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Old 07-11-2017, 12:21 AM   #1
MikalCarbine
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Default School me on AC compressor pressures

My AC has been less than stellar ever since I bought my SL2 3 years ago. With recirc on and blower on high my center vent temps are around 60* while driving and vent temps warm up a bit when stopped/idling. This is with outside ambient around 105* in Phoenix. From the service records that came with the vehicle, it had a complete AC overhaul (drier, expansion valve, compressor replaced and condenser/evap flushed) back in 2012.

I hooked up my AC manifold to check the pressures and they both seemed on the high side so I figured it might be overcharged. I recovered some of the 134a and lowered the pressures. Prior to this I noticed that my low pressure schrader valve cap was missing, I tried cleaning the crud out of it with brake cleaner and compressed air but I guess I didn't do a good enough job because when I started it up this morning my compressor wouldn't turn on.

I picked up some new valves and caps and the system had no pressure when I hooked back up to it. I replaced the valves, pulled vacuum and put 24 oz of 134a back into the system. The pressures still seem high but I'm not very experienced with AC and could use some input.

These were taken in my garage, ambient temps between 105-110F

Pressures at idle:
Low side: 61psi
High side: 255psi

Pressures at ~2000rpm
Low side: 38psi
High side: 315psi

It seems a bit better not but not much, while moving at 70mph temps are near 50* but still warm up a good 10+ degrees while idling. I noticed that the compressor would never cycle, at idle or 2000rpm
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Old 07-11-2017, 02:17 AM   #2
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Default Re: School me on AC compressor pressures

1-Big mistake assuming a fully restored system is overcharged without asking here or someone familiar with vehicle refrigeration that can help with honesty. Once you connect gauges, you become the ac specialist whether for good or bad. In this case, without knowing what you did and giving a repair report, you probably removed refrigerant that didn't need to be removed based on assumptions. Never assume anything.

2-The 45 degree temperature difference between outlet vent temps and outside temps are near perfect and cannot be improved on. When you removed refrigerant, you reduced the full amount, period. No one will overcharge a system, including repair shops. Refrigerant is expensive and no one is going to overfill any repaired system. When you removed refrigerant you reduced the full amount to less than full and simply made a perfectly good system operate at less than ideal conditions.

3-By posting initial pressures, presumed taken at idle rpm, you posted for everyone the reason never to assume anything about pressures taken at idle rpm. a)No one drives at idle speed and b)Saturn service manuals specify recording all pressures at 2k rpm and comparing actual pressures with temperature/pressure charts in manuals to assess system performance. As you already see, there's a difference in operating pressures between idle and 2k rpm; compressors vary operating pressures at all times so 2k rpm is the established criteria for performance measurements. Idle rpm pressure measurements are useless to anyone familiar with vehicle ac systems.

It's only a guess on my part but when you removed refrigerant, you may have removed so much that the pressure sensor automatically detected the lower pressure on the high side below 40 psi and sends a disable signal to the pcm. This prevent the compressor from operating at less than ideal amounts of refrigerant to protect the compressor from self destruction. Compressors will self destruct if allowed to run with less than half a pound of r134a. Refrigerant is a liquid under high pressure and moves lubricating oil - without refrigerant, oil won't circulate back to the suction side, enter and lube internal compressor parts. You emptied the system and the pressure sensor detected it, sent a low pressure signal to the pcm that disabled compressor operation.

4-Replacing the service valves, caps and performing an evacuation before refilling the system again with correct amounts of r34a hasn't improved on cooling because its running at maximum output. Your're in desert country and probably noticed many vehicles with dark tinted windows. The greater heat generated in desert country and larger infrared radiation simply heats up the interior, requiring more cooling in small cars. If you don't have tinted windows, either consider it to lower the heat loads on the ac system or use other materials to block the sun from entering the interior. 60F vent temps are a whole lot better than feeling 80F, 90F or 100F vent temps.

Unfortunately, while idling and infrared heat entering non tinted windows in small cars, the heat loads are greater while the ac is trying to maintain cold air. Using recirculation mode may help a little but dark windows is a distinct advantage in high heat areas. While not great for night vision thru dark side windows, using portable screens that are removable are options to consider besides tinting. Anything you can do to reduce the sun ray into the car lowers heat loads on the ac system.

5-Saturns do not cycle compressors. All assumptions are to be ignored whatever you know about other vehicles and how their compressor may cycle in normal running conditions. Saturns run with compressors on when ac is used.

Below are temperature/pressure charts with engine running at 2k rpm.
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Old 07-11-2017, 07:24 AM   #3
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Default Re: School me on AC compressor pressures

Honestly IMO your system was working at it's design limits IOW pretty normal. I wish we had R12 back again LOL. Back in the day we could pull AC temps just above freezing.
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Old 07-11-2017, 08:35 AM   #4
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Default Re: School me on AC compressor pressures

^^ 1+ We used to have FROST on the outside of the tubing! Those days are gone.
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Old 07-11-2017, 12:55 PM   #5
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Default Re: School me on AC compressor pressures

R134a makes ice. Its used in most home refrigerators when r12 was discontinued. R134a used in vehicle ac systems can make ice easily but better designs in refrigeration systems prevent it from occurring. Orifice tubes and thermal expansion valves are two ways to control vehicle refrigeration temperatures against ice box conditions.
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Old 07-11-2017, 03:05 PM   #6
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Default Re: School me on AC compressor pressures

Quote:
Originally Posted by fdryer View Post
R134a makes ice. Its used in most home refrigerators when r12 was discontinued. R134a used in vehicle ac systems can make ice easily but better designs in refrigeration systems prevent it from occurring. Orifice tubes and thermal expansion valves are two ways to control vehicle refrigeration temperatures against ice box conditions.
"No ice by design" - I get that. Sure would be nice if they let it get down to 38 F though...
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Old 07-11-2017, 04:59 PM   #7
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Default Re: School me on AC compressor pressures

Yeah, for sure!? I don't know why my L300 outputs no less than 42F at the center vents whether its 100F or 55F (high humidity). I don't remember gauge pressures when I repaired my last leak over a year ago (low side gauge pressure/temperature chart correlation) and low side tends to display low side pressures that's supposed to have outlet temps close to t/p charts. Vues tend to show outlet temps around 35F or a little lower, as seen from pics snapped by members of their thermometers. As far as txv settings, the engineers could have calibrated them to 35F. That would satisfy many but I'm not an HVAC engineer that designed the txv.
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Old 07-11-2017, 09:45 PM   #8
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Default Re: School me on AC compressor pressures

Thanks for all of the info guys, lesson learned. After recharging it and driving it today it was cooling amazing on the highway (below 50*F) with OAT over 100*

Is it normal to see such a difference when moving vs stopped? I'm going to clean up the bugs and gunk from my condenser to see if it helps at all, I went through this exercise last year and don't recall it helping much. Driving on the highway never gave me issues but putzing around town leaves something to be desired
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Old 07-11-2017, 10:36 PM   #9
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Default Re: School me on AC compressor pressures

You're only looking at half a system. The condenser coil is out front and subject to abuse including picking up blocking material - plastic grocery bags, papers, etc.. The other half is the evaporator coil buried inside the hvac box. Since GM and many car manufacturers didn't see fit to add a cabin air filter now popular on many cars and suvs, the S-series may suffer from long term cabin ventilation issues of collecting debris over the years. Guess which coil gets blocked first in the hvac airflow? Heater core or evaporator coil? Drop the blower motor and stick your cellphone in there for a few snapshots or video the inside. You may be surprised to see what may have collected over the years to block airflow. One recent member did and posted his pics.

At low engine rpm, the compressor runs slower and suction isn't as great. With higher suction pressure/lower suction volume/idle rpm, less refrigerant is sucked away from the evap coils, raising cooling temps. When compressor speed increases and stays above 1k rpm, suction pressure lowers and more refrigerant (volume) is sucked from the txv. The lower flow rate at low speed/idle means lower cooling from less refrigerant being sucked back into the compressor suction side. Higher compressor speed means much lower suction pressure as more refrigerant, by volume, is sucked thru the evap coils to absorb more heat resulting in greater cooling. Consistent compressor speed above 1k engine rpm increases the volume of refrigerant sucked thru the evap coils to absorb more heat. There's a delicate balance between too much compressor speed and too little (idle rpm vs redline) and the ac system tries to balance varying compressor speeds to cool interiors. Unfortunately, small cars suffer more from low speed, stop and go traffic in hot and humid conditions - the worst combinations possible. Parking under a shady tree and letting the ac system run for a few minutes helps as the decreased sun/infrared heat isn't absorbed to allow a hot interior cool off. A difference of a few degrees can be felt almost immediately and helped with any form of sun shielding; tinted windows, shades, whatever, to cut down on infrared heat absorption.
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Old 07-29-2021, 08:21 AM   #10
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Default Re: School me on AC compressor pressures

Same thing here. Thanks for sharing.
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Old 07-30-2021, 06:36 AM   #11
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Default Re: School me on AC compressor pressures

Ohh man, it sounds terrible. I think you should fix it somehow. If it were working like this, it would explode. Is your warranty still available? It would be best if you used it because this way you will fix it cheaper. A lot of people had problems like this, and they just bought a new one. Also, if you have issues, you could contact air conditioner servicing singapore. It might save you time because you don't how much time you would be looking for someone who can repair it. I hope that my advice will help you because I would never tell you about a random company.

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Old 08-03-2021, 02:49 PM   #12
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Default Re: School me on AC compressor pressures

Our cooling fans slow down with age. A new fan may help your idle performance.
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