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Need help settling a discussion about vehicle defrost using a/c?


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02-11-04, 05:17 AM   #1  
Need help settling a discussion about vehicle defrost using a/c?

I'm curious to know if all vehicles with a/c, cycle the compressor when in defrost mode or just a select few?
Do any vehicles use a temp control to cut out the compressor in sub-zero weather.
My wifes "88 Dynasty and friends '94 Olds rattle away at 20 below F but both still have original compressor.


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02-11-04, 05:25 AM   #2  
All of the fairly current vehicles I've had do cycle the A/C in defrost mode. My 01 Dakota, as far as I know, is the only vehicle I've had that will not use it if the temp is below a certain level.

 
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02-11-04, 05:30 AM   #3  
Greg...

...almost all A/C equipped vehicles operate the compressor automatically when in defrost mode...a few are selectable.

as for the compressor running at very low temps, it would depend on the type of system...CCOT systems wouldn't because compressor cycling is dependent on low side operating pressure and as you probably know, R12 and R134a wouldn't produce enough pressure at -20F to close the cycling switch. Also...many systems have low pressure cut-out switches to prevent operation at low charge levels to protect against compressor damage. i didn't look at the specific operating parameters of the vehicles you mentioned....but it does seem odd to me to operate the compressor at that low temp. i'll be checking back on this one as i'm interested in the opinions of others.

 
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02-11-04, 10:59 AM   #4  
I think the reason that the AC's are designed this way is to decrease the chance of compressor seal failure due to non-use during the cooler months.

 
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02-11-04, 12:20 PM   #5  
Originally posted by dirty dan
I think the reason that the AC's are designed this way is to decrease the chance of compressor seal failure due to non-use during the cooler months.
Not at all.
Air conditioning removes moisture from the air. That's the reason to use it on defrost........dryer air..........cleaner windsheild

 
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02-11-04, 04:19 PM   #6  
this has always been a serious pet peeve of mine.

When it is cold outside and your windows are fogged, cold a/c air is the last thing in hell I am going to ever use.

I in fact used to laugh at my friends for using a/c to defrost the windows,...but I guess technically that is what they are supposed to do. I use the heater to defrost my windows, works a lot better 99% of the time, and feels a lot better. Just my opinion.

So, you can imagine my fusteration when I bought my 98 tacoma, and I put the defroster on, and it automatically cycled the a/c. This pisses me off to no end. Is there any way to kill this feature??

my 86 mustang doesn't cycle the a/c on defrost

 
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02-11-04, 04:51 PM   #7  
I can see the benefit in using the ac to dry the air when the temperature of the air will allow moisture to condense on the evaperator coil. On cool rainy days for instance.
But when the air is sub-zero, moisture certainly won't form at those temps.

Quite a few around here just disconnect the wire to the compressor clutch for the winter.


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02-11-04, 05:11 PM   #8  
Jeremy on most cars the a/c cycles whether you pick cold or hot temp defrost setting.The air goes over the evap first and gets dried to some extent.A/C will not operate under a certain temp because the pressure gets to low.In very low temps moisture on the glass will take longer to remove due to the a/c not operating.Some cars cycle the a/c while in heater or vent mode also.

 
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02-11-04, 05:46 PM   #9  
Originally posted by jthompson
this has always been a serious pet peeve of mine.

When it is cold outside and your windows are fogged, cold a/c air is the last thing in hell I am going to ever use.
Who said it was COLD air?

It delivers hot, dry air. Melting frost on the windsheild will produce moisture. How about real rainy days, haven't you ever had a windsheild or side windows fog up. It is the best possible defrosting and window clearing you can get. Why on earth would you want to disconnect it?

 
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02-11-04, 05:58 PM   #10  
Originally posted by Desi501


Who said it was COLD air?

It delivers hot, dry air. Melting frost on the windsheild will produce moisture. How about real rainy days, haven't you ever had a windsheild or side windows fog up. It is the best possible defrosting and window clearing you can get. Why on earth would you want to disconnect it?
I was wondering that myself, I fail to see the logic in disconnecting the A/C for any reason, it helps to clear the windshield no matter what the temp outside.

 
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02-11-04, 09:39 PM   #11  
Ok.. There is no need to have the compressor on cold days colder than 40*. When you heat up cold air, the air will be dry.

Think about this.. Why do you think homes on a cold winter months are soo dry??

If the compressor was running in that cold of temp the coil will freeze up, and won't allow air to flow over it. Yes on a rainy day the temp is already above freezing, and the compressor is going to run to dry out the air to defog your windows.

I know back in school we had Auto A/C part of the program.. and they said the compressor don't run when it cold outside.

IF the compressor was running, your air temp would be much cooler!

 
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02-11-04, 10:02 PM   #12  
I suppose I should clear up a few things.

I live in california, where It never snows. Not only that, it seriously rains perhaps 10 times a year.

Given that, the only times the windows get fogged on me is usually when it is raining, or is 50 degrees in the morning. In that instance, I put on the defroster. In all of my other cars, the a/c compressor never kicked on, and I always used the hot air to clear the windows which worked great. However, I purchased my tacoma recently, and like to drive around with the windows up usually, but enjoy a little airflow to keep it from getting stagnent, so I put the defroster on so to get the air come out of the top vents.

The a/c compressor snaps on, and the truck lurches forward. Being an automatic, the increase in idle causes the truck to lurch forward, and as a result, I have to jab on the brake harder. (someday it is going to cause a collision) I have talked to the guys at the tacoma message board, and seems this is the common trait with these trucks.

To make a long story short, I would like to run the defroster vents without having to kick on the a/c compressor, therefore keeping the idle steady and no surging.

What I would like to do is disable the compressor ONLY on the defroster mode, but still come on when I choose air conditioning. Not sure if this is possible. Perhaps I could take a look at the defroster switch and see if this is possible

I kinda got off on a tangent on that one.

 
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02-12-04, 03:18 AM   #13  
Sounds like the problem is more in the "throttle up" setting than the compressor.
It is not adjustable, ECM controlled.

There is a TSB for '95-'97 2RZ-FE for a revised ECM with different idle logic.

 
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02-12-04, 04:48 AM   #14  
hold the phone!!

Ok.. There is no need to have the compressor on cold days colder than 40*. When you heat up cold air, the air will be dry.


you're comparing apples to oranges here...think about how much glass area there is in an automobile. more glass area, more condensation. also think about the volume of air inside a car versus a house. if condensation forms on your house windows, does it create an unsafe condition? it sure does in your car...and the colder the glass, the more condensation you're going to get. the average human exhales a pint of water a day...put 4 or more people in a car on a cool/cold damp day and just heated air won't dehumidify the air fast enough. in these days when fuel mileage is a critical design concern, believe me, the engineers wouldn't run the a/c compressor if it wasn't necessary.

 
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02-12-04, 09:24 AM   #15  
Originally posted by toyotaman11769


It is not adjustable, ECM controlled.

There is a TSB for '95-'97 2RZ-FE for a revised ECM with different idle logic.
Going to send you a pm, so I don't hijack the thread

 
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02-14-04, 07:40 AM   #16  
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I think the point is being missed here. In the winter months when it is cold the outside air is very dry. When you select defrost with outside air the air first goes over the A\C coils to be dried, then over the heater. Yes the air inside the vehicle may be wet, but the outside air you are using to defrost the window is already dry, the A/C cannot dry it more.

As long as it is cold (sub freezing) and you are using outside air there is no need whatsoever for a/c to defrost a window.

 
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02-14-04, 10:07 AM   #17  
Fireman hit it on the nail, that what I've been trying to say.. When you warm up a cold air, it dries up.

I looked at both of our cars. the compressor's cluch isn't cycling at all (about 15* outside). I even lookes at the new truck at work, and that didn't cycle either.

 
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02-14-04, 05:05 PM   #18  
Originally posted by 1fireman

As long as it is cold (sub freezing) and you are using outside air there is no need whatsoever for a/c to defrost a window.
The compressor could not engage at that temp anyway, even if it is wired to do so. The freon pressures will be so low, the switches won't let it engage anyway until it warms up some.

 
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02-14-04, 08:59 PM   #19  
Fascinating discussion.

Now, will someone tell me why the defroster always works faster on the passengers side?

 
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02-15-04, 03:45 AM   #20  
Originally posted by Dave_D1945
Fascinating discussion.

Now, will someone tell me why the defroster always works faster on the passengers side?
Because it's closer to the blower motor.

 
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02-16-04, 06:30 AM   #21  
I think the point is being missed here. In the winter months when it is cold the outside air is very dry.

you make it sound as if relative humidity would be the determining factor in whether or not the a/c compressor operates in cold weather. it's not...it's a function of the temperature/pressure relationship. as the temp of the refrigerant goes down, so does the pressure...when the temp reaches sufficient lack of heat, the pressure drops below the threshold to close the switch and operate the compressor. as far as humidity during cold weather...watch your hourly RH during changes in weather, it most definately goes up even during freezing weather. take your car and get it warm under the hood...i mean saturated...and then turn your defroster on again...most of the time, it will engage the compressor because the heat under the hood has been imparted to the refrigerant and now the pressure is high enough to cause compressor engagement.

 
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02-16-04, 10:23 AM   #22  
Hmm. what could I add..

Yes, the compressor runs when you select defrost. It's not _suppose_ to run if it's below about 40 degrees.
You should use defrost regularly, in the winter, to keep the a/c flowing come summer.

My stepson couldn't figure out why his defroster worked so poorly and was stupified to learn this. Since his a/c doesn't work, he has no defrost! I'm still not sure he believes me.
One of the primary functions of a/c is to remove the humidity.
A steady stream of water flows from my home a/c drain line whenever it's on.

You don't have to use the cold air setting though, slide that puppy over to heat, the window will still defrost and you'll be warm.

Anyone who thinks the humidity drops when it gets cold has never lived in florida..
It's 45 degrees right now with about 60% humidity, why do you think it's so fing cold in Florida when it's only in the 50's!?

 
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02-16-04, 11:14 AM   #23  
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Relative Humidity actually rises as the weather gets colder, it is not more vapor is is just that as air becomes colder it cant hold as much water vapor, so the colder you get, the relative humidity gets higher because the air is getting tighter in a sense of saying. Then you hit the ole dew point.

I think the main part of dryiness in colder areas is that water cant readily evaporate as much because it is in a frozen state. When your in a car of course breathing is just a water factory in a confined space. So as it sucks in air over the A/C coils it makes it hit its dew point inside the condensor area. And the air loses alot fo its humidity because the cldness made the air tighter.

Right now it is 21F here with a humidity level of 50% with a dew point at 8F (100% relative humidity) once it hits 8 F the air is going to constrict so much it cant hold anymore water and out will come the condensation.

 
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02-16-04, 06:22 PM   #24  
Some very good explanations. I hope we've finally made our point that it IS necessary to have a compressor wired to defrosters.

 
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