Troubleshooting help needed-AC pressures

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plumkrazee70 said:
Oh boy. I know how you feel. I'm going to have my system (classic auto air) redone. I had a shop charge it, but the tech didn't really seem to want to follow the instructions, and it doesn't blow nearly as cold as it should over 40* on the thermometer.

He only pulled a vacuum for 30 mins (new systems say 90 min).
Click to expand...

20-30 minutes after reaching 28-30 inches of vacuum is adequate for almost any system that wasn't at the bottom of a lake.
 
65wgn440 said:
20-30 minutes after reaching 28-30 inches of vacuum is adequate for almost any system that wasn't at the bottom of a lake.
Click to expand...

I was just going by what classic instructions stated. They said for a new system you want to be sure all the moisture is boiled out. When I have it done again, I'm going to o take note of the pressures, as I do believe it should be cooler coming out of th vents.
 
Unlikely the compressor outlet was lower than the suction side. You were likely reading on different gages and the HP one has less sensitivity. If you leave it sitting for a day, the two pressures should equalize, especially if it has just an open orifice tube, but even with an expansion valve it could leak around the other way past the flapper valves in the compressor, plus those flappers are one way check valves which should flow thru if suction side was higher (thinking out loud, never tested). So, after a day you expect the pressures should be the same which will tell you how the 2 gages agree. On a 70 F day, I see ~70 psig on both sides when a correct charge of refrigerant. With compressor running ~20 to 40 psig low-side and ~180 to 250 F high-side, depending on ambient temperature. I use Duracool in all my cars, but R-134A should give similar pressures.

With compressor running, if the outlet pressure doesn't increase and temperatures around the components don't change, the compressor isn't doing anything useful (i.e. compressing). Don't assume a new or rebuilt compressor is fine. I installed one in our 2002 Chrysler 3.8L from a big rebuilder in TN and it sounded funny and never worked right from the get-go but kind of worked. Next summer when it got worse, I took it apart and found they had put both thin flapper plates on one side (like to stick together). So how did it work at all? The backer plate was serving as a poor-man's flapper plate on one side as seen by witness marks from the port on it. Worked perfect with no sounds after I reassembled correctly. I used the center section from my old compressor which I had saved since much less slop between pistons and wobble-plate. That was a Denso compressor (similar to Sanden, 7-cylinders I recall). My OE compressor was fine but the clutch slipped and melted and a rebuilt one w/ clutch cost little more than clutch alone, so I went that way, but no good-deed go unpunished.
 
Not reading every post, i can tell you how I fill my R290 system (R290 (also known as hydrocarbon or HC) refrigerant, which is natural, non-toxic, and free of ozone-depleting properties. Approved in 2011, HC has a low GWP and is one of the most climate-friendly and cost-effective refrigerants available)

Ill start motor and turn on AC, I put the manifold on the low pressure port and read the gauge. Sometimes it will be low or as high as 90 psi but don't let that fool you it will drop to almost nothing when the compressor finally kicks on. I'll open supply side and let it idle for about 10 minutes and then make it a high idle for 10 minutes if needed to pull about 55 psi at 90F on the low side. That is usually all it will need to blow 50F or lower out the vents. Any more doesn't help. The gauge on the R134 bottles have a ambient temp compensation so you dial I the air temp and it will tell you where full is psi wise. I don't have an R290 guage I just use an old manifold I got for 10 on Craigslist but I'm sitting here idling in my R290 filled truck and it's 80F outside and it's nice and cool in here. I fill it about once every 3 months and could care less if I vent R290 as it's only propane!

 
ACME SS said:
Update:
So after I learned how the gauge set is valved internally and that the pressures probably were not reversed but simply affecting each other cause I didn't have the valves properly configured, I discovered I was loosing freon due to a sudden pressure leak. Because there was dye in the system, I quickly discovered it was the crimp connection between the hose and the LP compressor outlet tube (with the LP port on it). It got so bad at one point that it started to hiss and by the next morning the system was empty. This happened around 10pm so I could not take it anywhere to have what was left evacuated properly. I removed the hose and put it back in my crimp tool and was able to get a little bit more of a crimp on it. I pulled a vaccum and it held but I really wanted to pressurize it with argon or something but didn't feel like making up the connections so I decided to take it to a shop where they could pressure test it (not just pull a vacuum) since it had tested OK under vacuum but leaked under pressure.
The shop says sure, we can do that. Two hours latter I get a call that it is finished and I was a little miffed because I expected them to pressure test and pull a vacuum and to leave it long enough to be sure it isn't leaking in either condition. I highly doubt I will return to them since they didn't do what I asked. (not that I take my cars to shops more than once a decade as it is)
I pick it up and they say that I fixed it. I asked if they pressure tested it and they said no... I explained that the reason I brought it to them was to pressurize it in addition to evacuating it. Got that glassy stare back like I have 3 eyeballs.
Then I get home and see the invoice says they added a NAPA 134a with oil in it. I go to NAPA and get the specification on the part number and calculate they added 4 oz of oil to the system which is factory filled with 7oz and no more is to be added. At this point I wish I had just spent the time to plumb in argon or something and pressure test it myself. But I go back and kept my cool and confronted them concerning the oil and they say, "oh the computer just uses that part number (which is for a 14oz can) but we really used our cart which has 134a only in it".
And this my friends is why I would rather bumble around these things myself.
I'm not sure if it is really fixed yet. It blows cold but whether or not it will leak is to be seen. I cleaned some dye off the area this morning that I am hoping is from them when they disconnected their hoses.
Click to expand...
Sounds like you're not comfortable unless you understand it yourself. I can understand that. I have a story where I kept losing freon, blowing hot after a few days... I changed o rings, refilled.... junk yard compressor ....it started whining....pulled the belt. So I had to go to the valley where it is hot... sadi to myself I need ac..and so I changed the compressor 'new one' and bought oil... not really the right designation... too late...so I vac n fill it....it blows cold a couple days then luke again.i pull the belt, again.
Almost 6 months later..I go to fill it,using my manifold gauges ..and the fkrs already full?? 40-50psi low and 250lbs high side... has blown cold ever since.
All I did right before firing it...is... there's a couple things I do in a system is empty I either jump the pressure switch plug to activate the compressor clutch and get it spinning to draw in or I just hook the can to it when it's off and get a little in to kick it over. Hooking the can to it for maybe 10 seconds is what I did in this case.That can give some edgy high side #'s but it balances out after cycling.

Inquiring minds wanna know.. and I completely get that!
 
65wgn440 said:
20-30 minutes after reaching 28-30 inches of vacuum is adequate for almost any system that wasn't at the bottom of a lake.
Click to expand...
On a newly installed system, you are also checking for leaks... when I first pulled a vacuum on mine it didn't show up for a few hours and then I found it wasn't holding and found a leak. I actually let mine sit for a couple days under vacuum. I still had a leak but it was under pressure that it leaked.
 
1...Assemble system including initial oil charge
2...Pressure check using nitrogen (I actually often use CO2) and allow for temperature changes to judge leakage

When pressure checking, "sweep" system AKA inject test gas into high side with low side port open so that inert test gas "sweeps" most air/ moisture out under pressure.)
3....Evacuate and hold vacuum and re-check for leaks under vacuum It is helpful, if the engine will run, to warm both engine and interior as a warm system being evacuated will evaporate moisture more easily
4....If pass---charge system
 
JUST HOW BAD IT CAN GET and still work!!!
My 70RR came with no AC (440-6 car). The owner, a friend of mine, who was still kickin last I knew, added aftermarket "hang on knee knocker" aftermarket AC. When I swapped the 340 into the car, it took me awhile to realize that it was not difficult to adapt the pump to the small block.

I had bagged and taped up all connections. I did not even change the dryer. I just "red neck" baked the existing receiver dryer in a low temp oven for about half a day to drive out moisture.

When I charged the re-installed system now on the 340, I had a "mere barely basic" knowledge of AC. This was about ?1973? or so. I had no vacuum pump, but R12 was cheap. I installed the system, "swept" the system with a can of R12, and then "rigged" the thing to evacuate BY USING ENGINE VACUUM!!!! After evacuation, I re-swept the system with another can of R12, then re-evacuated using engine vacuum, and finally charged the system. That system functioned just fine--with about 1/2 can loss per year---until I sold the car about ?? 1979 ?? or so. The system used CLAMP ON hoses!!!
This poor faded (slide) photo was 73-74 VERY hot summers back then!!
70RR.jpg
 
ACME SS said:
On a newly installed system, you are also checking for leaks... when I first pulled a vacuum on mine it didn't show up for a few hours and then I found it wasn't holding and found a leak. I actually let mine sit for a couple days under vacuum. I still had a leak but it was under pressure that it leaked.
Click to expand...

So maybe a dumb question, but if my system is charged can I still pull a vacuum to check for leaks? My dad has a vacuum pump I can borrow.
 
plumkrazee70 said:
So maybe a dumb question, but if my system is charged can I still pull a vacuum to check for leaks? My dad has a vacuum pump I can borrow.
Click to expand...
When a vacuum is pulled on an AC system that process empties the internal passages of air and moisture and any other gasses in the internal passages.
Pulling a vacuum is a good way to verify that the AC system has integrity and does not leak. If it holds a vacuum it should hold the pressurized refrigerant. But if it does not hold a vacuum, you won’t know where the leak is.
Pulling a vacuum is not the way to empty a charged or partially charged system. AC repair shops have equipment that bleeds the refrigerant out and contains it. That is the right thing to do.
To check for leaks in a charged AC system you need to add dye. You can buy AC refrigerant that has dye in it.
Typically you also need an ultra violet light. Inexpensive pen sized UV lights can be purchased.
 
Last edited:
plumkrazee70 said:
Oh boy. I know how you feel. I'm going to have my system (classic auto air) redone. I had a shop charge it, but the tech didn't really seem to want to follow the instructions, and it doesn't blow nearly as cold as it should over 40* on the thermometer.

He only pulled a vacuum for 30 mins (new systems say 90 min).
Click to expand...
Are you saying the duct temp is 40 degrees? What are you complaining about. Most auto air systems will have a temp difference of 20-30 degrees between the intake air, and the discharge air. Outside air temp and humidity will make a difference. 40 degree discharge is a good safe temp. Anything colder and you could get evap freezup.
 
my68barracuda said:
When a vacuum is pulled on an AC system that process empties the internal passages of air and moisture and any other gasses in the internal passages.
Pulling a vacuum is a good way to verify that the AC system has integrity and does not leak. If it holds a vacuum it should hold the pressurized refrigerant. But if it does not hold a vacuum, you won’t know where the leak is.
Pulling a vacuum is not the way to empty a charged or partially charged system. AC repair shops have equipment that bleeds the refrigerant out and contains it. That is the right thing to do.
To check for leaks in a charged AC system you need to add dye. You can buy AC refrigerant that has dye in it.
Typically you also need an ultra violet light. Inexpensive pen sized UV lights can be purchased.
Click to expand...

The problem with vacuum leak checking is...........That hoses, especially older ones, sort of soak up oil (and refrigerant) AND moisture into the hose lining. They are porous, whether they leak to the outside, or not. So when you pull a deep vacuum, the hoses sit there and "boil off" whatever is soaked into the hoses lining, and the vacuum reading heads towards atmospheric. This can make leak checking difficult.

I'm not referring so much to gauge manifold (charging) hoses as more to the underhood hoses.
 
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