chart928s4

This might be a quick one word answer.

Background: On a ~50F rainy day I was driving my 91 S4 auto at normal operating engine temp. The windshield got fogged. I turned on the defrost which worked quickly. However, I could not leave the defroster on because the cabin got so warm. So I turned it off and of course the windshield quickly got fogged again.

The owners manual says the defrost setting will ignore the temperature, vent, and fan settings, max out the fan and heater temperature, direct all air flow to the windshield and door vents, and this little opaque tidbit with zero detail: "At temperatures above 32°F/0° C the air conditioner compressor switches on automatically."

Does that mean defroster air temperatures should be moderated by the A/C when the outside temp is warm, i.e. I have a malfunction somewhere?

Thanks to all.

fiatrn

In just means the compressor will run, which will act to dehumidify the air passing over the evaporator to help de-mist more efficiently.

The heater will overwhelm the a/c, though, and you'll still get warm/hot air.

chart928s4

Thanks. Warm air is one thing but a really hot cabin on a moderately cool humid day was tough.

A closer reading of the book shows the also-opaque behavior of the "v" setting will blow through the foot AND windshield vents, so I will use that next time. Obviously, I should RTFM.

Christopher Zach

Oddly enough running rhe rear ac does an excellent job of defogging as well.

Majestic Moose

I was caught in a downpour a couple years ago and this was the behavior of the defrost. For a luxury car it was an awful experience as my choices were to either crack the windows open and ruin the interior or suffer the blast of 200deg vent temps. Of course I chose to suffer.

Christopher Zach

Wait, why not just close the middle vent with the lever? Side vents can be closed as well, but to be honest if it's just fogged run the AC normally to quickly dehumidify the interior.And since it's not too hot out this is the time when the front and rear AC systems will both run at full defogging ability.

I've been running the car all winter/spring with a rear hatch leak (bad seal, replaced) and a damp-ish interior after hard rains. The AC system can quickly dry out the car in a jiffy.

chart928s4

Hot humid misery loves company, so thanks for weighing in Alex. My method was to alternate blast furnace/windows down/repeat.

I have a field report. The v setting does NOT push air through the windshield vents but does push through the console (and footwell). The ^ setting pushes through the console vents and a small amount through the windshield vents. I didn't test closing the console vent but that will probably push more air to the windshield.

Some combination of A/C (thanks Chris) and the ^ setting will probably work. But: Why is it so hard?

Alan

You could check that in DEF mode you have AC running - check the temp differential on the expansion valve, it should be obvious. Once the windshield is initially fully cleared - using just the AC with high fan should keep it clear (e.g. max fan and whatever temp you select for comfort with AC control loop active in ^ mode with closed console vent). The rear AC on full will help dehumidify the cabin too

The point of the DEF mode is really DEFogging not so much DEFrosting though it will work for that - just more slowly). If you have condensation inside the windshield you need warm dry air at a high rate blowing on it (= heat + AC for dehumidifying). But if you have frost on the outside of the windshield you will usually will want the hottest air available at high volume on the inside to melt the external frost - fogging on the inside will likely not be a major issue in that case (heated air can hold more water vapor). You will defrost faster with max heat & max fan airflow than with DEF mode. For this you need the ^ mode and close the central vent (close the door vents too for max windshield effect - but you'll probably want to defrost the side windows too...).

Alan

Majestic Moose

My '83 HVAC may have different settings then your car. I'm sure I closed the vents or aimed them away from me but in AUTO mode (AC on) there is no airflow directed at the windshield. The only effective mode was defrost.