Engine Size: 350
Refrigerant Type: R-12
Ambient Temp: 100
Pressure Low: 20-30
Pressure High: 230-240
Country of Origin: United States
Hey guys need some help with my R-12 a/c system. Short run down on what I have done. I have replaced everything on my 89 Chevy C2500 Standard Cab 5.7L Truck. New compressor, all new hoses, condenser, dryer, orifice tube, new o-rings the works. I did not change the evaporator but I did a serious flush on it and blew it out with shop air for about 2 hrs. I installed everything, put nitrogen in at 80 psi and waited for an hr no leaks, I then put a vacuum on it for an 1 ÃÂ½ waited for an hr no leaks. I installed 6 oz of mineral oil and 40 oz of R-12 per manufacture. A/c blows very cold. Maybe to cold.
Here are my reading at IDLE
Low pressure is 31 and high pressure is 240 is this OK at Idle
Humidity 20 percent
Temp outside 1 foot in front of condenser 100
Temp entering the condenser 161
Temp exiting the condenser 126
Temp entering the evaporator 24
Temp exiting the evaporator 28
Air temp at center duct in the truck is 40.4 Remember all of this is at IDLE with doors and windows closed a/c on high speed.
At 2000 RPM
Low pressure is at 24-20 after about 30-45 sec at 2000 rpm it hit the low pressure switch, pressures went to 40 psi and the compressor came back on. When it got down to 20 psi frost started to form on the dryer and as soon as the compressor hit the low pressure switch it immediately went away. The suction line at the compressor does not look like it freeze up though.
Inside duct at 2000 RPM was 36-37 degrees
Truck turned OFF
Low pressure 70 and High pressure 70
Can anyone give me any insight on this. Is this ok?? I checked my evaporator today and it is clean with no leaks and the temp on the fins where about 111-116 degrees. It just seems so weird that the inlet line for the evap. is 24 degress and the outlet line going to the dryer is about 28 degress. That to me seem to cold and would freeze the evap. but its not I check. The evap is not cold at all.
When checking your pressure do you do it at IDLE or 2000 rpms???
What should my pressures be at? Do I have to much R-12 or not enough in the system. Should I add a little more R-12 since the pressure at 2000 rpm is in the 20 psi range?
Thank you for reading this and for any help that can be given.
Are you sure you are checking the evaporator? 111f at the fins - with 37 degree vents? Does not seem right.
Your pressures look ok for 100f ambient. How is the airflow volume from the vents? I would suspect a weak cabin fan or clogged up air side on the evaporator.
"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.
It's good to see you made it over here. Your evap temp should be somewhere between your evap inlet temp and the outlet temp so I don't know if going through all of the trouble of trying to access the evap will tell you any different.
Usually when you have a correct charge, your evap inlet and outlet temps will be close to the same or within a few degrees of each other which yours are very close.
It's hard to think you are undercharged by your vent temps but if the compressor is cycling as much as you say, its possible.
I wonder if your cycling switch is malfunctioning. Is it original or has it been changed out.?
I don't think taking out the evap will tell you anything. I doubt the evap is dirty or else your vent temps would probably be higher than they are. You might just see about taking out your fan and seeing if you can at least see one side of the evap to see its condition. Does your fan mount under the dash?
Is your OT the correct one for your application? The OT meters a steady flow of refrigerant while the compressor is operating and meters the exact amount of refrigerant required to mantain proper heat transfer efficiency. This action also ensures that all of the liquid refrigerant will have changed to a vapor by the tiime it reaches the evaporator outlet. The cycling switch turns the compressor on and off. The intermittent compressor operation controls the refrigerant flow and pressure.
I've been reading my AC repair manual and found where it said that the evap outlet temp should be approximately 4 degrees higher than the inlet temp which is exactly what yours is so you are good there.
It also said that excessive moisture can accumulate on the evap coils causing the evap temp to be so low that the moisture would freeze on the evap before it could drain off. So, if that happens, its possible you have excessive moisture in your system.
I wouldn't worry about liquid refrigerant getting to the compressor since part of the job of the accumulator is to store any liquid refrigerant that exits the evaporator.
I personally would leave things as is for now because you have exceptional cooling and everything else seems pretty good.
Hopefully some others on this site with more experience than I can give you some input to determine if you have a problem or not.
Thanks Wayne for all the info. Yes the blower motor is underneath the dash. I might give that a try. I really hope I don't have moisture in the system. I vacuumed it down for 1 1/2 hrs with a 7cfm vacuum. You think that was enough our should I drain it and vacuum it down for 24 hrs?
As far as the right OT I bought a brand new line from the dealer and the OT that came in the line is the one I used. The line was for my year and model truck. Hope that was the right OT. It should have been. I agree with you if the evap. was plugged you would think the air flow would be down and the temps would be alot higher. Just not understanding how the inlet temps on the Evap. is so cold(24) coming into the evap. Maybe its normal for it to be that cold just seems with them being that cold it would freeze the evap especially with humidity in the air making the coil sweat it would freeze guess not.
Again thanks so much for your help and your answers. Noone else has responded so this must be a pretty tough question and you are the only one to seem to know anything about it. Congrats to you. You know more then you might think you do.
I think you vacuumed it down long enough. I don't even vacuum the ones that I have worked on for that long. I use a 6cfm vacuum pump so yours is stronger than that. I would not reclaim and start over because I don't feel you need to plus R12 is not cheap unless you have a way to reclaim and reuse what you remove.
I will have to assume that your OT is the correct one. Was it the same color as the original one.?
You have to remember that your fan is drawing the air from the inside of the car so even though your evap temps are below freezing, the air inside your vehicle is not. Also R12 usually is colder than R134 so that may be why you are seeing lower evap inlet and outlet temps.
I have only repaired two R12 vehicles and those are my two 92 Cavaliers and one of those I converted to R134. I do know that my Cavalier that I kept R12 will freeze me out of the car. I may have to take some evap inlet and outlet temps to see what they are. I do remember that when I fixed this system about three years ago, I was getting vent temps in the mid to high 30's.
There are a lot more knowledgable people than I am about this stuff on here, they must just be busy today and have not been here yet. The other guy that responded to you has been on this board for the five years I have been here and is very knowledgable about this stuff.
Wayne the new OT is the same color as the old one. I know there are pretty smart people on here I was just trying to make you feel good because you seem pretty smart on this. I thought the same way you did on all of these questions but it nice to hear the same thing from another person.
Thanks again for all the insight..
It seems to me that you're over-thinking this. By any measure, the system is kicking butt. If you do actually experience evaporator freeze-up you'll need to adjust or replace the cycling switch. The accumulator is designed with baffles to keep liquid out of the compressor.
Thanks mk378. Your right I am probably over thinking this. Just want to be safe. Just drop a couple of dollars in the ac and want to make sure I don't mess it up being so new.
The cycling switch is brand new and not adjustable. Do you know of an adjustable cycling switch I can pickup for R-12 1/4 thread?
Thanks for the info
Well, thanks for the kind words. Most of what I have learned, I learned here and reading a couple of auto AC manuals.
I've repaired around 10 vehicles in the last five years and some stuff has been a learning experience. I definately don't know it all when it comes to this stuff.
Wayne let me run something by you. I decided not to tear the dash apart. So I took the truck down the road for about 4 miles and the vent temps were right around 46-47 and stayed there. I got home and did another test by just having the truck in park and reved the motor to about 1500-1800 RPM's and let it sit there for about 3-4 mins. Vent temps went to 34.9. When it got that cold I jumped out of the truck to look at the dryer and lines and the lines on the dryer had ice all over them and so did the dryer. What do you think of that. The line coming out of the dryer heading back to the compressor was starting to get ice on it too. Now grant it the dryer and lines do sweat and I think it was getting cold enough to freeze the sweat. Think that will be ok and just leave it alone and just drive it? It seemed to get colder in park at about 1800 rpm's(34.9) then it did driving down the road at the same rpms(46-47) Whats your take on that?
Roger, it doesn't make much sense that you would have colder vent temps at idle than going down the road but you have to remember that the vent temps won't be as cold initially as they will be when the system runs for a little while and is why you might be seeing the colder vent temps when you got back home. Also initially the inside of your vehicle is going to be fairly warm and vent temps will be higher until the inside of the vehicle starts cooling off.
Like mk378 said up above, you might need to adjust or replace your cycling switch if you actually think that the evap is freezing up. I know you said its new but it could still be bad or not working correctly. For as long as you vacuumed down your system, I find it hard to believe that you would have still have moisture in the system. If you let it run for awhile, do the lines actually start forming a hard shell of ice or is it more like frost?.
I can't remember if you said, but how old is your fan clutch.? I don't really think its an issue since you are cooling so well but you never know.
Hey Wayne I agree. I am doing some new test and will get back with you on them. I went down and got another low pressure switch today. Fan clutch works excellant. I did some test during the day and it seems everything is werid now. I ordered a CPS TM250 Temp-Seeker and when it comes in I can test everything. I will let you know how it goes.
Hello all I am new to this site and this is my first post.
First off I dont know very much about automotive AC (this is why I joined this site) but I do have a Universal EPA certification and completed an HVAC course so I have the theory of refrigreration somewhere in these old brain cells lol.
Secondanly i dont have a clue what the problem is in this system but something did jump out at me, 4 degrees of superheat tells me that evaporater is nearly full of refrigerant which may be how they run in autos but in a typical home split system youde be about 10 degrees. and running a system that close to full seems to me that with a low heat load (i.e. low air flow across evap) it could freeze up quickly and as you know once it frost you have no heat transfer. could there possibly be a restriction in or at the input of the evaporater causing the pressure to drop to the point it freezes or could the fixed orifice tube be too restrictive? I know you said your gage read 26 psig and according to the r12 PT chart (linked to this site) for a 100 outside temp on the low side that would be correct, but if you got frost on your lines i suspect the pressure is lower than what your gage is telling you?
I cant see water being in the system as the head pressure is a little low according to the PT chart (linked to this site) for the temp you gave. honestly a sightglass would be a huge help in all this.
Like I said I dont know and aint trying to sound like I do, this will be interesting to see what the culprit is.
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