2nd Gen Non-Engine/Transmission Why does the A/C come on with the defroster?

[cyborg]

Do the carmakers remind you of the goverment? they feel the need to think for you and determine what is best and leaving you out of the loop. (tow\haul)



Why do they feel that the A/C needs to be on every time you turn on the defroster. Most of the time you just need to blow air near or at the same temp as the outside to equalize the differential.



Vent over.

 

[RustyJC]

It comes on to dehumidify (lower the dew point) of the air passing through the defroster. If the air hit the windshield, and the windshield were at a lower temperature than the dew point of the air, you would immediately fog up the inside of the windshield.



Rusty

 

[CumminSense]

yes, on warm humid days, you want the AC on w/defrost, and I'm OK with it coming on when you turn the knob to full defrost.



But I agree with cyborg -- there should be a way to direct air to the windshield without the AC coming on. On a raw wet Eastern cool day (not cold -- say, 50 degrees F and raining/foggy), I don't want the damn AC on making me colder in the cab. I think the feet/defrost combo setting should NOT turn on the AC, but allow the driver to crank up the heat and defrost that way -- like on a really cold day when you jump in and your breath fogs up the windshield before you leave the driveway.



Come on, Dodge, this is still the USA, let the people decide -- give us a way to defrost without the AC.

 

[RustyJC]

... ... and the first time some Darwin award nominee turned on the defroster on a -10 degF day without running the A/C compressor and immediately frosted up the inside of his windshield resulting in a wreck, the lawyers would be after DaimlerChrysler like white on rice. So, they do their best to ensure that can't happen by forcing dehumidification whenever the defroster is on.



Rusty

 

[JSellers]

Most of the time you just need to blow air near or at the same temp as the outside to equalize the differential. QUOTE]



That mode will do the trick - sometimes. It all depends on temperature and humidity conditions, inside and out of the vehicle. In most situations where windows fog, it's because the vent air contains enough humidity so it will condense on the interior glass. By operating the AC compressor, this humidity is removed before vent air is blown into the vehicle. So - no condensation on the windows.



I had to explain this to my brother a few years ago. He and his wife had a brand new Honda. My first ride with them was on a rainy night. I had to ask him why he kept a towel in the car, and why he needed to periodically wipe the windshield with it. Honda HVAC controls work so that the AC compressor won't run with the fan blower off. So, you CAN get the windows fogged, even with controls turned to defrost position. Most Honda owners have to learn this, since the operation is different than most all other vehicles. In fact, a lot of vehicles don't have an 'off' setting for the fan.



Hope this helps,

John

 

[cyborg]

Wow, you mean all those years (24?) that the defroster in my 1969 Power Wagon got me through those high sierra winters with moronic trail rides (4x4, 4 chained) we were defying physics?

I've been saved and I didn't even know it.

But if that is the case, why does it not work any better?

And in some ways worse.



(yes, I rant and rave)

 

[RustyJC]

Yep, in the old non-A/C cars (like my 56 Ford and 65 Mustang), we had to run enough heat in the defrost mode to actually heat the windshield above the dew point of the air coming out of the defroster. When the cars were just warming up and not enough heat was available to do that, all kinds of interesting stuff could happen. :-laf



Rusty

 

[Dl5treez]

They also have the system set up that way to keep the A/C system seals from drying out so they don't spew all the oil & refrigerant out the first time it's kicked on in the summer after sitting for the entire winter.

 

[Dane]

Why do they feel that the A/C needs to be on every time you turn on the defroster

Because the AC evaporator will pull the moisture out of the air and defrost the inside of the glass much faster.

 

[cyborg]

Now some one correct me here. I must be confused. I thought the evaporator, evaporated the freon into a gas to produce the "cold" in the coils. The only moisture removal occurs when condesation drips off the coils.



I think this might be as I suspected. Sounds good in theory, but needing to crank up the heat on an already warm night does not lend its self to passenger comfort. Reality sets in.



Now we need a switch to control the A/C

 

[PC12Driver]

Yep, that's what happens. And that's what Dane is getting at, without the step by step. But in case you're still confuesed:



The ambient temperature air is cooled as it passes the evaporator. Relatively cool air cannot hold as much moisture as relatively warm air, so some of the moisture is condensed out, which shows up on the evaporator coils and then drips from the bottom of the truck. Then the air is passed over the heater coil where the temperature is brought back up to a comfortable level, with less moisture.



So then, instead of blowing moisture-saturated air across the windscreen (which is what fogged up the window in the first place), you now have relatively dry air that will work as a sponge to pull moisture away from the windscreen. Or, at the very least, it already has had the moisture wrung out of it so as it passes the cooler surface of the windscreen it won't condense and fog things up.



I don't get why you need to crank up the heat. Maybe there's something wrong with your temp control (or you're the type that either sets it at full heat or full cold), but I've found that the 9-10 o'clock position will blow air close to ambient temperature. Plenty of room to vary the temp cooler and warmer depending on what is needed.



Having the compressor run makes the system work better for defrost/defog, the increase in fuel burn is negligeable, the pump gets some use, and the output temp can be whatever you want it to be. Does the click-on click-off noise bother you?

 

[rrausch]

I changed mine so the AC doesn't come on automatically with the Defrost. Hee Hee Hee!! It's not too hard to do--take the knob & facia off and remove a little metal tab as I recall--there are probably several dozen Threads about how to do it in the 2nd Gen trucks.

 

[BigEasy]

Some people just can't understand science, and therefore will forever act like 1965 was better... kinda like those "stupid Infinity amps" that drive some of the best sounding stock stereo systems up to that point...

 

[cyborg]

Well without the renditions on the latent heat of vaporization and such, I guess that for those who are unable to recognize conditions that would favor it off or on, such a thing is fine. But for myself, I don't need or want that decision made for me.



"I don't need or want that decision made for me" is the underlying subject. Kind of like the Tow/Haul button. For those who don't understand why, have trouble manipulating more than one control at a time, or are unable track/monitor more than one system at a time. Just leave everything "ON", so you don't need to think about what your doing and can devote your driving time to something more constructive, like talking on the cell.

Mediocrity is never an option.



Off Topic: The Infinity system is the best Chrysler has done. And not bad (it fills in all four corners of the hole). Except there is no bass, no amps to power to the rear speakers, and the speakers that are used are cheesy at best. Once you upgrade the speakers, the rest of the infinity runs out of gas pretty fast.

So the Infinity is Mediocre, at best. see above.



And by the way, my Iron Butterfly four-track "Inn-Godda-Davita" still plays fine.

 

[PC12Driver]

"I don't need or want that decision made for me" is the underlying subject. Kind of like the Tow/Haul button.

Or auto trannys...

:-laf

 

[Hohn]

One other aspect to the A/C operating is that it helps system reliability.


It's ironic, but A/C systems are often more reliable when used more often. If they arenn't used much, the seals don't get the oil they need and overall system lubricity is less.

jh

 

[Dl5treez]

Is there an echo?

 

[PC12Driver]

Do you mean in this thread or just this topic in general?

 

[Dl5treez]

Either/Or, take your pick.