unsafe behavior and have an accident because of window fogging that could have been avoided if they used the system as it was designed.

 

What I have been attempting to point out is that sudden windshield fogging will sometimes (often..?) result from using the system as designed. Yes, just as you discovered, the use of the A/C will often result in a serious level of dehumidification of the incoming airflow and the cabin atmosphere.

But that is not always the case, mother nature gets to play her hand FIRST.

But again, that is not really the subject matter I'm trying to address.

The windshield fogging problem I'm talking about RESULTS from the use of the system as designed, sometimes almost immediately and sometimes a day later, or even a few days later.

 

I was responding to the statement you made earlier, and seemd to state it as fact:

"Windshield fog/mist..?? Turn the HEAT up to HEAT the windshield, ALWAYS works, ALWAYS."

Apparently we are talking about two different things. And I have no idea what you mean when you say this:

"The windshield fogging problem I'm talking about RESULTS from the use of the system as designed, sometimes almost immediately and sometimes a day later, or even a few days later."

Is there a source where you can show this to be happening to others? For some reason, I think you are just a little obsessed with this. But that's OK. I have some obsessions also, they just don't involve tampering with safety equipment. And yes, the A/C as a dehumidifier is a piece of safety equipment.

I think it is foolish, no, unsafe, to be advocating disableing one's A/C based on the information that you have provided.

 

If some way could be found to reliably and safely clear the leftover condensate from these newer more complex and dense evaporators shortly after their use I would be in full agreement.

But as it stands today we are left with the potential HAZARD of sudden windshield fogging or less hazardous but more annoying is the random occurance of the horrid mold and mildew, "dirty gym socks", odor.

And who, which of us, knows what hazards to human health might result from the varous anti-microbial chemicals or fungicides the manufacturers have in trial use, evaporator coatings, to try and combat the growth of the microbes whose "leavings" result in that horrd odor.

The EED at airsept.com could be of great help in combating the mold and mildew odor issue. But it would require modification, it only works above 65F, to combat the cool/cold climate windshield fogging episodes.

 

I still don't see any reference anywhere else regarding your "HAZARD of sudden windshield fogging". You seem to be the only source. With all due respect, I will consider this discussion a dead end.

 

With respect to Jeff: I value his posts and he often has the right info when others do not.

But on this issue Willard has a valid point to make.

The sudden windshield fogging with A/C in the defrost mode is a known problem and it can be truly dangerous.

It has happened to me a few times, once in the TCH ! And it's really scary if you're moving.

It happens when the cabin air is very humid, there is no heat available, and the windshield is colder than the air coming out of the A/C / defrost, possibly also before the A/C evaporator coil has fully cooled down.

It seems possible to me that it would almost never happen in Arizona.

That being said, I must add that overall, the A/C in the defrost mode has always been the most effective de-fogger for me - particularly when both A/C and heat are active at the same time. I have more experience with cases in which heated air alone would not clear the windshield quickly enough for safety, but the addition of A/C did the job almost immediately.

 

I guess no answer to my original question. Does anyone know if the A/C always comes on when you turn on front defroster?

 

Yes, it comes on as long as the ambient outside temperatre is somewhere above freezing....I seem to recall that below 35 degrees is the point where it won't engage.

PS. jpezz, sorry for all the off topic chatter!

 

Tests on my TCH this afternoon with the engine warm and the outside air temperature approximately 80 F gave the following results:

1. Selection of (front) defrost always cleared both the recirc and the ECO mode if they were set. (This means that neither of these prior settings would affect what happens when defrost is on.)

2. With the (front) defrost on, air was delivered at the base of the windshield at a temperature governed by the (thermostat) temperature setting(s): cold (refrigerated) air at low temp settings to hot air at higher settings.

3. With the (front) defrost on, the light on the A/C button would turn on or off when the button was pushed, at any and all temperature settings, but this had no apparent effect on the temperature of the air being delivered.

4. With the (front) defrost on, the low side A/C refrigerant lines were cold at all times (indicating that the A/C was refrigerating), regardless of whether or not A/C button was lit and regardless of what temperature was set (including "HI") and regardless of whether hot or cold air was being delivered.

5. If the "auto" button light is lit when (front) defrost is selected, the "auto" light will be turned off and the blower speed will be reset to a generally higher speed, and If "HI" or "LO" is then selected, max blower speed will be set. On the other hand, if the "auto" light is not lit when (front) defrost is selected, the blower speed will not be changed from the speed set prior to defrost selection. In either case, the blower speed can then be changed manually while the defrost is still active.

I do not know whether all TCH models operate the same way, but this is how my US-built, early production, 2009 car works.

Thanks to Jpezz for raising the question in the first place, and for complaining when he did not gfet a straight answer.

My guilty feeling for not having been more helpful caused me to explore the operation much more carefully than I probably ever would have otherwise, and what I learned will surely help me deal much more quickly, safely and effectively in the future with any unsatisfactory defroster performance !

 

Sorry, I have been leaving out an important point.

With the NipponDenso(***), denso US, climate control design once the cabin temperature has been raised to your setpoint, or closely nearby, the temperature of the airflow reaching the interior surface of the windshield in defrost/defog/demist mode can be as much as 20F below your cabin temperature setpoint.

Once the cabin temperature reaches the setpoint, or closely nearby, these NipponDenso designed systems rely EXCLUSIVELY on the A/C for preventing windshield condensation or even removing windshield condensation.

Even worse is the fact that absent driver intervention these systems will automatically switch into COOLING mode, airflow only from the dash face and upper body outlets, once the cabin temperature has risen to your comfort setpoint. Your may or may not be discomforted by cool and dry airflow to your face and upper body on the coldest night in the depth of winter but you will most certainly be discomforted by the fact that NO WARMING AIRFLOW is routed to the windshield in this mode.

There are now c-best options that the dealer can set such that the system will not automatically switch into cooling mode and other options that allow you to turn off the A/C indefinitely and even unlink the A/C from automatically being switched on in defrost/defog/demist mode.

The latest standard mod is that the system will automatically switch out of cooling mode, into footwell mode wherein the windshield is somewhat heated, when the A/C is automatically disabled due to falling OAT.

I have a DIY modification for early LS400 climate controls that automatically turn up the HEAT, radically so, the instant the driver activates defrost/defog/demist mode, full or combined floor/windshield.

*** Used throughout the Toyota/Lexus/Scion product line and most other asian automotive manufacturers.