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How is this possible?

jed1154 on Tue June 03, 2008 11:19 AM User is offline

Year: 2002
Make: Ford
Model: F-150
Engine Size: 5.4
Ambient Temp: 98

37 degree vent temps, 65mph, 98 degree ambient temps.

How in teh world does my truck accomplish that and how do i duplicate it on others?

GM Tech on Tue June 03, 2008 3:58 PM User is offline

Might be icing -up due to a faulty cycling switch--- sometimes things can be too good to be true......will it do that for a 1 hour trip???

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

jed1154 on Tue June 03, 2008 4:20 PM User is offline

It will do it all day long!

HECAT on Tue June 03, 2008 4:35 PM User is offline

Good air flow?

What are the vent temps at hot idle w/ 98 ambient?

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jed1154 on Tue June 03, 2008 5:12 PM User is offline

hot idle, if you have been sitting for 10 mins or so, it will creep up to 44 at the absolute most. Great air flow. I just am trying to figure out how to duplicate the system on my other truck cause its absolutely fantastic.

HECAT on Tue June 03, 2008 5:18 PM User is offline

Every system seems to be somewhat unique and includes a myriad of variables.

May be difficult to get all the "stars to align" as they seem to be with this one.

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TRB on Tue June 03, 2008 5:22 PM User is offlineView users profile

Large evap core, large condenser and very well insulted vehicle with no air leaks.

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bohica2xo on Tue June 03, 2008 8:26 PM User is offline

Jed:

My wife will not go near my beat up F150 work truck in the summer without a sweater...

Getting vent temps like that are possible where the humidity is reasonable. 36f to 38f vent temp is right on the edge of the ice formation point above about 15% RH.

Have a look at the cycling switch settings, and you will find that you have a couple of things going on. First, you have a "good" switch with very little hysteresis. Second, the "on" pressure is set low enough that the average temp of the evaporator is right at 33f.

It can be done. You need to pay attention to the condition of the condensor & the fan clutch. Cycling switch adjustment can be done carefully to hold the average evaporator temperature as close to freezing as possible, without forming an ice buildup. As long as the condensor speds some time above 32f, it can go below that. Some cycling switches have a lot of hysteresis, and better results can be obtained by running two switches in that case.

B.

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"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

TRB on Tue June 03, 2008 11:19 PM User is offlineView users profile

Yes all true statements, but lets get real. These type of temps can't be achieved in all vehicles. Take that single unit setup and put it in a Excursion with black interior and exterior. Your 37 degree temps will be long gone. Take those old Isuzu Trooper's. Vent temp would be 42 degrees and with that sucker style system air flow was poor even on high blower. So that 42 vent temp was 65 when it hit you in the face.

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When considering your next auto A/C purchase, please consider the site that supports you: ACkits.com
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Edited: Tue June 03, 2008 at 11:20 PM by TRB

bohica2xo on Wed June 04, 2008 3:49 AM User is offline

Tim:

I agree that this only works in some cases. Single evaporator Excursions & Suburbans (whales) can have very cold vent temps, and still not get that cold inside.

You can maximize the cooling capacity for a given system & local conditions, but that may only be 50f, not 37f. Jed obviously lives in an area with reasonable humidity. The small cab volume of a fullsize pickup, with a barn door sized condensor is a big help for outstanding cooling. Thankfully most of those Troopers are in the scrapyard now....

I just got my thermo-hygrometer back from calibration a week ago. I saw an afternoon at 7% humidity, and could not ice up an evaporator if I tried. The same 16 psi low side would have turned solid with ice in New Orleans. Automakers must deliver a system that will work across 48 states, since it can be driven to anyplace in a few days. Local optimization only makes sense if the vehicle does not travel cross country on a regular basis.

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"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

TRB on Wed June 04, 2008 10:39 AM User is offlineView users profile

I'm with you Brad, my point is not all vehicles will cool to X degree. You can always fine tune a system but that still may not achieve a certain goal do to other factors. I just trying to insert a comment for the readers which may come by this thread and think all a/c systems perform the same.

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When considering your next auto A/C purchase, please consider the site that supports you: ACkits.com
Contact: ACKits.com

mk378 on Wed June 04, 2008 10:53 AM User is offline

Pick any two of the same brand new truck from a dealer lot and test them for zero to 60, mpg, A/C performance, whatever you want, and they will not be quite the same.

jed1154 on Wed June 04, 2008 11:23 AM User is offline

Very good answers. Though im no AC expert, they make sense and seem reasonable.

So when my vent temps are upper 30's, i should be close to FREEZING of the evaporator?

Im in Houston, TX right now, for those wondering.

In my F-250, i can get it down to about 44 out of the vents with 81 ambient temps, but that is if im going 55, if i speed up to 70-80 or if i sit still for a long time that temp goes to about 50...which really sucks in my opinion, but i suppose its reasonable.

Edited: Wed June 04, 2008 at 11:26 AM by jed1154

bohica2xo on Wed June 04, 2008 1:49 PM User is offline

Jed:

In your particular case, the evaporator is actually spending some time below freezing. As the system cycles, the evaporator spends just enough time above freezing to shed enough ice to maintain airflow. No automotive heat exchanger is 100% effecient. Blowing 80f air over a 35f surface will not give you 38f air - unless the surface is large, and the airflow small.

You could experiment with an adjustable cycling switch on the F250, but you may be looking at other issues as well. Depending on the year of the vehicle, the PCM may be adding hysteresis to the cycle. I fought this issue with an Explorer once. The compressor did not re-start as soon as the switch closed, the PCM waited another 5 to 7 seconds before restarting the compressor. This moved the average temperature of the evaporator way up, and robbed me of cooling capacity here in the desert.

As others have already said, no two vehicles are exactly alike. Just the differences between model years can be huge. There is no set temperature differential that can be achieved, and no "rule of thumb" that applies. If someone makes a blanket statement like "MVAC can only make a 20 degree drop" you should quit listening right there. R134a can & is used in freezers that go down below 0f. Some cars are lucky to make 65f vent air according to the factory specs under certain conditions. Your question was about a specfic vehicle, and my answers in no way apply to everything on the road.

B.

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"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

jed1154 on Wed June 04, 2008 4:30 PM User is offline

Good answers. Im happy enough.

I just asked here because all i have to go on is vent temps and what the techs tell me.

ajrothm on Thu June 05, 2008 2:43 PM User is offlineView users profile

I had a 2001 F150 with the 4.6l. That truck would idle all day at 38-39* vent temps here in Houston(95*). That one had a GREAT AC. Now I have an 05 w/ 4.6l and it will go into the mid 40s once its moving but never cooler then that. This also takes way longer to cool down and the compressor cycles off a lot more. I took this one to the dealer to get it checked and it tested within range on all the checks. Its just not as good as the older one.

I also had a Chevy Silverado(2000) and it would run at 40* pretty much all the time also. I even tested different thermometers in em to verify the results.

Some are just better then others I guess.

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1971 Corvette

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