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2001 Gmc 2500hd 8.1l Air Conditioning Question


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#1 toored

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Posted 10 June 2008 - 09:46 AM

When driving on the Interstate in hot weather with the A/C on, the output temp starts out very cold, but slowly gets warmer and warmer over 30-45 minutes until it's just cool. If I stop, I notice at first that there is very little water coming from the condensation drain. Then within 10 minutes there is a torrent of water coming out. Then back on the road this condition cycles all over again. I assume that the A/C is freezing up. I have heard that it may be low on freon and also that it may have too much freon. Which one is likely? What should the low/high side pressures be to indicate a full charge at about 88 degrees F? Any help is greatly appreciated!!
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#2 Killerbee

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Posted 10 June 2008 - 10:02 AM

When driving on the Interstate in hot weather with the A/C on, the output temp starts out very cold, but slowly gets warmer and warmer over 30-45 minutes until it's just cool. If I stop, I notice at first that there is very little water coming from the condensation drain. Then within 10 minutes there is a torrent of water coming out. Then back on the road this condition cycles all over again. I assume that the A/C is freezing up. I have heard that it may be low on freon and also that it may have too much freon. Which one is likely? What should the low/high side pressures be to indicate a full charge at about 88 degrees F? Any help is greatly appreciated!!


If your evaporator is freezing, you should switch to "recirculate". This should cut down on the amount of water that is getting phase changed, and may, by itself, solve your problem in high humidity times. If you have nice cold AC, I would hesitate to mess with refrigerant levels, until you do this.
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#3 xwire

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Posted 10 June 2008 - 10:05 AM

an undercharged AC system will cause the evaporator to freeze up, esp while driving at hwy speeds, as far as pressures I have always went by weight, I discharge the AC system and then vacuum it down for a good 20 mins (this gets any built up moisture out of the system), then fill with the manufactures recommend weight of freon, if I am not mistaken GM released a bulletin about their under hood stickers being off by about .5 lbs (low) a while back

Edited by xwire, 10 June 2008 - 10:07 AM.


#4 dewfpo

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Posted 10 June 2008 - 11:41 AM

The low side pressure switch controls the compressor. Typically keeps the low side between 25 and 40 psi; this is supposed to keep the evaporator from freezing. Is your compressor cycling? Can you put a gauge on the low side and see what the pressures are. The low pressure switch may be bad.

Just as a reference, at 72F ambient temp, engine cold and at ambient (72F), engine OFF, the 'static' pressure in system should be ~74 psi for R134a. At 75F/79psi, at 79F/85psi, etc...

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#5 toored

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Posted 10 June 2008 - 11:50 AM

Thanks for the info. I will check the low side and static pressures this evening and post a reply tomorrow. I do not have a vacuum pump so depleting and recharging the system (to ensure correct charge) is not an option. I may have to get someone to do it if I don't find an obvious problem.
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#6 dezman

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Posted 10 June 2008 - 10:11 PM

Check the cabin filter

#7 toored

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Posted 12 June 2008 - 06:25 AM

The static pressure is appx 75 PSI at 85 degrees. The low side pressure is 45 PSI at idle and 42 PSI at at 2500 rpm. The compressor is not cycling very often. Where is the cabin filter? Thanks again for any help.
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#8 toored

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Posted 12 June 2008 - 07:05 PM

I replaced the cabin air filters today. They were pretty dirty. This may help the problem. However, the compressor is not cycling at all at idle, and it rarely cycles on the road. Should I take some freon out and see if it begins cycling? The low side pressure is 45 psi at idle and 41-42 at 2500 RPM. Someone mentioned that the low side pressure switch may be bad. Where is it located? I'm doing a long road trip starting this weekend and would like to do what I can before hitting the road. Thanks again for any help.
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#9 dewfpo

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Posted 12 June 2008 - 09:25 PM

At 85F, the cold engine, engine off, static pressure should be ~95 psi. The low pressure switch typically turns the compressor on until the suction side of the compressor pulls the low side pressure down to ~25 psi, at which point the switch will turn the compressor off until the low side pressure rises to ~42 psi ay which point it then turns the compressor back on. On hot days, it's not uncommon for the compressor to stay on continuously at idle since the pressures are so high in the system but once the rpm comes up for a short while and air flows over the condenser, it should cycle.

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#10 Bob_fromLA

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Posted 15 June 2008 - 10:51 PM

R-134 leaks out much faster than Freon, all other things being equal. While the newer systems designed for R-134 cut down on the leak rate, I've found that most vehicles need a half can or so every 3-5 years. I just put about 1/3 can in my 2001 Sierra (first fill, the rest of the can went in the wife's Volvo) and the dash output is down near the spec 40 degrees for outside temp in the upper 80's.

#11 Beels

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Posted 30 June 2008 - 11:51 AM

Check the cabin filter

Yup....

My '01 8.1 was the same way. A/C worked great until you used it for a 1/2 hr or so. Cabin filters were plugged BAD and the evaporator was freezing up. Swapped the filters and it works great. I thought my refridgerant was low as well. I stuck the guages on it and it was right were it should be.

I felt real dumb because I didn't know it had those filters in there...

My buddy's Duramax was the same way.