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Does Your Ac Work At Idle?


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Just curious if your AC works when your truck is idling. Mine works sometimes but then will blow warm air and then back to cold air. Not sure if there is a problem as it works great when driving. Truck idles at about 500 rpm so maybe that is not enough to get compressor going. Anyone else have this situatiuon?

 

Rob

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Just curious if your AC works when your truck is idling. Mine works sometimes but then will blow warm air and then back to cold air. Not sure if there is a problem as it works great when driving. Truck idles at about 500 rpm so maybe that is not enough to get compressor going. Anyone else have this situatiuon?

 

Rob

 

Mine is only cold when driving. Seems like as soon as i come to idle it gets to be alot warmer. It never gets warm, just not really cold.

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yes mine too, and it is pretty normal. when you are sitting there, there is not as much air blowing over the condenser coils to remove the heat from the freon. Once you start moving more air is passed over them and more heat can be removed from the freon, hense the ac gets colder. It was explained to me by a AC tech that ACs do not cool air, they remove heat from it. That heat has to go somewhere, at idle there is just not enough cooler air flow on a hot day to move that heat from the freon to the outside air. The compressor has a pressure switch inside it, when you are at idle it takes longer for the compressor to build up the pressure needed to condense the freon, when you are moving and the engine is at higher RPM the compressor cycle is much shorter b/c it is turning faster and takes less time to build up that necessary pressure. Not that that was really your question, but it does explain a little too, why when you start and keep moving at a higher RPM and have more air blowing over the condenser coils why the AC feels colder. If you sat there and idled for a long time, the AC would probably get colder if the freon charge was correct.

 

Having said all that, you could be a little low on freon too. When you are low the system will not be able to transfer heat as well at the condenser coils because you pressures high and low will drop, and there isn't quite enough gas produced at the evaporator to compress into a liquid to transfer the heat, and the compressor running at idle speed may not be able to build the necessary pressures to triger the pressure switch or provide very good cooling. Once you are moving the compressor is able to get those pressures because it is spinning alot faster.

 

If you were over charged on freon the system would have too much pressure, the compressor would short cycle, since the pressure would build too quickly. Once that happens you are not moving that freon around from liquid to gas state as much if at all and there is no heat being removed from the air. And there is no room for the high pressure liquid to expand into a low pressure gas.

 

Here is how the cooling process was explained to me (I am a computer network engineer not a AC tech).

 

You have these basic parts of an AC system. Condensor coils, copressor, dryer, evaporator, oriface, fan (air handler)

 

The freon starts off at the compressor where it is a gas (larger line called the suction line or low pressure line), it is compressed down to a liquid (higher pressure line, or liquid line) which gives off heat (that is why your outside unit on your house blows hot air), then it passes thru a dryer (removes moisture and other contaminates) from the sytem. From there it goes to the oriface (some are fixed, some are adjustable due to pressure, and some use a system of capalary tubes) this high pressure liquid is forced thru this small opening. During that process it turns back into a gas ( it expands because it has a low boiling point and the system is in a slight vacuum so it has the available area to expand and turns cold (compressing a gas to a liquid produces heat (exothermic reaction), allowing a liquid of said gas to expand back to a gas removes heat (endothermic reaction), this cold gas circulates thru the evaporator coils where you ac fan blows air over them. The evaporator and more specifically the gaseous freon inside it removes the heat from the air blowing over it. Then that gas flows to the compressor where it is still cold. It gets compressed back into a liquid, giving off heat once again, then the cycle starts over again. To little or to much freon will throw off this balance of liquid to gas and gas to liquid, and the cooling will suffer.

 

The thing that trips most people I tell this too, is this. The freon flows in the opposite way than most would think. People see the cold and sweating line on their compressor/condenser and think that the compressor is putting out cold liquid to go into the house or car. When in fact it is the opposite. It is taking that cold gas and turning it into a hot liquid to put into the house or car to cool it. The hotter the day and the hotter the Air being cooled the hotter the liquid line will be. Not to mention pressures in the sytem vary somewhat due to inside and outside temperature so that has to be taken into account when readings are being taken.

 

I actually replaced the compressor on my sisters car last year, but it was when it was in early spring so we had hot days and cool to cold nights. We didn't get to vacuum and charge the system until that evening. The system was cooling great that night, but the next day in the heat it wouldn't cool at all. We based our pressure reading on what they should be on a hot day not a cold day. Therefore we overcharged the sytem and had to recover some of the freon for it to cool properly.

 

 

(Obviously someone had too much time on their hands at the office today, not to mention the whole AC thing has always been of interest to me. And if I am wrong on any of this someone please correct me like I said I am a Network Engineer not a AC Tech. )

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My 04 EXT Cab has cold air sitting or driving but then again i have a C5 cooling fan instead of a clutch fan. The other day it was 96F and was stopped in accident backup on I-75 for 30 minutes and still was cold and coolant temp actually went down a bit from normal.

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Just curious if your AC works when your truck is idling. Mine works sometimes but then will blow warm air and then back to cold air. Not sure if there is a problem as it works great when driving. Truck idles at about 500 rpm so maybe that is not enough to get compressor going. Anyone else have this situatiuon?

 

Rob

Mine does the exact same thing.............and so does the wifes CRV.

 

:)

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Just curious if your AC works when your truck is idling. Mine works sometimes but then will blow warm air and then back to cold air. Not sure if there is a problem as it works great when driving. Truck idles at about 500 rpm so maybe that is not enough to get compressor going. Anyone else have this situatiuon?

 

Rob

 

If your truck & PCM calibrations are stock your truck should be idling at 615-625 rpm w/ A/C on in "D" the tach will read about 550rpms but if you check with a tech 2 its digital and more accurate. Engine RPM's should be the same with A/C on or off. In 90+F temps its best to use the recirculate mode so as to not cool hot air. Using the recirculate mode save on fuel economy too.

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Sounds like you need an automatic thermostatic auxiliary fan on your condensor like a lot of other modern vehicles have. It will kick on when it senses that the condensor is getting too hot (at idle) then when you start to move and air starts to run accross the coils, it will turn off.

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Actually, I remember reading that this is a gas saving "feature" that is done on purpose. Running the AC at a stoplight requires an increase in RPM from slow idle, which takes more gas. To boost fuel economy, manufacturers are opting to cut off the AC compressor and slow idle at lights.

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I live in Florida and I do not notice any problem with the truck not cooling well at idle. As mentioned, I keep the system in he recirculating mode to avoid having to cool hot ambient air.

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