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Questions about A/C charging

3K views 18 replies 7 participants last post by  79jasper 
#1 ·
**** `95 with R134A refrigerant ****

I installed a new compressor, filter/dryer, orifice tube, and flushed the entire system. I evacuated it with a vacuum pump. I'm 99% confident I got all the trash out of the system.

It's 97 degrees outside, and the charts I'm looking at call for low pressure of 50-55 and high at 300-315.

I'm at 50 and 305 or so.

My suction line is very cool at the compressor, and the compressor itself is cool to the touch. Is this a sign that I'm over-charged? Shouldn't I want the suction line and the compressor to be warm? I'm concerned with liquid refrigerant entering the compressor.

And can I R&R the high pressure cutout switch without losing refrigerant?

Thanks
 
#2 ·
The suction line should be cool. It should be 54 degrees plus the heat it picked up at the evaporator. That's the superheat. The discharge line should be 161 degrees. That's according to the pressures you stated. What's your vent temperature?


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#4 ·
I think you are overcharged. Each refrigerant has a pressure - temperature chart. The higher the temperature, the higher the pressure and vice versa. By getting the low side to 50 psi, you've ensured the evaporator can not be cooler than 54 degrees. You want it to be closer to 35 -40 degrees. That means your low side pressure should be closer to 30-35 psi. That's in normal driving conditions. To mimic that, you open the doors, then set the fan to high- and sometimes even run a fan across the condenser. Then, have someone hold the engine at 1500-1800 RPM and see where your low side is. A better and easier way would be to weight the refrigerant in as the engineers have already done all the tests to see what refrigerant quantity gives the best results.


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#5 ·
Over charged for sure.
Yes, you can removed the high pressure cut-out and clutch cycling switch without losing refrigerant.

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#6 ·
Oops.
 
#8 ·
#9 ·
What are correct low and high side pressures?

Looking through posts people are claiming varying pressures. I see posts claiming 25-45 low side up to 45-50 low side and 200-240 to 275-300 high side. So what would correct charges be at 90-95F outside ? I repaired the A/C in my old F350 that I sold several years ago and the vent temps got into the 30's. I had the Ford manual on CD back then and it's been so long I do not remember what pressures I had.
I am replacing the orifice tube and drier after getting the system evacuated and I usually add dye before closing it up. I have a vacuum pump so I will pull it down and leave a vacuum on for 30-60 minutes. I place a thermometer in the vent and slowly charge up to recommended amount. I just want an idea of proper pressures.
 
#10 ·
There are just too many variables to give you an exact number. How gooked up is your evaporator? Your condenser? What condition is your fan clutch. Wet bulb temp on the evaporator? Engine speed? Fan speed? Best I can tell you is download a pressure temperature chart for your refrigerant. Your evaporator will never be colder than your low side pressure temperature equivalent. The conditions should mimic driving down the road- rpm, airflow, etc..


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#11 ·

Many charts out there. This is the one I went off of

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#13 · (Edited)
Many charts out there. This is the one I went off of

The problem with generic charts is they don't take individual vehicle characteristics into account. For example, an excursion with both an orifice tube up front and an expansion valve in the back will have dramatically different pressure than a Civic with its little undersized condenser. Air flow over both the evaporator and condenser affect pressure. Evaporator and condenser size and efficiency affects pressure. Orifice tube size affects pressures. Compressor size and efficiency affect pressures. RPM, insulation, tube size, etc. etc.

When I mention pressure/ temperature chart, I mean one like this- http://emea.forane.com/export/share...ls/forane-134a-pressure-temperature-chart.pdf


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#12 ·
I have learned through the years that there are certain things that I don't want to mess with and the AC system is one of them. I'll put a can of refrigerant into the system every now and then when needed but as for clearing the whole system and then recharging it I'll leave it to the pros.
 
#14 ·
Correct. But it'll get you pretty close.

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#15 · (Edited)
Exactly, I know there are numerous factors to take into consideration but if I can understand, maintain and repair a 200k mass spectrometer at work I can work on my own A/C system. I have converted two vehicles from R12 to 134a (which included clearing the entire system and replacing condenser, evap, drier, expansion valve etc.) and have fixed leaks on three different 134a vehicles I have owned.
All three of mine that were low ended up having minor leaks (high or low side valve, o-rings bad) and putting some UV dye helps find the leaks. I have a vacuum pump and gauges as well as other items I have collected over the years that helps to work on them.
Most people mechanically inclined can read up on how auto A/C's work and pick up tips off the net. Arizona AC used to have a pretty good forum where certified A/C techs would answer questions and I picked up a lot there. I do not see the reason to pay anyone to do what I can. I realize there are some concepts like latent heat and system design that play into it but I'm 5 for 5 right now in fixing auto A/C's. My current F250 is a bit low so I do not think it will be a problem getting it going like it should be.
Thanks for the help.
 
#16 ·
There is a hvac forum I came across. Haven't spent much time on there, so I don't know how nice they are.

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#18 ·
There is a hvac forum I came across. Haven't spent much time on there, so I don't know how nice they are.
If this is the one you are talking about - HVAC-Talk: Heating, Air & Refrigeration Discussion

They are real friendly until they get wind you are trying to do something yourself, then they get pretty crabby.

I don't want anyone to get me wrong - I believe everyone should do their own repair work within their comfort level. I find that when you are working on your own vehicle, you take the time to do it right as opposed to some (not all) shops who are just trying to run you in and out as fast as possible. Taking the time to wash out all the fins, or sit in front of a condenser for hours raking out bent fins is not something a shop is going to do. And, you've got the outright crooks in the business. Neighbor's son had an a/c go out on him. A/C repair guy quoted him $5,000 telling him the unit was beyond repair and needed replacing. I looked at it and found a $7 capacitor was blown. 7 bucks and it runs like new.
 
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#17 ·
Like usual I found the issue and it was fairly simple, the high side valve was leaking and I have been losing 134a. I just replaced it, the orifice tube (which was almost completely clear of debris) and the drier. I'm evacuating it now and as long as I do not have any leaks I'll be charging it tonight.
 
#19 ·
Oh. Lol
Like I said, hadn't really snooped around much. Just saw some info one day, and they seemed pretty civil.

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