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Old 06-30-2022, 12:24 AM   #21
steve seibel
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

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The compressor throttling is completely independent from the dash controls. The throttling is controlled by pressures within the compressor.

Have you evacuated the system and put in a known-correct charge yet?
No-- what is the correct charge?
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Old 06-30-2022, 12:39 AM   #22
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

I think it is 1.5#, but will have to look it up tomorrow to verify if somebody else doesn't.
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Old 06-30-2022, 03:39 AM   #23
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

steve seibel, below is a diagram of a car ac system. Saturns do not allow manual control of the ac system. In one sense, either ac works or its dead. Nothing in between. Once running, Saturn ac doesn't cycle its compressor - its running all the time. The variable displacement mechanism within the compressor helps to discharge anywhere from minimum displacement (volume of refrigerant) to maximum to reduce loads on the engine. The txv does the actual work of metering liquid refrigerant into the evaporator coils. With the compressor always pulling a vacuum, the metered refrigerant expands to absorb heat from the fins on the evaporator coils as warm air flows thru.

29 psi low side, 135 psi high side means the compressor is working. A stuck txv would have one of three possible symptoms.

1-If broken in the open position, low side pressures may be higher than normal, higher than normal vent temps. High side pressures are normal.
2-If broken in closed position, refrigerant can't flow so high side pressures ramp up while low side pressures drop with 0-5 psi from the vacuum created as the compressor tries to suck refrigerant from the evaporator coils that doesn't have any with a closed txv. High side pressures may be between 70-100 psi.
3-If partially restricted, suction pressure would be slightly lower, vent temps are higher than expected, high side pressures may be close to normal.

These are from Saturn service manuals. Service manuals never describe sealer use since sealer can damage a system by clogging passageways in a compressor, block the metering valve in the txv, clog evaporator coils, clog condenser coils, etc.

billr may be on to something not answered when he asked at the beginning of this thread - unless you know exactly how much refrigerant is in a system, the only way to know is removing refrigerant and evacuating the system to near absolute vacuum, 29.99 in hg then refilling with 1.5 lbs or 24 ozs or two cans (12oz/can) of r134a (no sealer). Pressures will never determine how much refrigerant is in a system. Basic ac systems start with the entire plumbing sealed and evacuated to hold a complete vacuum as close to absolute (29.99 in hg) when a vacuum pump is removed. A system capable of holding a near perfect vacuum should hold whatever refrigerant is used because refrigerant pressurizes a system before the compressor runs. If a system holds vacuum or pressure, the correct amount of refrigerant will not leak out, lowering cooling efficiency. A guess would be losing 1/3rd to 1/2 of a systems refrigerant when cooling is lost while pressures appear close to normal. To understand ac systems would require understanding gases used, temperature/pressure relationships of the refrigerant and expected pressures in an operating system. Service manuals provide values for anyone using gauges. Second guessing can be an expensive way of servicing ac systems.

This may be as simple as not enough refrigerant in this system. One 12 0z can of r134a can produce pressures you observed. If less than 24 ozs of r134a is in this system, adding refrigerant should show an immediate rise in discharge pressures, high side.
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Old 06-30-2022, 07:36 AM   #24
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

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Also noted utter lack of any evidence of cooling when I observe or touch the hoses leading to and from the condenser. They are not the least bit cool to the touch.
FYI, the condenser coils are in front of the radiator, to condense hot pressurized refrigerant gas discharged from the compressor. ...

There should be one output return hose going back to the compressor feeling cold and sweating from humid air condensing on this return line. The input to the evap coils should be lukewarm or the same as surrounding air. Both lines are on the firewall with two more lines from the engine cooling system. All four lines are grouped close to each other in the center of the firewall and downwards. On a running engine, you'll know the two hot coolant lines by feel. The other two lines are for ac.
Apologies, I meant to say "evaporator" not "condenser" in the quoted line. In the original post at very start of this thread I said it correctly, except might have been more clear if I had said hoses "to and from" evaporator, not just "to". So once again for clarity, I'm sure I did identify the correct hoses, and the return hose coming *from* the evaporator on the firewall, coming back *to* the condenser by the radiator, is not the least bit cold to the touch, and shows no evidence of moisture from humid atmosphere condensing on it. Surely that observation must play some role in helping us figuring out what is going on here? (And same is true of hose going *to* evaporator, but that's to be expected.)

Thanks again-- Steve
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Old 06-30-2022, 07:58 AM   #25
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

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Yes, the compressor runs all the time, and the expansion valve does not change. Cooling is varied by throttling the inlet to the compressor, a rotating port-plate inside the compressor. Some consider this throttling makes the compressor "variable-displacement" and call it that.
So in the Saturn a/c system, the thermal expansion "valve" is just a completely fixed orifice that is completely non-adjustable, as well as completely non-variable when the system is in operation? Or is that not an accurate description?

And if accurate, is it still true that the "valve" is prone to failure in some way-- perhaps by being blocked by some sort of debris or residue?

I keep coming back to observation that based on feeling temp of return hose coming from evaporator, there seems to be no evaporation happening at all-- the hose is not at all cool to the touch
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Old 06-30-2022, 11:33 AM   #26
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

billr is incorrect. The thermal expansion valve controls refrigerant flow, period. When metering liquid refrigerant, it controls actual temperatures occurring in the evaporator coils. There is no throttling in Saturn vane compressors. The complicated mechanism in Saturn vane compressors varies displacement. If you want to learn how vehicle ac systems work, do your research on how TXVs operate. If you aren't familiar with technical descriptions of refrigeration systems then you'll be on a steep learning curve. The best concise information for Saturn ac systems comes from Saturn service manuals, not hearsay. Guess where I get my info from to share with Saturnfans members?

Thermal expansion valves open and close a needle valve to meter liquid refrigerant. A diaphragm connects to the needle valve and regulates refrigerant flow by measuring temperatures; opening to let more refrigerant flow into the evaporator coils, close to restrict flow. It's constantly changing.

The compressor does two things; suck refrigerant from the evaporator coils and compresses gases. Most compressors fail by loss of refrigerant. It can't suck or compress when refrigerant leaks out.
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Old 06-30-2022, 12:07 PM   #27
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

Well, we agree on one thing: 24 oz = 1.5# for the correct charge.

I think we also agree that the TXV is not controlled by anything external, but can fail due to clogging or malfunction of parts internal to it.

Maybe we can agree on this: you can't just "add another can" of refrig, you have to fully evacuate and then put in two cans to get a known-good charge.
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Old 06-30-2022, 01:54 PM   #28
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

Yes, I agree the only way to know if 1.5 lbs of refrigerant is in a system is by removing what's in there (theoretically into a reclamation canister) then evacuating the empty system completely before refilling with two 12 oz cans of plain r134a.

Experienced hvac techs and diyers with many ac repairs can guess accurately by interpolation of operating pressures relative to ambient temperature and humidity in accordance to service manuals. Pros use electronic scales to weigh bulk canisters before and after filling a system (15, 30 lb canisters). At several hundred dollars a pound of other refrigerants for homes, offices, buildings, refrigerant by weight reduces waste.
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Old 07-01-2022, 02:48 PM   #29
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

Thatís how Iíve done it some times. I use the system performance chart and add until I get in the range of suction and discharge pressures based on outside temp.
The performance chart tells what RPM to have the engine at. Which is very important.

Iíve also vac them out and made sure it held a vacuum overnight. I have a 6cfm yellow and it pulls down small systems quick.

If I have an empty system I always put new Schrader valves in. They usually cause the leaks.
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Old 07-03-2022, 11:27 AM   #30
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

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billr is incorrect. The thermal expansion valve controls refrigerant flow, period. When metering liquid refrigerant, it controls actual temperatures occurring in the evaporator coils.
...

Thermal expansion valves open and close a needle valve to meter liquid refrigerant. A diaphragm connects to the needle valve and regulates refrigerant flow by measuring temperatures; opening to let more refrigerant flow into the evaporator coils, close to restrict flow. It's constantly changing.
And it does this in a manner that is completely self-contained, i.e. there is no tube going to a sensing bulb in the evaporator, or no electrical wire going to a temperature probe?

On page 3-14 of my Haynes manual (I don't have the actual service manual, obviously) in the description of how to replace the TXV, nothing is shown plumbed into the TXV besides the two refrigerant lines.

This is different than other cars I'm familiar with, where there is a line coming out of the TXV to a temperature-sensing bulb on the evaporator. So fdryer, can you confirm than that the temperature-sensing is completely internal to the TXV unit on this car?

Thanks for helping me understand this system--

Steve
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Old 07-03-2022, 12:57 PM   #31
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

Yes, completely self contained. The change from older style thermal expansion valves with a round diaphragm and capillary tubing to measure evaporator temps in r12 systems were replaced with an all contained aluminum block having the same round diaphragm assembly without needing the length of tube for the bulb temperature sensor. They're all contained within the rectangular aluminum block.

These blocks are very modular for fitment into many hvac enclosures by the firewall. Two thru holes from front to rear of the block and temperature sensing contained in the block allows liquid refrigerant to flow thru the metering valve/needle, entering the evap coils, expanding quickly from liquid to gas while absorbing heat from the coils and fins. Once making the circuit thru the coils, gas leaves thru the other thru holes to return back to the compressor.

Search rockauto for txvs for close up images or a general search of txvs with block style construction and explanation of how it works. The basic engineering principles hasn't changed from older style txvs, just redesign for fitting into a rectangular block. I remember an older txv in a sports car using r12 with a long temperature (capillary) tube when the evaporator coil failed. Reused the txv fitted to the new coil and cooling was restored.
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Old 07-08-2022, 10:34 AM   #32
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Default Do the Schreder valves used in the a/c system have special seals or something?

Do the Schreder valves used in the a/c system have special seals or something? Or can any Schreder valve that fits be used? Thanks

Context-- so, I recently found pressure in system had gone to zero, found a leaky Schreder valve (doesn't leak when protective plastic cap is tightened down in place.) Appear to be no other leaks. Will replace valve, attempt to fill with *correct* amount of R134a, proceed to see whether system provides any cool air...

Last edited by steve seibel; 07-08-2022 at 10:41 AM.
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Old 07-08-2022, 11:17 AM   #33
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

Do it right or don't do it at all. Ac repairs are unforgiving of mistakes.

It's too easy to use tire valve cores so its your dimes if a tire valve core used in ac service valves breaks down (sooner or later from incompatibility of seals to refrigerant and oil). There are replacement ac valve cores, maybe in local auto stores but certainly online. If they're made for ac use then there was thought that went into replacing ac/r134a valve cores with exact replacements and not tire valve cores otherwise dealers, repair shops and service manuals would declare them safe. As always, it's your choice.
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Old 07-08-2022, 01:24 PM   #34
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Default Re: Troubleshooting a/c problems-- pressure difference but no cooling

^^^^ +1

Those A/C Schrader valves are so inexpensive (and small) that they come in envelope packs of 5 or 10. Readily available at most FLAPS.
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