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My 2001 suburban (122,000) stopped cooling because it was low on Freon and would not turn the compressor on. I charge it back up and it worked for one day . I pulled a vaccumm on the system and it held overnight. What could be wrong with it? I read that sometimes the seals will seal under vaccum but will leak under pressure. Does this really happen? Next step is charging and using a leak detector?
Any ideas?


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yes, it is possible for the system to seal under vacuum but leak under pressure. But, to leak out in one day should be a leak you can find.

most ac systems use an ultraviolet dye in the oil so you can pinpoint where the leak is with a black light. You could also use an electronic leak detector. however, with a leak that goes out in one day you should be able to find the leak with a spray bottle with soapy water in it and spray the lines, connections, compressor and condenser. Se if bubbles show up anywhere. The only place you can't check is the evaporator.

what were the pressures when the system was working? What was the ambient temperature when you charged it up? What is the low side pressure now that is doesn't work again. Is the cycling or low side pressure switch working correctly?


   
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Ok, I need to clarify. I charged up the system on a sunday. It was not cooling, compressor not turning on... low freon.
I got it charged with one can of 134 + 4 0z of charge with red dye.
The temperature was approx. 96 and I had the high side about 240-250 PSI and low side 35-40 PSI . I think this is about what it should be. It was cooling fine with the front center vent temp. at about 50. I figured it would at least last for a couple of weeks if not more. My wife told me the next sunday (7 days later) that it was not cooling on Monday, so I than look at it Sunday (yesterday). The system had zero pressure . I checked all connections including the condenser, and compressor above and below. No red dye or oil. I have not checked the rear evaporator and connections and according to what I read you cannot get to the front evaporator unless you take the dash out. I ordered a sniffer last night to see if I can find it. If it is either evaporator it would seem to have to be the connection , since it is holding a vaccum. If that is the case the front evaporator's connection is on the firewall so I should have been able to see it. Hopely it is the rear evaporator connection because I can get to that.
The compressor pressure switch seems to be working correclty. Any thoughts?
Thanks alot for helping . I was hoping to get a hold of a knowlegdable person.


   
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vacuum and pressure don't act exactly the same. vacuum will only go to a certain amount. For most cases, 30 inches of vacuum (because a vacuum is an absence of pressure). So, air pressure being 14.7 lbs/in2 reads zero on a pressure gauge for AC purposes. While vacuum in the AC system is a fairly good check for leaks it is not as good as holding pressure that goes up to 250 on the high side or even 20-30 on the low side. Even static pressure of an AC system turned off could be 60-90 depending on ambient temperature.

the dye takes a while to circulate through the system. because the dye isn't a gas, it has to circulate with the oil in the system. The gas (refrigerant) pushes the oil around inside the system, but is isn't moving like the gas is. Kind of like when rain hits your windshield. The wind pushes the droplets of rain around on the windshield but it doesn't move the droplets at the same speed that the wind is hitting your glass, 50 - 60 mph.

the pressures look pretty good and the vent temperature looks pretty good too for a 96 degree day. So, it looks like you just have to find the leak. If you cannot find any leaks on the external parts it could be the evaporator. Because the evaporator sits inside the plenum and the condensation from the ac system collects in the plenum also, there can a debris build up in the plenum that can deteriorate the evaporator. If the evaporator is the leak it will usually have oil around the leak. the oil and debris can plug up the leak under vacuum but be pushed out under pressure. And the leak could be very very small. To use an electronic leak detector, you may have to add more r134a to the system and run the system to see if you can find the leak.

While the evaporator is hard to get at, you might be able to get a reading on the electronic leak detector by going through the plenum drain. But you have to make sure there isn't any water coming off the evaporator and draining out the drain tube.


   
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Strange I can't seem to reply with my user's name because it tells me its already in use...duhh!!!
So, I'll have to do what I did last time and reply via "GUEST"

Yea, I was planning on that. I got my leak detector in today so I will charge it up and hope I find the leak on the rear evaporator.
Thanks for the info. I'll let you know what happens.
If it is the front evaporator I'm going to think real hard how to get in replace without tearing out the dash even if I have to do some Southern Engineering. :)


   
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ok, let me know what you find out...


   
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I recharged the system and then started using the sniffer to detect the leak. I found out that the Exhaust from the engine activates the sniffer so you have to be careful when sniffing the Rear evaporator housing. I thought I had found the leak there but it was jus the exhaust creeping into the back since I had the back doors opened.:(
I could not find the leak on the exterior of the AC system ...damn. I finially got the sniffer to detect the leak in the front Air vents.
With the fan on full for 30secs. the leak detection went away, so I turned off the fan which turned off the compressor and waited for about a minute to see if there was a leak it would fill up the duct. I then turned the fan back on and sure enough the sniffer activated again. I did this five or six times , so I am convinced that it is in the front evaporator. Double Damn!
So I beleive I am going to use the "cut in the Plentum" method to remove it. I watched a video on taking out the dash and then the whole AC/Heater assy. Even if I paid to have this done I have no way of knowing if everything got put back together right.
Any way thanks for the help, hopefully I will be sucessfuly in fixing it.


   
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Oh, one more thing. When I put the gauges on the system before I recharged it I still had 30 in. of vaccum. It boggles the mind to think that would happen. I think your right about some debris sealing the leak under vaccum.


   
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