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Weird AC pressures Pages: 12

CyFi on Thu July 31, 2008 8:28 PM User is offline

Year: 1987
Make: Toyota
Model: Supra
Engine Size: 3.0 GE
Refrigerant Type: R134a
Ambient Temp: 75-80F
Pressure Low: 20
Pressure High: 230

Hello again everyone, i seem to be having some issues with my AC. Just recently i had HC refrigerant in my system and wasn't happy, so i decided to convert to R134a. Doing so, i removed every component from the system including all the lines and did a full system flush. Flushed all the lines, the condenser, evaporator, and "drained" the compressor, even though about 2 drop came out (yes i rotated the compressor with a drill in both directions while upside down and hardly got any oil out). I also replaced the expansion valve and attached and wrapped the pickup probe properly. I filled the compressor with about 4oz of oil (ester), and filled the receiver dryer(also new) with about 2oz of oil making a total of about 6oz. I replaced all the seals in the system, and finally, i got the system evacuated to 29 inches by a shop, and filled the system myself.

Here is an overview:

-Replaced expansion valve
- Replaced receiver dryer
- Flushed entire system
- Got system evacuated

Now here are my issues. I filled the system with approximately 2 cans of plain old 134a (no dye or sealer), which would be a total of 24oz. This seemed like a little too much, but my low side pressure said otherwise. Here are some pictures of the gauges, the rpm the measure was taken at, and the vent temperature. I could not get anything that would hold the rpm at 2k, so this rpm is what i have my readings at.

Picture 1
Picture 2
Picture 3
Picture 4

The weird part is that just letting it sit like that, ac full, windows open, recirculation on, the pressures would vary quite a bit. Low side can go anywhere from 10psi all the way to 25 or 30 psi, but most of the time it is around 20. High side would get as high as 250 psi (this is when the condenser fan would kick on and help bring the pressure down) and could get as low as 200 or 215, but mostly stayed around as pictured. There seemed to be too much variation in pressures, it would go up and down randomly. When spraying the condenser with water the high side pressure would stay around where it is in the picture. While driving i am consistently getting 45-50 degree vent temps no matter what the conditions (cruising, idling, highway). Now, i am fine with these vent temps, but i hope to see them stay the same when i move to Arizona, and hope they don't get worse with the heat, but my pressures don't seem right, it acts as though there is a restriction in the system, but when flushing my components, everything flowed through fine.

Also, when i flushed the system, it was to get old oil etc out, not because of any mechanical failure, the system was 100% clean before and after flushing.

Thanks so much for all your help.

Edited: Thu July 31, 2008 at 8:31 PM by CyFi

94RX-7 on Thu July 31, 2008 10:14 PM User is offline

What's the factory charge supposed to be?

You say you had the system did you have it evacuated, drive home, hook up your gauges and charge it? If so, then how did you get the air out of your gauges before charging the system? I'm thinking you may have air in the system if the pressures are bouncing around somewhat randomly.

CyFi on Thu July 31, 2008 10:22 PM User is offline

some air did get in, but not a lot, it was still under a vacuum when i filled it. I used a short can tap and fill line to fill the system, not the big gauge set, so not much air got in. I don't think that little amount of air that got in would make it act like that, but of course i don't know a whole lot so I'm not sure.

mk378 on Thu July 31, 2008 11:43 PM User is offline

Does your compressor cycle on and off? Once the evaporator temperature switch has decided to cycle the compressor off, the vents won't get any colder no matter what you do to the refrigeration system.

Random pressure variation could be the expansion valve. When replacing the valve, the sensor bulb must be clamped to the line and insulated exactly as specified.

CyFi on Fri August 01, 2008 12:46 AM User is offline

my compressor only cycles if it is left idleing for a long time and the temp gets cold enough, and even then, it is very brief. The sensing bulb on my new expansion valve is perfectly fine. It is clamped down tight at the same spot the old one used to be and wrapped tightly and securely with proper insulation.

bohica2xo on Fri August 01, 2008 1:59 AM User is offline

Calibrate your vent thermometer. Use a thermos bottle with ice & water, and let it stand for 5 minutes with at least 3 inches of the probe immersed. I have found pocket thermometers off by more than 10 degrees on occasion...

You will probably see more consistent pressures above 2000 engine rpm with that car. Doors open, cabin fan on highest speed & 2000 engine rpm for a few minutes will give you a worst case evaporator load for your ambient.

System performance at 80f is not always a perfect predictor of 110f performance.


"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.

CyFi on Fri August 01, 2008 2:09 AM User is offline

Thanks, are you saying just to immerse it in ice water to calibrate it and that is all? I will do this. After i do this and re-evaluate my temperatures i will hook up the gauges again and take pictures at 2k rpm. I remember when doing the reading at 2k rpm, the high side seemed high and the low side seemed low, around 10-15 psi on low side and about the same or a little less than pictured on the high side. But again, i will re-test it and try to get pictures.

CyFi on Fri August 01, 2008 3:52 AM User is offline

The more i think about it the more i think i got air in the system. Everytime i have to get the system evacuated it costs almost $100 dollars, my mistakes are starting to become very costly haha.

94RX-7 on Fri August 01, 2008 7:59 AM User is offline

You can buy your own vacuum pump for $100!

NickD on Fri August 01, 2008 8:19 AM User is offline

I am new at this AC stuff, but how can you possibly go to a shop to get your system vacuumed, drive home, attempt to charge the system without expecting to get air into it? That negative pressure works exactly opposite to positive pressure and air can leak in, not to mention when the vacuum pump is removed, more air is added to the system. Air also has moisture in it and R-134a and moisture form a acid that will cause further problem down the road like locking up your TXV.

You have to be able to completely vacuum the system clear up to the refrigerant tank and have a means to valve off that tank and valve in refrigerant before moving any hoses, your pressures definitely show air in your system and a lot of it.

I have an 88 Supra, it will only get R-12, it was designed for that and have perfect use of the sight glass, R-12 costs a couple of bucks more, but no where near the price to fill the tank with gas that is 77 bucks now. I also use HC's, seems to work very well in the LP tank of my motorhome.

mk378 on Fri August 01, 2008 10:51 AM User is offline

You can't tell anything from high side pressure while the condenser fan is cycling on and off. It will just go up and down to the cycle points. See if there's an easy way to hotwire the fan so it is always on for testing. You can probably jumper the pressure switch or the coolant temperature switch on the radiator or engine.

NickD on Fri August 01, 2008 11:48 AM User is offline

He has the none-turbo, clutch driven condenser fan, turbo has that plus a couple of extra electric fans. Supra is the only vehicle that lists full pressures over a wide operating temperature range and actually recommends adjusting charge to those pressures to get the correct vent temperatures, but only if you are sticking with R-12. Other vehicles tell you to charge by weight where the pressures, vent temperatures can vary all over the place due to production tolerances.

CyFi on Fri August 01, 2008 2:39 PM User is offline

Well that was my only option, i dont have a vacuum pump of my own, and im only doing this once, so if i bought one i would never use it again and i dont have space to store it, so what am i left to do? Now i have to get my refrigerant recovered and then i have to buy my own pump and pull a vacuum myself and then figure out what to do with the vacuum pump once im done with it, its a mess. If there are some good shops in Az maybe i will just cough up the cash and tell them to do all the work for me.

And actually the condenser does have one small electric fan that cycles on and off, instead of the two larger ones that the turbo model has.

Edited: Fri August 01, 2008 at 2:47 PM by CyFi

mk378 on Fri August 01, 2008 2:45 PM User is offline

If you're paying someone to do it, have them evacuate and recharge in the same operation so air doesn't get in. Once you're sure that your car doesn't leak, find a shop that works with R-12 and switch back to R-12, it will work better. You can keep using the ester oil with R-12 even though the can says not to.

The little 1.5 cfm pumps look like they will almost fit in a shoe box. Some parts stores have rental or loaner vacuum pumps.

CyFi on Fri August 01, 2008 9:25 PM User is offline

Ok well i retested today, a lot of my weird pressures had to do with the fan cycling and such, so i took a hose and ran water over teh condenser the entire time i ran my test. So basically the hose was flowing water over the condenser the whole time. Hose on condenser, fan high, windows down, coldest setting, recirculation on, 2000 rpm i have

High side: ~200psi
Low side: ~10-15 psi
Vent temp was a pretty consistent ~45 degrees F which is pretty good.

Something doesn't seem right, it sounds like the high side is too high and the low side is too low, which would indicate a restriction, but i just flushed the entire system to get rid of oil, there was no previous failure, i replaced the TXV and i saw absolutely no signs of contamination or any kind of particles/dirt/anything, it was all very clean. Also, the system took almost 24oz of R134a to get to this point, and that is a full charge of R12 for this car.

Also, if i remove the hose from the condenser and do the same thing, the high side pressure quickly rises to 250+ and then the small electric fan kicks on and brings the pressure back down to around 200 before it turns off once again and climbs back. ALso, while this high side is doing this, the low side seems to sink lower, and at one point it got as low as 5 psi and then the fan came on, high side went down and low side rose back up to around 15psi (which still seems low.)

I am a bit confused as i am still getting good vent temps, so any input is welcomed.
I really appreciate all of the help you guys have given me and it has given me a chance to learn many things.

TRB on Fri August 01, 2008 9:42 PM User is offlineView users profile

System Type Cycling Clutch
Compressor Type Nippondenso 10-Cyl.
R-12 Capacity 23-26 oz.
System Pressures
Low Side 21-28 psi (1.5-2.0)
High Side 206-213 psi (14.4-15.0)


When considering your next auto A/C purchase, please consider the site that supports you:

CyFi on Sat August 02, 2008 3:45 PM User is offline

How do i translate R12 pressures to r134a pressures? SHould they be about the same? Thanks.

TRB on Sat August 02, 2008 3:50 PM User is offlineView users profile

PSI is PSI no matter which refrigerant you are using. Issue is your vehicle is not designed to use R134a. So you have to make adjustments in refrigerant components and capacity for things to work correctly.


When considering your next auto A/C purchase, please consider the site that supports you:

CyFi on Sat August 02, 2008 4:19 PM User is offline

Ok, then in that case something seems pretty wrong with my system. Its acting as though i have a blockage somewhere but i just had the entire system apart and everything is good. High pressure is too high and low pressure is too low. Any advice on what else to look for?

94RX-7 on Sat August 02, 2008 6:34 PM User is offline

I'm wondering if the expansion valve you put in might be defective....that or you have a blockage in the system....but you flushed everything, so my money would be on a bad part.

Your pressures with R-134a will be a bit higher than the factory spec for R-12. 2.2 to 2.5 times ambient temperature on the high side.

Edited: Sat August 02, 2008 at 6:38 PM by 94RX-7

TRB on Sat August 02, 2008 7:09 PM User is offlineView users profile

Why an expansion valve? You have a refrigerant that operates at a higher pressure when using R12 components. Both low and high high pressures are up. Put money on it that a little water across the condenser and the temps will drop. Not really questioning your suggestion! But this seems like the typical R134a conversion post to me.


When considering your next auto A/C purchase, please consider the site that supports you:

Edited: Sat August 02, 2008 at 7:10 PM by TRB

CyFi on Sat August 02, 2008 7:14 PM User is offline

Well i thought pressures were supposed to be the same no matter the refrigerant? The problem is there is no circumstance where the condenser will be as cool as it is with water flowing across it, and when there is none flowing across it my low side pressures are pretty low and my high side pressures are pretty high. But regardless my vent temps are pretty good which i dont understand.

TRB on Sat August 02, 2008 7:41 PM User is offlineView users profile

That's why you need a more efficient condenser or more air flow! The water test just helps prove it's not a defective component. You are failing to make the transfer from a gas to a liquid. You can reduce the amount of refrigerant but then you lose the amount of refrigerant needed to pull the ambient temp from the vehicle along with poor oil circulation.


When considering your next auto A/C purchase, please consider the site that supports you:

CyFi on Sun August 03, 2008 1:25 AM User is offline

The stock condenser takes up all the available space and is already a parallel flow, so i don't see how that can really be improved by much but what do i know. I do have another question though, is there a way to easily lock the thermostatic fan clutch? I am worried about my engine temps also and it would be great to have a locked fan clutch, but at the same time i dont want it to be permanent i would like to be able to remove it, any thoughts?

94RX-7 on Sun August 03, 2008 9:17 AM User is offline

Originally posted by: TRB
Why an expansion valve? You have a refrigerant that operates at a higher pressure when using R12 components. Both low and high high pressures are up. Put money on it that a little water across the condenser and the temps will drop. Not really questioning your suggestion! But this seems like the typical R134a conversion post to me.

Low pressure is up? Huh? Didn't he say he was seeing low side pressures of 10-15 psi going as low as 5 psi? That doesn't jive with ~200psi on the high side in ambient temperatures of 75 to 80 degrees.

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