I guess I'll throw it back at you. Maybe I'm not understanding. Here's the facts as I see it.

1. The freezing temperature we're referring to is for water, at +32 deg F. Not 0 degF, not -32degF.
2. He's in NY and it's cold. +5 degF is well below freezing (+32 degF).
__Note this only applies to vehicles with automatic A/C with automatic temp-sensing a/c disengagers)
3. It's probably not EXTERNAL moisture (although there are such things as cold and humid days).
4. It is *probably* INTERNAL sources (shower, melted snow on the floormat, etc) that a properly operating compressor would remove.

I said "different model cars handle it differently" and "Your Mileage May Vary." As you said, I like the "most." That meas we also agree that all vehicles with automatic compressors (no blue "a/c" button) do NOT handle it the same. Which contridicts this next quote: 1. Not on a vehicle where he can turn the compressor on and off (a/c button). As far as I can tell with the Amigo (The Rodeo is in AZ), when the A/C (compressor) button is pushed, it just keeps cycling. Even in the cold.

2. Not on those automatic compressor vehicles that that (by your own admission) do not have the ambient temp sensor.

3. And of course, not if the outdoor ambient temp sensor isn't working.

But none of this is even a troubleshooting option here becuase:

This is a Rodeo, and my 93 and 04 both have the blue "A/C" compressor button. The years bracket his model, and I'm assuming it's the same. It should ALWAYS come on when he turns it on, and ALWAYS turn off when he turns it off. Even in the cold outside, with the internal temp hot, the A/C exhanger should dry the air running over it. If he's fogging, A/C should help if it's on recirculated air. It *might* help dry the air a little if he's drawing in outside air that is warmed some coming through the firewall.

Since you're a Texas boy, we'll forgive your ignorance of bitter cold temps...
Born and riased in Chicago, left at 19 for Plattsburg, NY, then Pease, NH for a few years. I currently have residences in Phoenix, Dallas/Fort Worth, and a new home in Richmond, Virginia. The Ford is in Virginia. Two days ago: +16 degF. Yesterday: +20 degF. Today: +12 degF. Freezing temp of water: +32 degF.

The Ford compressor came on every time. Period. (Although, as said earlier, I have my doubts as to them saying "That's the way it's supposed to be.")

If I have the mechanics wrong, correct me. I'm trying to relate this to his Rodeo, and not how all vehicles operate differently. I've still got my money on the perfume test on the air source selector. Fresh air should clear the windows, just not as quickly.

I'm not saying you are wrong, I'm just saying it doesn't matter what the AC does.

When the dewpoint is around, say, 15F, as it probably was with a temp of 5F, the AC system can't get cold enough to produce fog without freezing up entirely. Now if the heater core is leaking, there could conceivably be enough moisture picked up to produce a fog. BUT, the defrost/fog scenario was presented as an alternative explanation to a heater core leak, not as a symptom of it. My point was simply that at that temp/relative humidity and without an internal source of moisture, the AC/defrost system could not produce fog.

My apologies to the board for sidetracking this topic with such minutiae...as a consultant for indoor air quality, I work with humidity issues quite a bit. <img src="/forums/images/graemlins/ignore.gif" alt="" />

NOW i get it. Since he was driving it a little, I was assuming it had warmed up enough to evaporate any internal moisture (frozen floor mats), and that the A/C condensor would be able to remove it.

If I implied the A/C produced fog at any point, that was in error.