A/C randomly blows off pressure when Idling

Tiny
KEITH WARKENTIN
  • MEMBER
  • 2003 GMC SIERRA
  • 6.6L
  • V8
  • 4WD
  • AUTOMATIC
  • 285,000 MILES
Very odd situation, I own a shop and have a new A/C machine. System holds vacuum and has no leaks. When properly charged, system blows nice and cold. This vehicle has a brand new compressor, accumulator, O-tube, fan clutch, and the condenser has been flushed and blown out. Also has a new high pressure switch, which apparently does not actually shut off the compressor when the pressure gets too high.
What it does is randomly blow off the Freon. Usually if I catch it quick enough it will only lose about a half pound of the 1.7 this system holds. But I even drove it with gauges on it and watched the pressures, which were perfect while driving, and most of the time even while stopped. But occasionally would climb to 450 psi plus and I would shut it off so it wouldn't blow and as soon as I started moving it seemed to drop the pressures. Also, while in the shop I had it start increasing pressures, like there was not enough air flow, but then the fan started pulling a bit more, still at idle, but the pressures dropped, and it was blowing cold.
Everything is in a stock configuration, all original components are in place.
More oddly, my brother has a 2003 Chevrolet Duramax, his is manual, mine is automatic system, that just recently did the same thing. Blew off most of the Freon at idle.
Of note, the High and Low ports are only about a foot apart, with the O-tube separating them and the pressure issue shows on the gauges, so it is not a blockage in the condenser or elsewhere. It appears to be an air flow issue. But should cut itself off I expect that it was doing that previously, because it would go into swamp cooler mode at idle? Warmer humid air, but now, instead it blows off the pressure.
Thoughts of ideas are appreciated. I am going to look at the electrical to try to see why the over pressure does not cut off the compressor clutch.
Thanks,
Keith
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Tuesday, September 3rd, 2019 AT 10:16 AM

35 Replies

Tiny
KEN L
  • ADMIN
Hello,

Try a new pressure switch to see if that stops the high pressure, also unplug it while running to see if the system shuts off. Next it sounds like you have too much oil in the system which if you added any without a large rupture to disperse the oil, you have too much in there. The compressors come with oil in them) High pressure comes from high temperature as well, do you ever here the clutch fan come on? Please get back to me.
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Wednesday, September 4th, 2019 AT 6:17 PM
Tiny
KEITH WARKENTIN
  • MEMBER
As stated in the question, the pressure switch (high pressure transducer) is new, along with a new fan clutch, and the new compressor, but I will replace it again in case it is a bad/defective one. I replaced it as I suspected it to have been causing this issue, but the new one had the same response as the original one, or should I say lack of response. While running in the normal pressure range of about 250 psi it was around 2.5v, and it does shut off when unplugged. I had previously confirmed this part. Although I cannot find any specifications on pressure to voltage for this component. It was also right at 1 volt when I last charged it, before turning the system on, so static pressure.
Thanks
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Thursday, September 5th, 2019 AT 1:04 PM
Tiny
KEN L
  • ADMIN
Lets check the connection between the pressure sensor and the PCM. here is a guide to help and the air conditioner wiring diagrams to help (below). It could be a bad PCM.

https://www.2carpros.com/articles/how-to-check-wiring

Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.
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Thursday, September 5th, 2019 AT 1:22 PM
Tiny
KEITH WARKENTIN
  • MEMBER
I have already checked values, but my MODIS only tells me A/C "granted" yes or no. I would like it if it had a voltage reading also, but it does not. I already had looked through the wiring diagrams, 3 wires straight to the PCM. But it seems nearly unfathomable that the PCM would fail in that manner, when it will turn it off by unplugging it. I think I will feed it 5v by jumpering the sensor connector and see if it says not granted. That would tell me most quickly if it keeps it off, or allows it to turn on. I will keep you posted.
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Thursday, September 5th, 2019 AT 1:34 PM
Tiny
KEITH WARKENTIN
  • MEMBER
So I just confirmed that it has 5v reference, ground, and signal return. Also that the system will deny the A/C when fed 5v from the reference to the signal return. So I will get a factory sensor (transducer) and see what that does. Although it still won't answer why it randomly goes high in the first place. It makes no difference if it is completely cold, fully hot.. But always at idle while stopped. Sitting at a stop light, sitting in traffic, waiting for a train. Where it used to just randomly cut off the compressor until you began moving again.
Attached are the values. Static, system off and running at Idle.
Again, I'll keep you posted.
If anyone has a voltage to pressure value, that could be useful, but I have not found one yet. (Although it appears to be about 1v to 100 psi)
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Thursday, September 5th, 2019 AT 2:29 PM
Tiny
KEITH WARKENTIN
  • MEMBER
To clarify that last test, it was done while the engine was running and A/C on, working normally. Then fed the 5v to the sig ret. As though it got there suddenly on its own. While there was a several second delay, it did deny the A/C while fed, then after about 15 seconds after removing the 5v it granted and began running the compressor again.
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Thursday, September 5th, 2019 AT 2:57 PM
Tiny
KEN L
  • ADMIN
The pressure looks a little low. so you say it goes to over 350 and blows the pressure valve? sounds like too much oil. Have you checked that? Also, once the pressure valve blows you need to replaced it because it becomes weak after the first event. lastly I have seen PCM's go out in every way possible so this would not surprise me FYI. Here is how you would replace the PCM. Here are the air conditioner wiring diagrams as well:

https://www.2carpros.com/articles/how-to-check-wiring

Check out the diagrams (below). Please let us know what happens.
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Saturday, September 7th, 2019 AT 12:31 PM
Tiny
KEITH WARKENTIN
  • MEMBER
So, the over pressure valve does not appear weak (and holds), and the aftermarket compressor does not come prefilled with oil (per warnings on the box, and installation instructions), so it is unlikely that it is overfilled with oil. Beyond that, that should be a consistent issue, and not random. The new AC Delco high pressure transducer has corrected the blow off issue, apparently the aftermarket does not meet specs (or was simply defective), and the original one had failed as I suspected. So the PCM is not the issue.
Currently, it still randomly goes into the "swamp cooler mode". Where the compressor cuts out due to high pressure, and you get very humid air, and no A/C. But this is not predictable as to when, and works normally as soon as the vehicle starts moving again. It may do it right out of the driveway, at the first signal, or go several days before doing it and when it began doing it in the shop, it seemed to be headed towards blow off. Achieving over 450psi, when the fan started pulling a little more air, and the pressures dropped back to normal. The pressures in the pictures were a little low, as I had not realized it had blown off about 5 ounces of Freon.
So currently I have not found an explanation for the random high pressure, as it seems to simply be an air flow issue. But has a new fan clutch and all (radiator, inter-cooler, condenser)components were blown out, and do not appear to have any restriction.
I am considering putting an auxiliary fan in front of the condenser to "fix" the issue, but that should not be needed.
(Also, the pressure does not blow until about 550 PSI, and is supposed to cut the compressor at 426 PSI).
Thanks,
Keith
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Monday, September 9th, 2019 AT 11:15 AM
Tiny
KEN L
  • ADMIN
Yep, sounds like too much oil in the system the oil being forced through the orifice causing the high pressure. I would vacuum the system to take oil out then remove the low side hose to look inside to see how oily the system is. Can you please shoot a quick video with your phone so we can see what's going on? That would be great. You can upload it here with your response.
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Monday, September 9th, 2019 AT 11:19 AM
Tiny
KEITH WARKENTIN
  • MEMBER
It appears to call for 7.1 ounces. And I have not added much back to the system since all the blow off, and loss at the compressor. During the recovery process, it had only removed a few ounces, and I have not been adding it back, concerned about too much oil. Last time I recharged it I added back 3 ounces. After not doing so previously.
I have never experienced a random issue like this, where all it takes is air flow to correct the issue. And searching for symptoms of being over charged with oil, I do not find anything like this, as it blows completely cold, with no symptoms except the random high pressure issue. (Note:I have been working on A/C systems since I was 17 years old, so the last 36 years)
As stated, this only happens at Idle, when stopped, so I'm wondering if there is a symptom chart that suggests it is over charged with oil? Perhaps something unique to Duramax systems, even though they are just a standard GM configuration.

(SYMPTOMS of too much oil
Noisy compressor;
Low oil level in the sight glass on the compressor's crankcase;
TXV has a hard time controlling superheat (hunting);
Low evaporator and compressor superheat; and.
Warmer-than-normal box temperatures with loss of capacity.)
(Also If you add too much oil to the system, it may pool in various places of the system and can coat the inner walls of the evaporator and condenser coils. This will diminish their ability to absorb or dissipate heat, thereby bringing down the overall cooling performance.)

Essentially, everything I have seen and experienced about excessive oil are performance issues. Not random over pressure, but continuous poor performance, instead of random. Where over pressure issues are normally caused by air flow, or plugged fins on the condenser.

Also, if that were the case, it seems the O-tube would be saturated each time I removed it to inspect, and while it has oil on it, there has never been oil coming out of the line when opened up.

Also, to make sure I cleared any blockages. Last time I opened up the system, I pulled the orifice tube and activated the compressor. Then put my thumb over the line to build pressure, then released it several times to make sure there was nothing plugging the system. Which should have also gotten extra oil out of the system.

Just trying to understand what suggests to you that oil is the issue.
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Monday, September 9th, 2019 AT 1:25 PM
Tiny
KEN L
  • ADMIN
I noticed the Denso replacement comes with 8 ounces of oil in it. Did you add oil when doing the job? Check out the diagrams (below). Please run this test and get back to us.
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Tuesday, September 10th, 2019 AT 10:45 AM
Tiny
KEITH WARKENTIN
  • MEMBER
Not sure which test you are referring to? Apparently the Murray compressor I installed comes pre-charged with 3 ounces of oil (not 8 as you said the Denso does), and after flushing the condenser, and replacing the accumulator, and having blown pressure off at the compressor several times. I have added approximately 6 ounces total. Which means that (if anything) the system would be low on oil.
Again, is there some symptom causing you to believe that the system can work completely normally, blowing cold, with no issues. Except randomly goes high, when stopped at Idle. Apparently with less air flow. And the system cuts off (again now with the new high pressure transducer, instead of blowing the pressure relief.) Until you start moving again, and the pressures go back to normal?
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Tuesday, September 10th, 2019 AT 1:22 PM
Tiny
KEN L
  • ADMIN
You cannot get the oil out of the system it sits in all the low spots, I would judge the systems oil by inspection. Open the low side hose and look inside if it is super oily you have too much. Please undo the low side and upload a video or picture of the inside of the tube for me please. It is the only thing that can cause this issue.
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Wednesday, September 11th, 2019 AT 10:04 AM
Tiny
KEITH WARKENTIN
  • MEMBER
So to answer all the questions, I got about 3 drops of oil from the compressor end of the low side hose, as shown, there was about a drop on the orifice tube. The video is of me putting 150 pounds of shop air through the condenser to see if any oil would come out, which it did not. As noted on the white paper. The only oil was from the orifice tube sitting on it prior to blowing out the condenser.
If there were too much oil, I should have gotten some from the condenser. As that is one of the places oil will puddle.
Your suggestion that oil going through the orifice tube would cause the random high pressure should require there to be significant oil running through the system. Yet that is not what I find.
I cannot find any other suggestion that oil will cause this. Usually it causes a performance issue.
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Wednesday, September 11th, 2019 AT 12:39 PM
Tiny
KEN L
  • ADMIN
Thanks for the pictures and video, the oil looks good, I will put this one out to the community to see what comes back but, does the clutch fan kick in can you hear it come on? The only other thing that can cause this is high temperatures. Only can you not fill the system all the way to see how it performs? Just a little to much Freon can cause this as well.
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Thursday, September 12th, 2019 AT 9:35 AM
Tiny
ASEMASTER6371
  • EXPERT
Okay, first, Can you recover the Freon from the system and see how oil comes out after it is recovered?

I do not think it is an oil issue as it would do it all the time not randomly.

On the pressure switch, was there a color on the old one? A lot of the high-pressure switch came in colors. The colors were for the open circuit pressure. Some were high, some were low. The compressor should kick off at 375. The compressor blow-off is 425.

I almost sound like there is a piece of debris floating around the high side that causes a blockage at times.

What is the low pressure?

Roy
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Thursday, September 12th, 2019 AT 10:03 AM
Tiny
WRENCHTECH
  • EXPERT
The issue here is the pressure is intermittently spiking. There are a couple of causes for this.
First and foremost, these condensers are bypass type and cannot be flushed. It may appears to be clear because your air is flowing freely but in reality the condenser can be 90% blocked and still appear that way due to design.
The other thing I have seen numerous times is the discharge hose being ruptured internally and "flapping" causing an intermittent blockage. I have even seen this happen when the pressures appear normal due to the location of the service port being downstream of the obstruction.
The third thing is I believe this fan is Pulse Width Modulated so use the scan tool to test that it is capable of switching to high speed.
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Thursday, September 12th, 2019 AT 10:17 AM
Tiny
HMAC300
  • EXPERT
To me it sounds as if the screen at orfice which on older models was replaceable. Partially blocked or getting that way.
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Thursday, September 12th, 2019 AT 11:28 AM
Tiny
WRENCHTECH
  • EXPERT
A plugged orifice will not cause high discharge pressure. It's on the suction side of the system.
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Thursday, September 12th, 2019 AT 11:30 AM
Tiny
BMDOUBLE
  • EXPERT
It's also possible to have the dessicant come out of the receiver drier and clog part of the system, and also the rubber lines can come apart from the inside. Just a thought.
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Thursday, September 12th, 2019 AT 11:43 AM

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